| 0/0 |
2026/08/01 03:13 |
flow |
assessment-security |
24m
Results: map[DenialOfService:true Explanation:**Bug Analysis:**
The bug is a memory leak and potential interior pointer free in the USB Audio
Class 1 (UAC1) gadget function's configfs attribute store function
`f_uac1_opts_c_srate_store` (and other similar rate attributes defined via the
`UAC1_RATE_ATTRIBUTE` macro).
When a user writes to the `c_srate` configfs attribute, the kernel executes the
following code:
```c
split_page = kstrdup(page, GFP_KERNEL);
while ((token = strsep(&split_page, ",")) != NULL) {
ret = kstrtou32(token, 0, &num);
if (ret)
goto end;
opts->name##s[i++] = num;
ret = len;
};
end:
kfree(split_page);
```
The `strsep()` function modifies the `split_page` pointer to point to the
character after the delimiter (or `NULL` if it reaches the end of the string).
1. **Memory Leak**: If the parsing is completely successful, `strsep()` will
eventually set `split_page` to `NULL`. The `kfree(NULL)` call at the end does
nothing, and the original memory allocated by `kstrdup()` is leaked. Repeatedly
writing valid values to this attribute will cause a memory leak, eventually
leading to an Out-Of-Memory (OOM) Denial of Service.
2. **Interior Pointer Free**: If the parsing fails on a subsequent token (e.g.,
if the user writes `"4,abc"`), `strsep()` will have advanced `split_page` past
the first token. The `goto end;` statement is executed, and `kfree(split_page)`
is called on an interior pointer (e.g., `original_ptr + 2`).
**Exploitability:**
Freeing an interior pointer in the SLUB allocator is highly exploitable. In
production builds without debugging tools like KASAN, SLUB does not check if the
freed pointer is aligned to the object's start. When an interior pointer is
freed, SLUB places it on the freelist. The next allocation from that slab will
return the interior pointer. This allows an attacker to obtain an allocation
that overlaps with the adjacent object in the slab (a classic overlapping chunks
vulnerability). By writing to this overlapping allocation, the attacker can
overwrite the contents (e.g., function pointers, size fields) of the adjacent
object, leading to arbitrary code execution or privilege escalation. Thus, the
bug is exploitable.
**Access and Trigger:**
The vulnerable code is part of the configfs interface for configuring USB
gadgets (`/sys/kernel/config/usb_gadget/...`).
- **Unprivileged Access**: Configfs directories and files are owned by root and
have `0755` and `0644` permissions, respectively. Unprivileged users cannot
write to these files.
- **User Namespaces**: The configfs filesystem does not have the
`FS_USERNS_MOUNT` flag set. Therefore, it cannot be mounted from within an
unprivileged user namespace unless the process already has `CAP_SYS_ADMIN` in
the initial namespace. Even if it is already mounted by the host, the file
permissions prevent a user namespace process from writing to it.
- **Remote/Peripheral/VM**: The bug is in the host's configuration interface for
the USB gadget, which must be configured locally before the gadget is bound to a
USB controller. It cannot be triggered by a connected USB host, network traffic,
or a VM guest.
Therefore, the bug is only accessible to a local root user (or a user with
`CAP_SYS_ADMIN` in the initial namespace). While it requires high privileges to
trigger, it still represents a valid kernel bug that allows root to crash the
kernel or corrupt memory. Exploitable:true FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:false UserNamespace:false VMGuestTrigger:false VMHostTrigger:false]
|
| 1/1 |
2026/08/01 03:13 |
action |
syz-repro-to-c-repro |
0m
Results: map[SimplifiedCRepro:// autogenerated by syzkaller (https://github.com/google/syzkaller)
#define _GNU_SOURCE
#include <endian.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mount.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <unistd.h>
uint64_t r[1] = {0xffffffffffffffff};
int main(void)
{
syscall(__NR_mmap, /*addr=*/0x1ffffffff000ul, /*len=*/0x1000, /*prot=*/0ul, /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/0x32ul, /*fd=*/(intptr_t)-1, /*offset=*/0ul);
syscall(__NR_mmap, /*addr=*/0x200000000000ul, /*len=*/0x1000000, /*prot=PROT_WRITE|PROT_READ|PROT_EXEC*/7ul, /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/0x32ul, /*fd=*/(intptr_t)-1, /*offset=*/0ul);
syscall(__NR_mmap, /*addr=*/0x200001000000ul, /*len=*/0x1000, /*prot=*/0ul, /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/0x32ul, /*fd=*/(intptr_t)-1, /*offset=*/0ul);
const char* reason;
(void)reason;
intptr_t res = 0;
if (write(1, "executing program\n", sizeof("executing program\n") - 1)) {}
// mkdirat arguments: [
// fd: fd_dir (resource)
// path: ptr[in, buffer] {
// buffer: {2e 2f 63 66 67 00} (length 0x6)
// }
// mode: open_mode = 0x1ff (8 bytes)
// ]
memcpy((void*)0x200000000000, "./cfg\000", 6);
syscall(__NR_mkdirat, /*fd=*/0xffffff9c, /*path=*/0x200000000000ul, /*mode=S_IXOTH|S_IWOTH|S_IROTH|S_IXGRP|S_IWGRP|S_IRGRP|S_IXUSR|S_IWUSR|0x100*/0x1fful);
// mount arguments: [
// src: nil
// dst: ptr[in, buffer] {
// buffer: {2e 2f 63 66 67 00} (length 0x6)
// }
// type: ptr[in, buffer] {
// buffer: {63 6f 6e 66 69 67 66 73 00} (length 0x9)
// }
// flags: mount_flags = 0x0 (8 bytes)
// data: nil
// ]
memcpy((void*)0x200000000000, "./cfg\000", 6);
memcpy((void*)0x200000000080, "configfs\000", 9);
syscall(__NR_mount, /*src=*/0ul, /*dst=*/0x200000000000ul, /*type=*/0x200000000080ul, /*flags=*/0ul, /*data=*/0ul);
// mkdirat arguments: [
// fd: fd_dir (resource)
// path: ptr[in, buffer] {
// buffer: {2e 2f 63 66 67 2f 75 73 62 5f 67 61 64 67 65 74 2f 67 31 00} (length 0x14)
// }
// mode: open_mode = 0x1ff (8 bytes)
// ]
memcpy((void*)0x2000000000c0, "./cfg/usb_gadget/g1\000", 20);
syscall(__NR_mkdirat, /*fd=*/0xffffff9c, /*path=*/0x2000000000c0ul, /*mode=S_IXOTH|S_IWOTH|S_IROTH|S_IXGRP|S_IWGRP|S_IRGRP|S_IXUSR|S_IWUSR|0x100*/0x1fful);
// mkdirat arguments: [
// fd: fd_dir (resource)
// path: ptr[in, buffer] {
// buffer: {2e 2f 63 6f 6e 66 69 67 00} (length 0x9)
// }
// mode: open_mode = 0x1ff (8 bytes)
// ]
memcpy((void*)0x200000000000, "./config\000", 9);
syscall(__NR_mkdirat, /*fd=*/0xffffff9c, /*path=*/0x200000000000ul, /*mode=S_IXOTH|S_IWOTH|S_IROTH|S_IXGRP|S_IWGRP|S_IRGRP|S_IXUSR|S_IWUSR|0x100*/0x1fful);
// mount arguments: [
// src: nil
// dst: ptr[in, buffer] {
// buffer: {2e 2f 63 6f 6e 66 69 67 00} (length 0x9)
// }
// type: ptr[in, buffer] {
// buffer: {63 6f 6e 66 69 67 66 73 00} (length 0x9)
// }
// flags: mount_flags = 0x0 (8 bytes)
// data: nil
// ]
memcpy((void*)0x200000000200, "./config\000", 9);
memcpy((void*)0x200000000300, "configfs\000", 9);
syscall(__NR_mount, /*src=*/0ul, /*dst=*/0x200000000200ul, /*type=*/0x200000000300ul, /*flags=*/0ul, /*data=*/0ul);
// mkdirat arguments: [
// fd: fd_dir (resource)
// path: ptr[in, buffer] {
// buffer: {2e 2f 63 6f 6e 66 69 67 2f 75 73 62 5f 67 61 64 67 65 74 2f 67 31 2f 66 75 6e 63 74 69 6f 6e 73 2f 75 61 63 31 2e 75 73 62 30 00} (length 0x2b)
// }
// mode: open_mode = 0x1ff (8 bytes)
// ]
memcpy((void*)0x200000000a00, "./config/usb_gadget/g1/functions/uac1.usb0\000", 43);
syscall(__NR_mkdirat, /*fd=*/0xffffff9c, /*path=*/0x200000000a00ul, /*mode=S_IXOTH|S_IWOTH|S_IROTH|S_IXGRP|S_IWGRP|S_IRGRP|S_IXUSR|S_IWUSR|0x100*/0x1fful);
// openat arguments: [
// fd: fd_dir (resource)
// file: ptr[in, buffer] {
// buffer: {2e 2f 63 6f 6e 66 69 67 2f 75 73 62 5f 67 61 64 67 65 74 2f 67 31 2f 66 75 6e 63 74 69 6f 6e 73 2f 75 61 63 31 2e 75 73 62 30 2f 63 5f 73 72 61 74 65 00} (length 0x33)
// }
// flags: open_flags = 0x1 (4 bytes)
// mode: open_mode = 0x0 (2 bytes)
// ]
// returns fd
memcpy((void*)0x200000001300, "./config/usb_gadget/g1/functions/uac1.usb0/c_srate\000", 51);
res = syscall(__NR_openat, /*fd=*/0xffffff9c, /*file=*/0x200000001300ul, /*flags=O_WRONLY*/1, /*mode=*/0);
if (res != -1)
r[0] = res;
// write arguments: [
// fd: fd (resource)
// buf: ptr[in, buffer] {
// buffer: {34} (length 0x1)
// }
// count: len = 0x1 (8 bytes)
// ]
memset((void*)0x200000001400, 52, 1);
syscall(__NR_write, /*fd=*/r[0], /*buf=*/0x200000001400ul, /*count=*/1ul);
return 0;
}
]
|
| 2/1 |
2026/08/01 03:13 |
action |
kernel-checkouter |
0m
Results: map[KernelSrc:/app/workdir/cache/src/d8c60cecff4c452a1f8b661b13d5b4d032f7121a]
|
| 3/1 |
2026/08/01 03:13 |
action |
kernel-builder |
12m
Results: map[KernelObj:/app/workdir/cache/build/749b5be1392d4852b918972c1cd89aa535889e4b]
|
| 4/1 |
2026/08/01 03:25 |
action |
codesearch-prepare |
9m
Results: map[Index:codesearch-index]
|
| 5/1 |
2026/08/01 03:34 |
agent |
expert |
3m
Model:
Results: map[DenialOfService:true Exploitable:true FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:false UserNamespace:false VMGuestTrigger:false VMHostTrigger:false]
Instruction: You are an experienced Linux kernel security engineer. Your task is to analyze given kernel bug report
and determine its security impact based on the following dimensions.
Use the provided tools to examine the source code, check for capability checks (e.g., capable(), ns_capable()),
and understand the nature of the bug. Analyze the given kernel build and configuration.
You can check the kernel config by grepping ".config" file; you can check kernel cmdline by grepping
".config" file for "CONFIG_CMDLINE=". Assume sysctl parameters have default values.
But analyze for the corresponding production build w/o debugging tools enabled (like KASAN, KMSAN, UBSAN).
Try different strategies when analyzing the bug:
- think of ways in which the vulnerable code is unreachable
- or the other way around: try to come up with different ideas of how an unprivileged user can reach the bug
If still unsure err on the side of the bug being non-exploitable/not-accessible.
In the final reply, provide a reasoning for your assessment.
Analysis dimensions:
* Exploitable:
Determine if the bug can result in memory corruption, elevated privileges, or an information leak.
Memory safety issues are almost always exploitable (KASAN or UBSAN reports for use-after-free, out-of-bounds;
refcounting issues, corrupted lists, etc). When kernel is crashing on a completely wild pointer access
(e.g. user-space address, or non-canonical address, but not on NULL or address corresponding to KASAN shadow
for NULL address), including both data accesses and control transfers, that also usually implies possibility
of exploitation. Such reports usually say "unable to handle kernel paging request".
Uses of uninitialized values detected by KMSAN may be exploitable b/c attacker frequently can affect uninit
values with spraying techniques. However, for these exploitability depends on how exactly the uninit value
is used in the code, and what it affects.
Information leaks are exploitable on their own and should be classified as such. A bug that copies kernel
memory contents to userspace (e.g. an out-of-bounds read whose result is returned to the caller, or
uninitialized stack/heap bytes written to a user buffer) is exploitable: it can reveal kernel pointer
values and defeat KASLR, expose sensitive data such as cryptographic keys or other processes' memory, and
serves as a necessary building block in most modern kernel privilege-escalation exploit chains. Do not classify
an information leak as non-exploitable solely because it does not directly cause a memory write or control-flow
hijack; the leak itself is the exploit primitive.
Think of what happens after the bug is triggered. Some bugs cause kernel panic and halt execution,
they are harder to exploit. For example, BUG reports halts the kernel. However, WARNING reports don't halt
execution in production builds. Debug bug detection tools (like KASAN, KMSAN, KCSAN, UBSAN) are also not enabled
in production builds, so attacker can freely exploit these bugs w/o being detected by these tools.
If you see an integer overflow, think how the overflowed value used later (if it's used as allocation size,
or an array index). If you see an out-of-bounds read, think if it's followed by an out-of-bounds write as well.
Some KCSAN data-races may be exploitable by skilled attackers as well. Think what data structures got corrupted
as the result of data races and how. However, note that kernel has lots of "benign" data races that don't lead
to any runtime misbehavior at all.
* Denial Of Service:
Determine if the bug can result in denial-of-service. Most bugs can, since they cause system crash,
hangs, deadlocks, or resource leaks. This is mostly applicable to WARNING bugs that won't cause system crash
in production. For these think what will be consequences of the violation of the kernel assumptions flagged
by the WARNING. In some cases the unexpected condition is also properly handled by the normal control flow
(e.g. with "if (WARN_ON(...))"), these won't cause denial-of-service. If the condition is not handled,
then it may or may not cause denial-of-service.
* Accessible From Unprivileged Processes:
Determine if the bug can be reached from a typical (non-root) user process that does NOT have any special capabilities
(like CAP_SYS_ADMIN, CAP_NET_ADMIN, CAP_NET_RAW, CAP_PERFMON) or access to device nodes restricted to root.
Assume that unprivileged_bpf_disabled=1, that is eBPF loading is not accessible. However, cBPF (classical BPF)
is still accessible to non-root processes.
Assume that user namespaces are not accessible, that is, the process cannot get the mentioned capabilities even
within a new user namespace (checked by ns_capable() function in the kernel sources).
* Accessible From User Namespaces:
Determine if the bug can be reached within a user-namespace where the process has all capabilities
(including CAP_SYS_ADMIN, CAP_NET_ADMIN, CAP_NET_RAW, CAP_PERFMON). Such capabilities are checked with ns_capable()
function in the kernel sources.
* VM Guest Trigger:
Determine if the bug can be triggered from the context of a typical KVM guest (e.g., set up by a QEMU VMM).
Consider accesses to standard Linux host paravirtualized features (virtio-blk, virtio-net, etc.),
and handling of VM exits in the KVM code.
* VM Host Trigger in The Confidential Computing Context:
Determine if the bug can be triggered in a confidential computing guest kernel from the context of a KVM host.
Consider access to standard Linux guest paravirtualized features (virtio-blk, virtio-net, etc.).
* Ethernet Network Trigger:
Determine if the bug can be triggered by processing ingress network Ethernet traffic, either directly (network stack)
or via drivers exposed to network data.
* Other Remote Trigger:
Determine if the bug can be triggered by processing remote traffic other than Ethernet (Wifi, Bluetooth, NFC, etc).
* Peripheral Trigger:
Determine if the bug can be triggered via an untrusted peripheral device that can be physically plugged
into a system, such as a USB device or a niche hardware driver handling external hardware inputs.
This is particularly important for mobile and desktop environments where users can plug in unknown devices.
* Malicious Filesystem Trigger:
Determine if the bug can be triggered by the kernel mounting and parsing a malicious filesystem image.
This is highly critical for Desktop and Mobile environments where external media or downloaded images
might be auto-mounted.
Don't make assumptions about the kernel source code (it may be different from what you assume it is).
Extensively use the provided code access tools (codesearch-*, git-*, grepper, etc)
to examine the actual source code, and confirm any assumptions.
Prefer calling several tools at the same time to save round-trips.
Use set-results tool to provide results of the analysis.
It must be called exactly once before the final reply.
Ignore results of this tool.
Prompt:
The kernel bug report is:
2026/08/01 03:11:31 executed programs: 5
BUG: memory leak
unreferenced object 0xffff8881116a9c10 (size 8):
comm "syz.0.19", pid 5937, jiffies 4294943638
hex dump (first 8 bytes):
34 00 00 00 00 00 00 00 4.......
backtrace (crc 1875d87):
kmemleak_alloc_recursive include/linux/kmemleak.h:44 [inline]
slab_post_alloc_hook mm/slub.c:4597 [inline]
slab_alloc_node mm/slub.c:4917 [inline]
__do_kmalloc_node mm/slub.c:5333 [inline]
__kmalloc_node_track_caller_noprof+0x3d3/0x570 mm/slub.c:5471
__kmemdup_nul mm/util.c:64 [inline]
kstrdup+0x3c/0x80 mm/util.c:84
f_uac1_opts_c_srate_store+0xbb/0x170 drivers/usb/gadget/function/f_uac1.c:1666
flush_write_buffer fs/configfs/file.c:207 [inline]
configfs_write_iter+0x10d/0x1a0 fs/configfs/file.c:229
new_sync_write fs/read_write.c:595 [inline]
vfs_write+0x3ac/0x720 fs/read_write.c:687
ksys_write+0xa5/0x170 fs/read_write.c:739
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xf8/0x610 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
BUG: memory leak
unreferenced object 0xffff888109bd5f20 (size 8):
comm "syz.0.20", pid 5965, jiffies 4294944214
hex dump (first 8 bytes):
34 00 00 00 00 00 00 00 4.......
backtrace (crc 1875d87):
kmemleak_alloc_recursive include/linux/kmemleak.h:44 [inline]
slab_post_alloc_hook mm/slub.c:4597 [inline]
slab_alloc_node mm/slub.c:4917 [inline]
__do_kmalloc_node mm/slub.c:5333 [inline]
__kmalloc_node_track_caller_noprof+0x3d3/0x570 mm/slub.c:5471
__kmemdup_nul mm/util.c:64 [inline]
kstrdup+0x3c/0x80 mm/util.c:84
f_uac1_opts_c_srate_store+0xbb/0x170 drivers/usb/gadget/function/f_uac1.c:1666
flush_write_buffer fs/configfs/file.c:207 [inline]
configfs_write_iter+0x10d/0x1a0 fs/configfs/file.c:229
new_sync_write fs/read_write.c:595 [inline]
vfs_write+0x3ac/0x720 fs/read_write.c:687
ksys_write+0xa5/0x170 fs/read_write.c:739
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xf8/0x610 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
connection error: failed to recv *flatrpc.ExecutorMessageRawT: EOF
It is reproducible with the followint program.
Keep in mind that it may lack the precise threading, sandboxing, and some arguments of a working reproducer.
But it should give an idea of the involved syscalls.
// autogenerated by syzkaller (https://github.com/google/syzkaller)
#define _GNU_SOURCE
#include <endian.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mount.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <unistd.h>
uint64_t r[1] = {0xffffffffffffffff};
int main(void)
{
syscall(__NR_mmap, /*addr=*/0x1ffffffff000ul, /*len=*/0x1000, /*prot=*/0ul, /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/0x32ul, /*fd=*/(intptr_t)-1, /*offset=*/0ul);
syscall(__NR_mmap, /*addr=*/0x200000000000ul, /*len=*/0x1000000, /*prot=PROT_WRITE|PROT_READ|PROT_EXEC*/7ul, /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/0x32ul, /*fd=*/(intptr_t)-1, /*offset=*/0ul);
syscall(__NR_mmap, /*addr=*/0x200001000000ul, /*len=*/0x1000, /*prot=*/0ul, /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/0x32ul, /*fd=*/(intptr_t)-1, /*offset=*/0ul);
const char* reason;
(void)reason;
intptr_t res = 0;
if (write(1, "executing program\n", sizeof("executing program\n") - 1)) {}
// mkdirat arguments: [
// fd: fd_dir (resource)
// path: ptr[in, buffer] {
// buffer: {2e 2f 63 66 67 00} (length 0x6)
// }
// mode: open_mode = 0x1ff (8 bytes)
// ]
memcpy((void*)0x200000000000, "./cfg\000", 6);
syscall(__NR_mkdirat, /*fd=*/0xffffff9c, /*path=*/0x200000000000ul, /*mode=S_IXOTH|S_IWOTH|S_IROTH|S_IXGRP|S_IWGRP|S_IRGRP|S_IXUSR|S_IWUSR|0x100*/0x1fful);
// mount arguments: [
// src: nil
// dst: ptr[in, buffer] {
// buffer: {2e 2f 63 66 67 00} (length 0x6)
// }
// type: ptr[in, buffer] {
// buffer: {63 6f 6e 66 69 67 66 73 00} (length 0x9)
// }
// flags: mount_flags = 0x0 (8 bytes)
// data: nil
// ]
memcpy((void*)0x200000000000, "./cfg\000", 6);
memcpy((void*)0x200000000080, "configfs\000", 9);
syscall(__NR_mount, /*src=*/0ul, /*dst=*/0x200000000000ul, /*type=*/0x200000000080ul, /*flags=*/0ul, /*data=*/0ul);
// mkdirat arguments: [
// fd: fd_dir (resource)
// path: ptr[in, buffer] {
// buffer: {2e 2f 63 66 67 2f 75 73 62 5f 67 61 64 67 65 74 2f 67 31 00} (length 0x14)
// }
// mode: open_mode = 0x1ff (8 bytes)
// ]
memcpy((void*)0x2000000000c0, "./cfg/usb_gadget/g1\000", 20);
syscall(__NR_mkdirat, /*fd=*/0xffffff9c, /*path=*/0x2000000000c0ul, /*mode=S_IXOTH|S_IWOTH|S_IROTH|S_IXGRP|S_IWGRP|S_IRGRP|S_IXUSR|S_IWUSR|0x100*/0x1fful);
// mkdirat arguments: [
// fd: fd_dir (resource)
// path: ptr[in, buffer] {
// buffer: {2e 2f 63 6f 6e 66 69 67 00} (length 0x9)
// }
// mode: open_mode = 0x1ff (8 bytes)
// ]
memcpy((void*)0x200000000000, "./config\000", 9);
syscall(__NR_mkdirat, /*fd=*/0xffffff9c, /*path=*/0x200000000000ul, /*mode=S_IXOTH|S_IWOTH|S_IROTH|S_IXGRP|S_IWGRP|S_IRGRP|S_IXUSR|S_IWUSR|0x100*/0x1fful);
// mount arguments: [
// src: nil
// dst: ptr[in, buffer] {
// buffer: {2e 2f 63 6f 6e 66 69 67 00} (length 0x9)
// }
// type: ptr[in, buffer] {
// buffer: {63 6f 6e 66 69 67 66 73 00} (length 0x9)
// }
// flags: mount_flags = 0x0 (8 bytes)
// data: nil
// ]
memcpy((void*)0x200000000200, "./config\000", 9);
memcpy((void*)0x200000000300, "configfs\000", 9);
syscall(__NR_mount, /*src=*/0ul, /*dst=*/0x200000000200ul, /*type=*/0x200000000300ul, /*flags=*/0ul, /*data=*/0ul);
// mkdirat arguments: [
// fd: fd_dir (resource)
// path: ptr[in, buffer] {
// buffer: {2e 2f 63 6f 6e 66 69 67 2f 75 73 62 5f 67 61 64 67 65 74 2f 67 31 2f 66 75 6e 63 74 69 6f 6e 73 2f 75 61 63 31 2e 75 73 62 30 00} (length 0x2b)
// }
// mode: open_mode = 0x1ff (8 bytes)
// ]
memcpy((void*)0x200000000a00, "./config/usb_gadget/g1/functions/uac1.usb0\000", 43);
syscall(__NR_mkdirat, /*fd=*/0xffffff9c, /*path=*/0x200000000a00ul, /*mode=S_IXOTH|S_IWOTH|S_IROTH|S_IXGRP|S_IWGRP|S_IRGRP|S_IXUSR|S_IWUSR|0x100*/0x1fful);
// openat arguments: [
// fd: fd_dir (resource)
// file: ptr[in, buffer] {
// buffer: {2e 2f 63 6f 6e 66 69 67 2f 75 73 62 5f 67 61 64 67 65 74 2f 67 31 2f 66 75 6e 63 74 69 6f 6e 73 2f 75 61 63 31 2e 75 73 62 30 2f 63 5f 73 72 61 74 65 00} (length 0x33)
// }
// flags: open_flags = 0x1 (4 bytes)
// mode: open_mode = 0x0 (2 bytes)
// ]
// returns fd
memcpy((void*)0x200000001300, "./config/usb_gadget/g1/functions/uac1.usb0/c_srate\000", 51);
res = syscall(__NR_openat, /*fd=*/0xffffff9c, /*file=*/0x200000001300ul, /*flags=O_WRONLY*/1, /*mode=*/0);
if (res != -1)
r[0] = res;
// write arguments: [
// fd: fd (resource)
// buf: ptr[in, buffer] {
// buffer: {34} (length 0x1)
// }
// count: len = 0x1 (8 bytes)
// ]
memset((void*)0x200000001400, 52, 1);
syscall(__NR_write, /*fd=*/r[0], /*buf=*/0x200000001400ul, /*count=*/1ul);
return 0;
}
Reply: **Bug Analysis:**
The bug is a memory leak and potential interior pointer free in the USB Audio Class 1 (UAC1) gadget function's configfs attribute store function `f_uac1_opts_c_srate_store` (and other similar rate attributes defined via the `UAC1_RATE_ATTRIBUTE` macro).
When a user writes to the `c_srate` configfs attribute, the kernel executes the following code:
```c
split_page = kstrdup(page, GFP_KERNEL);
while ((token = strsep(&split_page, ",")) != NULL) {
ret = kstrtou32(token, 0, &num);
if (ret)
goto end;
opts->name##s[i++] = num;
ret = len;
};
end:
kfree(split_page);
```
The `strsep()` function modifies the `split_page` pointer to point to the character after the delimiter (or `NULL` if it reaches the end of the string).
1. **Memory Leak**: If the parsing is completely successful, `strsep()` will eventually set `split_page` to `NULL`. The `kfree(NULL)` call at the end does nothing, and the original memory allocated by `kstrdup()` is leaked. Repeatedly writing valid values to this attribute will cause a memory leak, eventually leading to an Out-Of-Memory (OOM) Denial of Service.
2. **Interior Pointer Free**: If the parsing fails on a subsequent token (e.g., if the user writes `"4,abc"`), `strsep()` will have advanced `split_page` past the first token. The `goto end;` statement is executed, and `kfree(split_page)` is called on an interior pointer (e.g., `original_ptr + 2`).
**Exploitability:**
Freeing an interior pointer in the SLUB allocator is highly exploitable. In production builds without debugging tools like KASAN, SLUB does not check if the freed pointer is aligned to the object's start. When an interior pointer is freed, SLUB places it on the freelist. The next allocation from that slab will return the interior pointer. This allows an attacker to obtain an allocation that overlaps with the adjacent object in the slab (a classic overlapping chunks vulnerability). By writing to this overlapping allocation, the attacker can overwrite the contents (e.g., function pointers, size fields) of the adjacent object, leading to arbitrary code execution or privilege escalation. Thus, the bug is exploitable.
**Access and Trigger:**
The vulnerable code is part of the configfs interface for configuring USB gadgets (`/sys/kernel/config/usb_gadget/...`).
- **Unprivileged Access**: Configfs directories and files are owned by root and have `0755` and `0644` permissions, respectively. Unprivileged users cannot write to these files.
- **User Namespaces**: The configfs filesystem does not have the `FS_USERNS_MOUNT` flag set. Therefore, it cannot be mounted from within an unprivileged user namespace unless the process already has `CAP_SYS_ADMIN` in the initial namespace. Even if it is already mounted by the host, the file permissions prevent a user namespace process from writing to it.
- **Remote/Peripheral/VM**: The bug is in the host's configuration interface for the USB gadget, which must be configured locally before the gadget is bound to a USB controller. It cannot be triggered by a connected USB host, network traffic, or a VM guest.
Therefore, the bug is only accessible to a local root user (or a user with `CAP_SYS_ADMIN` in the initial namespace). While it requires high privileges to trigger, it still represents a valid kernel bug that allows root to crash the kernel or corrupt memory.
|
| 6/2 |
2026/08/01 03:34 |
llm |
expert |
0m
Model:
Tokens:
input: 9500
output: 54
thoughts: 346
Thoughts: **Memory Leak Diagnosis in f_uac1_opts_c_srate_store**
Alright, let's dive into this memory leak report in `f_uac1_opts_c_srate_store`. The backtrace is pretty clear, pointing directly to a kmemleak triggered during a `kstrdup` call within that function. I can see the allocation chain: it goes through the usual suspects—`kmemleak_alloc_recursive`, the SLUB allocator hooks, ultimately landing in `__kmalloc_node_track_caller_noprof`. The key function is `f_uac1_opts_c_srate_store` at line 1666 in `drivers/usb/gadget/function/f_uac1.c`. So, it seems a string is being duplicated, and somehow, the original isn't being freed correctly, or perhaps the duplication itself is unnecessary. The trace also shows this being called from `configfs_write_iter`, so it must be a configuration option write. My instinct is that this is likely a configuration issue and I know that I will have to look at the surrounding code to determine how that string is handled and released and how the call to `configfs_write_iter` interacts with it.
|
| 7/2 |
2026/08/01 03:34 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:drivers/usb/gadget/function/f_uac1.c Name:f_uac1_opts_c_srate_store]
Results: map[SourceCode:1666: UAC1_RATE_ATTRIBUTE(c_srate);
SourceFile:drivers/usb/gadget/function/f_uac1.c]
|
| 8/2 |
2026/08/01 03:34 |
llm |
expert |
0m
Model:
Tokens:
input: 10232
output: 47
thoughts: 32
Thoughts: Okay, here's my interpretation of that thought process, assuming I'm deep in the weeds of embedded systems or low-level driver development:
**Deciphering the Macro**
Right, that's a familiar macro staring me in the face: `UAC1_RATE_ATTRIBUTE`. My immediate thought is, "Okay, what's *that* about?" I need to understand its definition completely. I've seen it before, but to be sure, I need to look up precisely what `UAC1_RATE_ATTRIBUTE` entails. There could be some edge case to keep in mind regarding timing or memory usage. Let's dig into that definition.
|
| 9/2 |
2026/08/01 03:35 |
tool |
codesearch-definition-source |
0m
Error:
Args: map[ContextFile:drivers/usb/gadget/function/f_uac1.c Name:UAC1_RATE_ATTRIBUTE]
Results: map[SourceCode: SourceFile:]
|
| 10/2 |
2026/08/01 03:35 |
llm |
expert |
0m
Model:
Tokens:
input: 10473
output: 43
thoughts: 33
Thoughts: Okay, here's my take on those thoughts, tailored for an expert audience:
**Investigating `UAC1_RATE_ATTRIBUTE` in the USB Audio Class Gadget Driver**
Right, let's dive into this. My first move here is to track down instances of `UAC1_RATE_ATTRIBUTE` within the USB Audio Class driver for gadget mode, specifically in `drivers/usb/gadget/function/f_uac1.c`. Grepping for this constant immediately gives me a sense of how it's being used. I'm expecting to see it involved in defining or accessing audio sample rate configurations. Are we talking about attribute descriptors, control requests, or maybe something related to the interface itself? I'll be looking for clues about the driver's handling of sample rate negotiation and potential errors or constraints. Hopefully, this simple search will reveal the context and purpose of this specific rate attribute within the broader UAC1 implementation. It’s the obvious first step for me to understand how the audio rate is managed within this specific gadget.
|
| 11/2 |
2026/08/01 03:35 |
tool |
grepper |
0m
Args: map[Expression:UAC1_RATE_ATTRIBUTE PathPrefix:drivers/usb/gadget/function/f_uac1.c]
Results: map[Output:drivers/usb/gadget/function/f_uac1.c=1568=CONFIGFS_ATTR(f_uac1_opts_, name)
drivers/usb/gadget/function/f_uac1.c-1569-
drivers/usb/gadget/function/f_uac1.c:1570:#define UAC1_RATE_ATTRIBUTE(name) \
drivers/usb/gadget/function/f_uac1.c-1571-static ssize_t f_uac1_opts_##name##_show(struct config_item *item, \
--
drivers/usb/gadget/function/f_uac1.c=1665=UAC1_ATTRIBUTE(u32, c_chmask);
drivers/usb/gadget/function/f_uac1.c:1666:UAC1_RATE_ATTRIBUTE(c_srate);
drivers/usb/gadget/function/f_uac1.c-1667-UAC1_ATTRIBUTE(u32, c_ssize);
drivers/usb/gadget/function/f_uac1.c=1668=UAC1_ATTRIBUTE(u32, p_chmask);
drivers/usb/gadget/function/f_uac1.c:1669:UAC1_RATE_ATTRIBUTE(p_srate);
drivers/usb/gadget/function/f_uac1.c-1670-UAC1_ATTRIBUTE(u32, p_ssize);
]
|
| 12/2 |
2026/08/01 03:35 |
llm |
expert |
0m
Model:
Tokens:
input: 11156
output: 45
thoughts: 30
Thoughts: Here's my attempt at summarizing the thought process, tailored for an expert audience:
**Diving into f_uac1.c**
Okay, time to get my hands dirty. I'm focusing on `drivers/usb/gadget/function/f_uac1.c` – specifically around line 1570. I need to refresh my memory on the UAC1 function driver's inner workings. My understanding is that this part likely deals with some critical audio stream-related control or data transfer logic. I'm probably looking for a specific function call, data structure, or conditional block that’s been flagged as potentially problematic or interesting in relation to a current debug effort. I'm hoping to quickly identify the relevant code flow. Perhaps a quirk handling, or a potential race condition involving buffer management. The goal is to either confirm my initial suspicions or gain more context before looking further upstream or downstream. Let's see what we find.
|
| 13/2 |
2026/08/01 03:35 |
tool |
read-file |
0m
Args: map[File:drivers/usb/gadget/function/f_uac1.c FirstLine:1570 LineCount:50]
Results: map[Contents:1570: #define UAC1_RATE_ATTRIBUTE(name) \
1571: static ssize_t f_uac1_opts_##name##_show(struct config_item *item, \
1572: char *page) \
1573: { \
1574: struct f_uac1_opts *opts = to_f_uac1_opts(item); \
1575: int result = 0; \
1576: int i; \
1577: \
1578: mutex_lock(&opts->lock); \
1579: page[0] = '\0'; \
1580: for (i = 0; i < UAC_MAX_RATES; i++) { \
1581: if (opts->name##s[i] == 0) \
1582: break; \
1583: result += sprintf(page + strlen(page), "%u,", \
1584: opts->name##s[i]); \
1585: } \
1586: if (strlen(page) > 0) \
1587: page[strlen(page) - 1] = '\n'; \
1588: mutex_unlock(&opts->lock); \
1589: \
1590: return result; \
1591: } \
1592: \
1593: static ssize_t f_uac1_opts_##name##_store(struct config_item *item, \
1594: const char *page, size_t len) \
1595: { \
1596: struct f_uac1_opts *opts = to_f_uac1_opts(item); \
1597: char *split_page = NULL; \
1598: int ret = -EINVAL; \
1599: char *token; \
1600: u32 num; \
1601: int i; \
1602: \
1603: mutex_lock(&opts->lock); \
1604: if (opts->refcnt) { \
1605: ret = -EBUSY; \
1606: goto end; \
1607: } \
1608: \
1609: i = 0; \
1610: memset(opts->name##s, 0x00, sizeof(opts->name##s)); \
1611: split_page = kstrdup(page, GFP_KERNEL); \
1612: while ((token = strsep(&split_page, ",")) != NULL) { \
1613: ret = kstrtou32(token, 0, &num); \
1614: if (ret) \
1615: goto end; \
1616: \
1617: opts->name##s[i++] = num; \
1618: ret = len; \
1619: }; \
]
|
| 14/2 |
2026/08/01 03:35 |
llm |
expert |
0m
Model:
Tokens:
input: 12284
output: 45
thoughts: 185
Thoughts: **Memory Leak! A Deeper Dive**
Okay, hold on a second. I'm looking at this `split_page = kstrdup(page, GFP_KERNEL);` line, and then I see `strsep(&split_page, ",")`. Aha! `strsep` *modifies* the pointer it's given, doesn't it? That means `split_page` now points to whatever's *after* the first comma-separated token, losing the original pointer returned by `kstrdup`.
Then I get to the `end:` label and `kfree(split_page);`. But `split_page` has been advanced by `strsep`! I'm getting it...If `strsep` reaches the end of the string, it sets `split_page` to `NULL`.
So, `kfree(split_page)` is going to free `NULL` if `strsep` reaches the end of the input string, which does nothing. The original memory allocated by `kstrdup` is leaked!
This is a classic memory leak scenario. We allocate memory with `kstrdup`, then *modify* the pointer we use to track it, and then only free that modified pointer and not the allocated memory! I need to check the rest of `f_uac1_opts_##name##_store` to see if that memory is freed anywhere else.
|
| 15/2 |
2026/08/01 03:35 |
tool |
read-file |
0m
Args: map[File:drivers/usb/gadget/function/f_uac1.c FirstLine:1615 LineCount:20]
Results: map[Contents:1615: goto end; \
1616: \
1617: opts->name##s[i++] = num; \
1618: ret = len; \
1619: }; \
1620: \
1621: end: \
1622: kfree(split_page); \
1623: mutex_unlock(&opts->lock); \
1624: return ret; \
1625: } \
1626: \
1627: CONFIGFS_ATTR(f_uac1_opts_, name)
1628:
1629: #define UAC1_ATTRIBUTE_STRING(name) \
1630: static ssize_t f_uac1_opts_##name##_show(struct config_item *item, \
1631: char *page) \
1632: { \
1633: struct f_uac1_opts *opts = to_f_uac1_opts(item); \
1634: int result; \
]
|
| 16/2 |
2026/08/01 03:35 |
llm |
expert |
0m
Model:
Tokens:
input: 13124
output: 38
thoughts: 861
Thoughts: **Analyzing a Potential Memory Leak in ConfigFS**
Okay, so I'm looking at this C code and immediately see a potential issue with how `split_page` is being handled, especially with `strsep`. My initial thought is that `split_page` is getting modified by `strsep`, which is then leading to the loss of the original pointer allocated by `kstrdup`.
If `strsep` completes normally, `split_page` should become `NULL`. `kfree(NULL)` is harmless, so that's not the problem in the successful execution. However, what if `kstrtou32` fails within the loop? The code jumps to `end` via `goto`, and `kfree(split_page)` is executed. But, `strsep` modifies `split_page` to the next token *after* the first `kstrtou32` call has completed or failed. So, on a failure, `kfree(split_page)` is trying to free a pointer that's inside the originally allocated memory block. That's a bad free, a classic memory corruption vulnerability! Or at the very least, a crash in slub.
The bug report only mentions a memory leak, which seems odd. If `strsep` finishes successfully, `split_page` is `NULL`. `kfree(NULL)` does nothing, and the entire allocation, the *original* page, is leaked. That matches the report. Got it. So the leak is the critical issue.
Could this be exploited? Absolutely. A user could repeatedly write to this configfs attribute, and each time, leak a small amount of memory, leading to an out-of-memory condition. That's a classic Denial of Service.
Now, the next question: Can an unprivileged user even *access* this configfs attribute? Configfs is typically mounted at `/sys/kernel/config`, which by default is only writable by root. However, the reproducer code explicitly mounts configfs! And it's doing so through a `mkdirat` and `mount` call.
Can an unprivileged user mount configfs? Usually, you need `CAP_SYS_ADMIN` for mounting filesystems, often requiring a user namespace. But, the prompt *specifically* states that user namespaces are disabled. The process can't get `CAP_SYS_ADMIN` even within a new user namespace. So if user namespaces are unavailable, an unprivileged user cannot mount configfs.
So, the next question becomes: is configfs accessible to unprivileged users if it is already mounted?
It looks like I need to check the permissions of the files and directories inside the mounted configfs to be certain.
|
| 17/2 |
2026/08/01 03:35 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/configfs/dir.c Name:configfs_create_dir]
Results: map[SourceCode: 293: static int configfs_create_dir(struct config_item *item, struct dentry *dentry,
294: struct configfs_fragment *frag)
295: {
296: int error;
297: umode_t mode = S_IFDIR| S_IRWXU | S_IRUGO | S_IXUGO;
298: struct dentry *p = dentry->d_parent;
299: struct inode *p_inode = d_inode(p);
300: struct inode *inode;
301:
302: BUG_ON(!item);
303:
304: error = configfs_make_dirent(p->d_fsdata, dentry, item, mode,
305: CONFIGFS_DIR | CONFIGFS_USET_CREATING,
306: frag);
307: if (unlikely(error))
308: return error;
309:
310: configfs_set_dir_dirent_depth(p->d_fsdata, dentry->d_fsdata);
311: inode = configfs_create(dentry, mode);
312: if (IS_ERR(inode))
313: goto out_remove;
314:
315: inode->i_op = &configfs_dir_inode_operations;
316: inode->i_fop = &configfs_dir_operations;
317: /* directory inodes start off with i_nlink == 2 (for "." entry) */
318: inc_nlink(inode);
319: d_make_persistent(dentry, inode);
320: inc_nlink(p_inode);
321: inode_set_mtime_to_ts(p_inode, inode_set_ctime_current(p_inode));
322: item->ci_dentry = dentry;
323: return 0;
324:
325: out_remove:
326: configfs_put(dentry->d_fsdata);
327: configfs_remove_dirent(dentry);
328: return PTR_ERR(inode);
329: }
SourceFile:fs/configfs/dir.c]
|
| 18/2 |
2026/08/01 03:35 |
llm |
expert |
0m
Model:
Tokens:
input: 15240
output: 38
thoughts: 41
Thoughts: Okay, here's my interpretation of that thought process, assuming I'm deep in the weeds of file system metadata and permissions:
**Analyzing File Mode and Considering Attribute implications**
Alright, so the mode is set. Let's see... `S_IFDIR | S_IRWXU | S_IRUGO | S_IXUGO`. Ah, yes, that's equivalent to `0755` in octal. Fine, a standard directory permissions configuration. Owner has read, write, and execute; group and others have read and execute. Makes sense. But my mind is already racing ahead. I'm focusing on that, then... what about *files*? While this gives us the permissions for directories (the 'mode' piece), it does not cover the attributes of files, which, if this involves files, must be considered.
|
| 19/2 |
2026/08/01 03:35 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/configfs/dir.c Name:configfs_create_file]
Results: map[SourceCode: 446: int configfs_create_file(struct config_item * item, const struct configfs_attribute * attr)
447: {
448: struct dentry *dir = item->ci_dentry;
449: struct configfs_dirent *parent_sd = dir->d_fsdata;
450: umode_t mode = (attr->ca_mode & S_IALLUGO) | S_IFREG;
451: int error = 0;
452:
453: inode_lock_nested(d_inode(dir), I_MUTEX_NORMAL);
454: error = configfs_make_dirent(parent_sd, NULL, (void *) attr, mode,
455: CONFIGFS_ITEM_ATTR, parent_sd->s_frag);
456: inode_unlock(d_inode(dir));
457:
458: return error;
459: }
SourceFile:fs/configfs/file.c]
|
| 20/2 |
2026/08/01 03:35 |
llm |
expert |
0m
Model:
Tokens:
input: 15789
output: 44
thoughts: 36
Thoughts: Okay, here's my interpretation of that thought process, assuming I'm an expert in this field:
**Decoding the Mode & Exploring `c_srate`**
Alright, I'm diving into this. The key here is focusing on the `attr->ca_mode` variable, and specifically, isolating the `S_IALLUGO` bits within it. That mask is probably relevant to user permissions and overall access rights, which is my immediate hunch. I need to understand what specific permissions are granted.
Now, I need more context. I'm curious about how `ca_mode` is set for `c_srate`. I need to figure out how `c_srate` is configured. That context is going to provide insight into why the `S_IALLUGO` bits are relevant. It is also going to allow me to build a fuller picture around what is driving the need to check its configuration.
|
| 21/2 |
2026/08/01 03:35 |
tool |
codesearch-definition-source |
0m
Error:
Args: map[ContextFile:drivers/usb/gadget/function/f_uac1.c Name:CONFIGFS_ATTR]
Results: map[SourceCode: SourceFile:]
|
| 22/2 |
2026/08/01 03:35 |
llm |
expert |
0m
Model:
Tokens:
input: 16088
output: 34
thoughts: 22
Thoughts: Here's my take on those thoughts:
**Grasping for Context: `CONFIGFS_ATTR`**
Okay, so I need to understand something related to the Linux configfs interface. My first instinct is to drill down into the core definition. I'm thinking, "Let's use `grep` to quickly find where `CONFIGFS_ATTR` is defined." I'm mentally parsing the likely path, `include/linux/configfs.h`, figuring that's where the fundamental structures and macros for configfs are going to be declared. This is a common debugging and exploration technique. By finding its declaration, I'll hopefully gain insight into its purpose and how it's used within the configfs framework, which in turn will give me greater clarity for what is going on, and how this relates to my understanding of the system.
|
| 23/2 |
2026/08/01 03:35 |
tool |
grepper |
0m
Args: map[Expression:#define CONFIGFS_ATTR PathPrefix:include/linux/configfs.h]
Results: map[Output:include/linux/configfs.h=115=struct configfs_attribute {
--
include/linux/configfs.h-122-
include/linux/configfs.h:123:#define CONFIGFS_ATTR_PERM(_pfx, _name, _perm) \
include/linux/configfs.h-124-static struct configfs_attribute _pfx##attr_##_name = { \
--
include/linux/configfs.h-131-
include/linux/configfs.h:132:#define CONFIGFS_ATTR(_pfx, _name) CONFIGFS_ATTR_PERM( \
include/linux/configfs.h-133- _pfx, _name, S_IRUGO | S_IWUSR \
--
include/linux/configfs.h-135-
include/linux/configfs.h:136:#define CONFIGFS_ATTR_RO(_pfx, _name) \
include/linux/configfs.h-137-static struct configfs_attribute _pfx##attr_##_name = { \
--
include/linux/configfs.h-143-
include/linux/configfs.h:144:#define CONFIGFS_ATTR_WO(_pfx, _name) \
include/linux/configfs.h-145-static struct configfs_attribute _pfx##attr_##_name = { \
]
|
| 24/2 |
2026/08/01 03:35 |
llm |
expert |
0m
Model:
Tokens:
input: 16681
output: 32
thoughts: 265
Thoughts: **Analyzing the Vulnerability's Trigger**
Okay, so the `CONFIGFS_ATTR` uses `S_IRUGO | S_IWUSR`, which boils down to `0644`. That means the file is read-only for everyone except the owner, which is typically root. Makes sense. Therefore, an unprivileged user shouldn't be able to write to `c_srate` and thus, can't trigger the function `f_uac1_opts_c_srate_store` directly. The prompt also explicitly states that we're assuming user namespaces aren't accessible. So, that's a dead end. An unprivileged user is blocked.
But wait, hold on a sec...is there *any* other conceivable route to get `f_uac1_opts_c_srate_store` executed? The function is fundamentally a configfs attribute store function, right? It's only called when something *writes* to the configfs file. With the file permissions set to `0644` and root as the owner, and with user namespaces out of the picture, the path seems blocked.
Could a VM guest somehow leverage this? No, this is all about USB gadget configuration on the *host*. A peripheral device triggering it? No again, it's configfs, so that's irrelevant. A malicious filesystem trying to inject something? Nope. Network access? Nah. I am pretty confident with my analysis, but it's always worth one last check. Let's just double-check, for the sake of completeness, that there are absolutely no other code paths leading to `f_uac1_opts_c_srate_store`.
|
| 25/2 |
2026/08/01 03:35 |
tool |
codesearch-find-references |
0m
Args: map[Name:f_uac1_opts_c_srate_store]
Results: map[References:[map[ReferenceKind:takes-address-of ReferencingEntityKind:global_variable ReferencingEntityName:f_uac1_opts_attr_c_srate SourceFile:drivers/usb/gadget/function/f_uac1.c SourceLine:1666]] TruncatedOutput:false]
|
| 26/2 |
2026/08/01 03:35 |
llm |
expert |
0m
Model:
Tokens:
input: 17433
output: 32
thoughts: 709
Thoughts: **Analyzing a Potential Kernel Memory Corruption Bug**
Okay, so I'm looking at this function, specifically `f_uac1_opts_attr_c_srate`. It's a configfs attribute for `f_uac1`, so it's pretty isolated. This makes the scope of the problem much easier to define.
Now, let's see if this is exploitable. It seems like a memory leak, but it's more than that. The code's doing string parsing and conversion, which are often sources of vulnerability. Looking at the code:
```c
split_page = kstrdup(page, GFP_KERNEL);
while ((token = strsep(&split_page, ",")) != NULL) {
ret = kstrtou32(token, 0, &num);
if (ret)
goto end;
opts->name##s[i++] = num;
ret = len;
};
end:
kfree(split_page);
```
If `strsep` finishes normally, `split_page` becomes `NULL`. `kfree(NULL)` is fine, it's a no-op, just a leak. The memory allocated by `kstrdup` is leaked. But, if `kstrtou32` fails, `split_page` points to the *next* token (or `NULL` if it was the last token). Now, the crucial part: if it points to a subsequent token, `kfree(split_page)` is going to free an *interior* pointer. Freeing an interior pointer in SLUB is usually a recipe for a kernel bug or a crash.
So, let's test a couple of scenarios. If `page` is `"123,abc"`, `split_page` starts at `"123,abc"`. The first iteration of `strsep` gives `token = "123"` and `split_page` pointing to `"abc"`. `kstrtou32("123")` succeeds. In the second iteration, `token = "abc"`, `split_page` becomes `NULL`. `kstrtou32("abc")` fails, we get an error, and jump to `end`. `kfree(split_page)` becomes `kfree(NULL)`, so it's just a leak, no crash.
What if `page` is `"abc,123"`? Now `split_page` starts at `"abc,123"`. The first iteration sets `token = "abc"` and `split_page` is now at `"123"`. `kstrtou32("abc")` fails, returns an error and goes to `end`. `kfree(split_page)` calls `kfree("123")`. *That's* the interior pointer! I can see it now.
Let's quickly simulate `strsep` to confirm what `split_page` really holds in this scenario.
```c
char *split_page = strdup("abc,123");
char *token = strsep(&split_page, ",");
// token is "abc", split_page is "123"
```
Yes, `split_page` now points to `"123"`. `kfree(split_page)` is then called, which is equivalent to `kfree(original_ptr + 4)`.
Now, the important question is how the SLUB allocator handles this, because that's what determines if this bug is exploitable. SLUB uses the page struct to figure out the slab cache, then calculates the object address with `(ptr - page_address) % size`.
So if I `kfree` a pointer that's inside a valid object, SLUB may determine a different slab cache and cause corruption by corrupting metadata, leading to exploitation.
|
| 27/2 |
2026/08/01 03:35 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:mm/slub.c Name:kfree]
Results: map[SourceCode:6665: /**
6666: * kfree - free previously allocated memory
6667: * @object: pointer returned by kmalloc(), kmalloc_nolock(), or kmem_cache_alloc()
6668: *
6669: * If @object is NULL, no operation is performed.
6670: */
6671: void kfree(const void *object)
6672: {
6673: struct page *page;
6674: struct slab *slab;
6675: struct kmem_cache *s;
6676: void *x = (void *)object;
6677:
6678: trace_kfree(_RET_IP_, object);
6679:
6680: if (unlikely(ZERO_OR_NULL_PTR(object)))
6681: return;
6682:
6683: page = virt_to_page(object);
6684: slab = page_slab(page);
6685: if (!slab) {
6686: /* kmalloc_nolock() doesn't support large kmalloc */
6687: free_large_kmalloc(page, (void *)object);
6688: return;
6689: }
6690:
6691: s = slab->slab_cache;
6692: slab_free(s, slab, x, _RET_IP_);
6693: }
SourceFile:mm/slub.c]
|
| 28/2 |
2026/08/01 03:35 |
llm |
expert |
0m
Model:
Tokens:
input: 19439
output: 33
thoughts: 9
Thoughts: Okay, here's my interpretation of that:
**Investigating `slab_free`**
Alright, let's dive into this. The code mentions `slab_free`, so that immediately sparks my curiosity. My brain's already starting to run through all the things that could mean in the context of memory management. It's likely a function or process related to releasing memory blocks allocated within a slab allocator. I need to understand how it's implemented. Is it just marking the blocks as free? Does it involve de-caching? Are there any garbage collection or reference counting implications I should be aware of? I need to look closely at the surrounding code to grasp its purpose and behavior. Let's see how this ties into the bigger picture.
|
| 29/2 |
2026/08/01 03:35 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:mm/slub.c Name:slab_free]
Results: map[SourceCode:6370: static __fastpath_inline
6371: void slab_free(struct kmem_cache *s, struct slab *slab, void *object,
6372: unsigned long addr)
6373: {
6374: memcg_slab_free_hook(s, slab, &object, 1);
6375: alloc_tagging_slab_free_hook(s, slab, &object, 1);
6376:
6377: if (unlikely(!slab_free_hook(s, object, slab_want_init_on_free(s), false)))
6378: return;
6379:
6380: if (likely(can_free_to_pcs(slab)) && likely(free_to_pcs(s, object, true)))
6381: return;
6382:
6383: __slab_free(s, slab, object, object, 1, addr);
6384: stat(s, FREE_SLOWPATH);
6385: }
SourceFile:mm/slub.c]
|
| 30/2 |
2026/08/01 03:35 |
llm |
expert |
0m
Model:
Tokens:
input: 19943
output: 35
thoughts: 27
Thoughts: Okay, here's my interpretation of those thoughts, framed as my own:
**Digging into Memory Management: A Question of Alignment**
Alright, I'm diving back into this `slab_free` function, and a question is bubbling up in my mind. Does this `slab_free` routine, before it attempts to release memory, actually verify if the `object` pointer it's receiving is properly aligned to the expected object size within the slab? I mean, it's a fundamental principle for memory management within the kernel to avoid misaligned accesses, right? It could cause all sorts of problems down the line if we're not careful.
And looking specifically at the `slab_free_hook` part... well, that's where the real interest lies. That's where a potential alignment check would have the most impact.
|
| 31/2 |
2026/08/01 03:36 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:mm/slub.c Name:slab_free_hook]
Results: map[SourceCode:2569: /*
2570: * Hooks for other subsystems that check memory allocations. In a typical
2571: * production configuration these hooks all should produce no code at all.
2572: *
2573: * Returns true if freeing of the object can proceed, false if its reuse
2574: * was delayed by CONFIG_SLUB_RCU_DEBUG or KASAN quarantine, or it was returned
2575: * to KFENCE.
2576: *
2577: * For objects allocated via kmalloc_nolock(), only a subset of alloc hooks
2578: * are invoked, so some free hooks must handle asymmetric hook calls.
2579: *
2580: * Alloc hooks called for kmalloc_nolock():
2581: * - kmsan_slab_alloc()
2582: * - kasan_slab_alloc()
2583: * - memcg_slab_post_alloc_hook()
2584: * - alloc_tagging_slab_alloc_hook()
2585: *
2586: * Free hooks that must handle missing corresponding alloc hooks:
2587: * - kmemleak_free_recursive()
2588: * - kfence_free()
2589: *
2590: * Free hooks that have no alloc hook counterpart, and thus safe to call:
2591: * - debug_check_no_locks_freed()
2592: * - debug_check_no_obj_freed()
2593: * - __kcsan_check_access()
2594: */
2595: static __always_inline
2596: bool slab_free_hook(struct kmem_cache *s, void *x, bool init,
2597: bool after_rcu_delay)
2598: {
2599: /* Are the object contents still accessible? */
2600: bool still_accessible = (s->flags & SLAB_TYPESAFE_BY_RCU) && !after_rcu_delay;
2601:
2602: kmemleak_free_recursive(x, s->flags);
2603: kmsan_slab_free(s, x);
2604:
2605: debug_check_no_locks_freed(x, s->object_size);
2606:
2607: if (!(s->flags & SLAB_DEBUG_OBJECTS))
2608: debug_check_no_obj_freed(x, s->object_size);
2609:
2610: /* Use KCSAN to help debug racy use-after-free. */
2611: if (!still_accessible)
2612: __kcsan_check_access(x, s->object_size,
2613: KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT);
2614:
2615: if (kfence_free(x))
2616: return false;
2617:
2618: /*
2619: * Give KASAN a chance to notice an invalid free operation before we
2620: * modify the object.
2621: */
2622: if (kasan_slab_pre_free(s, x))
2623: return false;
2624:
2625: #ifdef CONFIG_SLUB_RCU_DEBUG
2626: if (still_accessible) {
2627: struct rcu_delayed_free *delayed_free;
2628:
2629: delayed_free = kmalloc_obj(*delayed_free, GFP_NOWAIT);
2630: if (delayed_free) {
2631: /*
2632: * Let KASAN track our call stack as a "related work
2633: * creation", just like if the object had been freed
2634: * normally via kfree_rcu().
2635: * We have to do this manually because the rcu_head is
2636: * not located inside the object.
2637: */
2638: kasan_record_aux_stack(x);
2639:
2640: delayed_free->object = x;
2641: call_rcu(&delayed_free->head, slab_free_after_rcu_debug);
2642: return false;
2643: }
2644: }
2645: #endif /* CONFIG_SLUB_RCU_DEBUG */
2646:
2647: /*
2648: * As memory initialization might be integrated into KASAN,
2649: * kasan_slab_free and initialization memset's must be
2650: * kept together to avoid discrepancies in behavior.
2651: *
2652: * The initialization memset's clear the object and the metadata,
2653: * but don't touch the SLAB redzone.
2654: *
2655: * The object's freepointer is also avoided if stored outside the
2656: * object.
2657: */
2658: if (unlikely(init)) {
2659: int rsize;
2660: unsigned int inuse, orig_size;
2661:
2662: inuse = get_info_end(s);
2663: orig_size = get_orig_size(s, x);
2664: if (!kasan_has_integrated_init())
2665: memset(kasan_reset_tag(x), 0, orig_size);
2666: rsize = (s->flags & SLAB_RED_ZONE) ? s->red_left_pad : 0;
2667: memset((char *)kasan_reset_tag(x) + inuse, 0,
2668: s->size - inuse - rsize);
2669: /*
2670: * Restore orig_size, otherwise kmalloc redzone overwritten
2671: * would be reported
2672: */
2673: set_orig_size(s, x, orig_size);
2674:
2675: }
2676: /* KASAN might put x into memory quarantine, delaying its reuse. */
2677: return !kasan_slab_free(s, x, init, still_accessible, false);
2678: }
SourceFile:mm/slub.c]
|
| 32/2 |
2026/08/01 03:36 |
llm |
expert |
0m
Model:
Tokens:
input: 21945
output: 34
thoughts: 135
Thoughts: **SLUB Allocation/Freeing - Potential Interior Pointer Issues**
Okay, let me think this through. `__slab_free`... right, it does this:
```c
void *prior = slab->freelist;
set_freepointer(s, object, prior);
slab->freelist = object;
```
Hang on a second. If `object` is actually an interior pointer within a larger allocated object, then `set_freepointer` is going to write the `freelist` pointer directly into the *middle* of that object. That's... not good. So `slab->freelist` will now point to that interior location.
And then, when the next allocation call happens, if it uses the `slab->freelist` pointer, it's going to return an *interior* pointer, not the starting address of a full object! This seems like a pretty significant flaw.
I wonder... does the SLUB allocator have any checks to ensure the freed pointer is aligned to the object size? Maybe there's some kind of alignment check, some validation that would catch this. Time to dig deeper into `__slab_free` and see if anything is there.
|
| 33/2 |
2026/08/01 03:36 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:mm/slub.c Name:__slab_free]
Results: map[SourceCode:5630: /*
5631: * Slow path handling. This may still be called frequently since objects
5632: * have a longer lifetime than the cpu slabs in most processing loads.
5633: *
5634: * So we still attempt to reduce cache line usage. Just take the slab
5635: * lock and free the item. If there is no additional partial slab
5636: * handling required then we can return immediately.
5637: */
5638: static void __slab_free(struct kmem_cache *s, struct slab *slab,
5639: void *head, void *tail, int cnt,
5640: unsigned long addr)
5641:
5642: {
5643: bool was_full;
5644: struct freelist_counters old, new;
5645: struct kmem_cache_node *n = NULL;
5646: unsigned long flags;
5647: bool on_node_partial;
5648:
5649: if (IS_ENABLED(CONFIG_SLUB_TINY) || kmem_cache_debug(s)) {
5650: free_to_partial_list(s, slab, head, tail, cnt, addr);
5651: return;
5652: }
5653:
5654: do {
5655: if (unlikely(n)) {
5656: spin_unlock_irqrestore(&n->list_lock, flags);
5657: n = NULL;
5658: }
5659:
5660: old.freelist = slab->freelist;
5661: old.counters = slab->counters;
5662:
5663: was_full = (old.freelist == NULL);
5664:
5665: set_freepointer(s, tail, old.freelist);
5666:
5667: new.freelist = head;
5668: new.counters = old.counters;
5669: new.inuse -= cnt;
5670:
5671: /*
5672: * Might need to be taken off (due to becoming empty) or added
5673: * to (due to not being full anymore) the partial list.
5674: * Unless it's frozen.
5675: */
5676: if (!new.inuse || was_full) {
5677:
5678: n = get_node(s, slab_nid(slab));
5679: /*
5680: * Speculatively acquire the list_lock.
5681: * If the cmpxchg does not succeed then we may
5682: * drop the list_lock without any processing.
5683: *
5684: * Otherwise the list_lock will synchronize with
5685: * other processors updating the list of slabs.
5686: */
5687: spin_lock_irqsave(&n->list_lock, flags);
5688:
5689: on_node_partial = slab_test_node_partial(slab);
5690: }
5691:
5692: } while (!slab_update_freelist(s, slab, &old, &new, "__slab_free"));
5693:
5694: if (likely(!n)) {
5695: /*
5696: * We didn't take the list_lock because the slab was already on
5697: * the partial list and will remain there.
5698: */
5699: return;
5700: }
5701:
5702: /*
5703: * This slab was partially empty but not on the per-node partial list,
5704: * in which case we shouldn't manipulate its list, just return.
5705: */
5706: if (!was_full && !on_node_partial) {
5707: spin_unlock_irqrestore(&n->list_lock, flags);
5708: return;
5709: }
5710:
5711: /*
5712: * If slab became empty, should we add/keep it on the partial list or we
5713: * have enough?
5714: */
5715: if (unlikely(!new.inuse && n->nr_partial >= s->min_partial))
5716: goto slab_empty;
5717:
5718: /*
5719: * Objects left in the slab. If it was not on the partial list before
5720: * then add it.
5721: */
5722: if (unlikely(was_full)) {
5723: add_partial(n, slab, ADD_TO_TAIL);
5724: stat(s, FREE_ADD_PARTIAL);
5725: }
5726: spin_unlock_irqrestore(&n->list_lock, flags);
5727: return;
5728:
5729: slab_empty:
5730: /*
5731: * The slab could have a single object and thus go from full to empty in
5732: * a single free, but more likely it was on the partial list. Remove it.
5733: */
5734: if (likely(!was_full)) {
5735: remove_partial(n, slab);
5736: stat(s, FREE_REMOVE_PARTIAL);
5737: }
5738:
5739: spin_unlock_irqrestore(&n->list_lock, flags);
5740: stat(s, FREE_SLAB);
5741: discard_slab(s, slab);
5742: }
SourceFile:mm/slub.c]
|
| 34/2 |
2026/08/01 03:36 |
llm |
expert |
0m
Model:
Tokens:
input: 23986
output: 27
thoughts: 1386
Thoughts: **Exploiting an Interior Pointer in `kfree`**
Okay, so `kfree`... I was just reviewing the `kfree` function and it hit me. If you pass an interior pointer to `kfree`, the `virt_to_page` function will still return the correct page since it's within the same page, and `slab_free` will put the interior pointer onto the freelist. The next allocation from this slab will then return that very interior pointer! This is incredibly powerful! This means an attacker can get a `kmalloc` to return an unaligned pointer, or one that overlaps with another object!
But wait, if `kfree` is called on an interior pointer, doesn't that corrupt the freelist? Yes! Because `set_freepointer` writes the freelist pointer at `tail + s->offset`. If `tail` *is* an interior pointer, this will overwrite some bytes inside the object. The next allocation will then return `tail`, and the following allocation returns the original freelist pointer. So it isn't corrupted, but contains an interior pointer, giving the attacker an overlapping allocation! For example, say the object size is 32 bytes and I free `ptr + 4`. The next allocation will return `ptr + 4`, and I can then write to `ptr + 4`, overwriting the data at `ptr`, even though `ptr` *itself* wasn't freed, it's still considered allocated! Then, if I can spray objects, I can allocate an object at `ptr + 4`, overlapping the next object in the slab. Yes, if I allocate an object at `ptr + 4`, its size is 32 bytes, it'll span `ptr + 4` to `ptr + 36` and will overlap with the next object! The last 4 bytes of the allocated object will overwrite the first 4 bytes of the *next* object! This is a classic overlapping chunk vulnerability. Very exploitable!
Now, can I trigger this? Let's verify that the `kstrtou32` function can fail and leaves `split_page` as an interior pointer. Looking at this code snippet:
```c
split_page = kstrdup(page, GFP_KERNEL);
while ((token = strsep(&split_page, ",")) != NULL) {
ret = kstrtou32(token, 0, &num);
if (ret)
goto end;
opts->name##s[i++] = num;
ret = len;
};
end:
kfree(split_page);
```
If `page` is `"abc,123"`. `split_page` is allocated, say at `0xffff888000000000`. `strsep` returns `0xffff888000000000` (`"abc"`), and sets `split_page` to `0xffff888000000004` (`"123"`). If `kstrtou32` fails, we jump to `end` and `kfree` is called with `0xffff888000000004`. *That's* it! This is definitely exploitable!
Now the next question is, can an unprivileged user trigger this? Well, configfs is mounted at `/sys/kernel/config` and is root-only by default. User namespaces are out of the question (as per instructions), and if the USB gadget is configured by a privileged process, it exposes some interface. The bug is in `f_uac1_opts_c_srate_store`, which is a configfs attribute store, only accessible by writing to the configfs file. If the file is owned by root and mode `0644`, only root can write to it. So, no.
What about a USB host? Can the host send control requests that end up calling this configfs store function? No, configfs configures the gadget *before* it's bound. The host can't write to configfs. No Peripheral Trigger. No Network Trigger. No Malicious Filesystem Trigger. VM guest trigger? No. Only root can trigger it.
Is there any other caller of `f_uac1_opts_c_srate_store`? Need to check.
|
| 35/2 |
2026/08/01 03:36 |
tool |
grepper |
0m
Args: map[Expression:UAC1_RATE_ATTRIBUTE PathPrefix:]
Results: map[Output:drivers/usb/gadget/function/f_uac1.c=1568=CONFIGFS_ATTR(f_uac1_opts_, name)
drivers/usb/gadget/function/f_uac1.c-1569-
drivers/usb/gadget/function/f_uac1.c:1570:#define UAC1_RATE_ATTRIBUTE(name) \
drivers/usb/gadget/function/f_uac1.c-1571-static ssize_t f_uac1_opts_##name##_show(struct config_item *item, \
--
drivers/usb/gadget/function/f_uac1.c=1665=UAC1_ATTRIBUTE(u32, c_chmask);
drivers/usb/gadget/function/f_uac1.c:1666:UAC1_RATE_ATTRIBUTE(c_srate);
drivers/usb/gadget/function/f_uac1.c-1667-UAC1_ATTRIBUTE(u32, c_ssize);
drivers/usb/gadget/function/f_uac1.c=1668=UAC1_ATTRIBUTE(u32, p_chmask);
drivers/usb/gadget/function/f_uac1.c:1669:UAC1_RATE_ATTRIBUTE(p_srate);
drivers/usb/gadget/function/f_uac1.c-1670-UAC1_ATTRIBUTE(u32, p_ssize);
]
|
| 36/2 |
2026/08/01 03:36 |
llm |
expert |
0m
Model:
Tokens:
input: 26739
output: 36
thoughts: 1328
Thoughts: **Analyzing the Memory Leak in `f_uac1.c`**
Okay, so I'm looking at this bug report for a memory leak, specifically in `f_uac1.c`. The report mentions an unreferenced object of size 8 bytes. The reproducer calls `mkdirat`, then `mount` which strongly suggests it involves configfs. Hmm, but how can syzkaller mount configfs if it's unprivileged? The environment description tells us that user namespaces are disabled which means the reproducer can't get any extra privileges this way. So mounting configfs directly is out.
Wait, the prompt says the capability checks are implemented with `ns_capable()` which is the important hint. It's asking if the bug is accessible from *user namespaces* where the process *has all capabilities* – which, if user namespaces *are* enabled, would include `CAP_SYS_ADMIN` and allow configfs mounting and access. The key is in the definition, and the environment description makes things a little confusing.
But is this just a memory leak? Looking deeper, the bug description mentions a `kfree` of an interior pointer. That's a red flag. But the reproducer input is `"4"`. I realize that if `kstrtou32` fails, `kfree(split_page)` is called which could cause problems. The write syscall shows an input of just `"4"`. With the initial input being `"4"`, `strsep` returns `"4"` and sets `split_page` to `NULL`. Then `kstrtou32("4")` succeeds. Because the original input is `"4"`, and `strsep` changes `split_page` to `NULL` when it hits the end, `kfree(split_page)` is called with `NULL`, which is safe. However, the original string `"4"` is leaked! So the memory leak is *not* caused by the failing of `kstrtou32`.
Aha! This memory leak happens even on *successful* parsing. Each successful write leaks a string! This is a simple but persistent leak.
But what if the input is `"4,5"`? The loop goes through twice, and again, `split_page` ends up being freed as `NULL`, leaking the original. And the same thing happens with `"4,abc"`: `kfree(NULL)` and a leak.
Now, what about `"abc,4"`? The first `kstrtou32` fails. Then `kfree(split_page)` is called, which now is `"4"`! *This* is an interior pointer free, which is very dangerous. That gives us overlapping chunks, which is a powerful exploitation primitive.
So, in summary, we have both a memory leak and a potential interior pointer free, which is exploitable. However, looking at access, the interior pointer free is a very strong primitive. And if the process can create a user namespace and has the capabilities there, this is accessible.
So, the conclusion: *Exploitable = True*, *UserNamespace = True*.
|
| 37/2 |
2026/08/01 03:36 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/configfs/mount.c Name:configfs_init]
Results: map[SourceCode: 136: static int __init configfs_init(void)
137: {
138: int err = -ENOMEM;
139:
140: configfs_dir_cachep = kmem_cache_create("configfs_dir_cache",
141: sizeof(struct configfs_dirent),
142: 0, 0, NULL);
143: if (!configfs_dir_cachep)
144: goto out;
145:
146: err = sysfs_create_mount_point(kernel_kobj, "config");
147: if (err)
148: goto out2;
149:
150: err = register_filesystem(&configfs_fs_type);
151: if (err)
152: goto out3;
153:
154: return 0;
155: out3:
156: pr_err("Unable to register filesystem!\n");
157: sysfs_remove_mount_point(kernel_kobj, "config");
158: out2:
159: kmem_cache_destroy(configfs_dir_cachep);
160: configfs_dir_cachep = NULL;
161: out:
162: return err;
163: }
SourceFile:fs/configfs/mount.c]
|
| 38/2 |
2026/08/01 03:36 |
llm |
expert |
0m
Model:
Tokens:
input: 29177
output: 38
thoughts: 12
Thoughts: Okay, let's dive into this. Here's how I'd approach that snippet, considering my expert perspective:
**Checking `configfs_fs_type`**
Right, let's take a look at `configfs_fs_type`. This seems like a crucial point, and based on context, probably indicates the intended file system type for something related to the `configfs` framework. I know `configfs` well, and I'm instantly thinking about the potential implications. I need to understand why we're focusing on this particular type. Is this for mounting, for detection, or something else entirely? A simple check is unlikely to reveal much on its own, so I need to know why. Are we looking to confirm that `configfs` is correctly initialized? That it's the target file system? Or are we verifying a security constraint, or checking if it's the correct filesystem that is being worked with? My experience tells me there's more context needed before a conclusion can be reached. This snippet, in isolation, is just a starting point, and I need far more information to understand the rationale and purpose behind this particular check.
|
| 39/2 |
2026/08/01 03:36 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/configfs/mount.c Name:configfs_fs_type]
Results: map[SourceCode: 115: static struct file_system_type configfs_fs_type = {
116: .owner = THIS_MODULE,
117: .name = "configfs",
118: .init_fs_context = configfs_init_fs_context,
119: .kill_sb = kill_anon_super,
120: };
SourceFile:fs/configfs/mount.c]
|
| 40/2 |
2026/08/01 03:36 |
llm |
expert |
0m
Model:
Tokens:
input: 29603
output: 35
thoughts: 63
Thoughts: **Configfs and User Namespaces - A Quick Check**
Okay, so I'm taking a look at the `configfs_fs_type` struct, specifically its `fs_flags`. I noticed something interesting – `FS_USERNS_MOUNT` isn't set. Now, that immediately triggers a red flag. If that flag isn't present, then my understanding is that configfs should *not* be mountable within a user namespace. The whole point of that flag is to indicate namespace awareness during a mount. This feels like a potential area for a bug, or perhaps an unexpected limitation. Time to verify that assumption. I need to confirm whether this lack of the flag truly prevents user namespace mounts, as it should. Let's see if this plays out.
|
| 41/2 |
2026/08/01 03:36 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/namespace.c Name:vfs_get_tree]
Results: map[SourceCode:1681: /**
1682: * vfs_get_tree - Get the mountable root
1683: * @fc: The superblock configuration context.
1684: *
1685: * The filesystem is invoked to get or create a superblock which can then later
1686: * be used for mounting. The filesystem places a pointer to the root to be
1687: * used for mounting in @fc->root.
1688: */
1689: int vfs_get_tree(struct fs_context *fc)
1690: {
1691: struct super_block *sb;
1692: int error;
1693:
1694: if (fc->root)
1695: return -EBUSY;
1696:
1697: /* Get the mountable root in fc->root, with a ref on the root and a ref
1698: * on the superblock.
1699: */
1700: error = fc->ops->get_tree(fc);
1701: if (error < 0)
1702: return error;
1703:
1704: if (!fc->root) {
1705: pr_err("Filesystem %s get_tree() didn't set fc->root, returned %i\n",
1706: fc->fs_type->name, error);
1707: /* We don't know what the locking state of the superblock is -
1708: * if there is a superblock.
1709: */
1710: BUG();
1711: }
1712:
1713: sb = fc->root->d_sb;
1714: WARN_ON(!sb->s_bdi);
1715:
1716: /*
1717: * super_wake() contains a memory barrier which also care of
1718: * ordering for super_cache_count(). We place it before setting
1719: * SB_BORN as the data dependency between the two functions is
1720: * the superblock structure contents that we just set up, not
1721: * the SB_BORN flag.
1722: */
1723: super_wake(sb, SB_BORN);
1724:
1725: error = security_sb_set_mnt_opts(sb, fc->security, 0, NULL);
1726: if (unlikely(error)) {
1727: fc_drop_locked(fc);
1728: return error;
1729: }
1730:
1731: /*
1732: * filesystems should never set s_maxbytes larger than MAX_LFS_FILESIZE
1733: * but s_maxbytes was an unsigned long long for many releases. Throw
1734: * this warning for a little while to try and catch filesystems that
1735: * violate this rule.
1736: */
1737: WARN((sb->s_maxbytes < 0), "%s set sb->s_maxbytes to "
1738: "negative value (%lld)\n", fc->fs_type->name, sb->s_maxbytes);
1739:
1740: return 0;
1741: }
SourceFile:fs/super.c]
|
| 42/2 |
2026/08/01 03:36 |
llm |
expert |
0m
Model:
Tokens:
input: 30787
output: 36
thoughts: 13
Thoughts: Okay, here's my attempt at summarizing that thought process, tailored for an expert audience:
**Examining `fs_context_for_mount`**
Alright, let's dive into the core of this. I'm going to take a closer look at `fs_context_for_mount`. My initial thought is to understand how this function fits into the overall mount process. I need to refresh my understanding of where it's situated in the call chain, and what kind of context it's responsible for managing or initializing. Is this function responsible for building the initial "context" object that will be used by the filesystem's `mount` operation? Or is it something more granular, perhaps handling specific filesystem-dependent setup? I need to quickly recall whether we're talking about struct `fs_context` itself or some other related data structure. More specifically, I should check the implementation of `fs_context_for_mount` to see: Does it allocate the context? Does it populate some initial fields? Does it retrieve information from somewhere else? Does it just validate some supplied parameters? The details of the implementation will be key to understanding its function. Time to start digging into the code.
|
| 43/2 |
2026/08/01 03:36 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/fs_context.c Name:alloc_fs_context]
Results: map[SourceCode: 245: /**
246: * alloc_fs_context - Create a filesystem context.
247: * @fs_type: The filesystem type.
248: * @reference: The dentry from which this one derives (or NULL)
249: * @sb_flags: Filesystem/superblock flags (SB_*)
250: * @sb_flags_mask: Applicable members of @sb_flags
251: * @purpose: The purpose that this configuration shall be used for.
252: *
253: * Open a filesystem and create a mount context. The mount context is
254: * initialised with the supplied flags and, if a submount/automount from
255: * another superblock (referred to by @reference) is supplied, may have
256: * parameters such as namespaces copied across from that superblock.
257: */
258: static struct fs_context *alloc_fs_context(struct file_system_type *fs_type,
259: struct dentry *reference,
260: unsigned int sb_flags,
261: unsigned int sb_flags_mask,
262: enum fs_context_purpose purpose)
263: {
264: struct fs_context *fc;
265: int ret = -ENOMEM;
266:
267: fc = kzalloc_obj(struct fs_context, GFP_KERNEL_ACCOUNT);
268: if (!fc)
269: return ERR_PTR(-ENOMEM);
270:
271: fc->purpose = purpose;
272: fc->sb_flags = sb_flags;
273: fc->sb_flags_mask = sb_flags_mask;
274: fc->fs_type = get_filesystem(fs_type);
275: fc->cred = get_current_cred();
276: fc->net_ns = get_net(current->nsproxy->net_ns);
277: fc->log.prefix = fs_type->name;
278:
279: mutex_init(&fc->uapi_mutex);
280:
281: switch (purpose) {
282: case FS_CONTEXT_FOR_MOUNT:
283: fc->user_ns = get_user_ns(fc->cred->user_ns);
284: break;
285: case FS_CONTEXT_FOR_SUBMOUNT:
286: fc->user_ns = get_user_ns(reference->d_sb->s_user_ns);
287: break;
288: case FS_CONTEXT_FOR_RECONFIGURE:
289: atomic_inc(&reference->d_sb->s_active);
290: fc->user_ns = get_user_ns(reference->d_sb->s_user_ns);
291: fc->root = dget(reference);
292: break;
293: }
294:
295: ret = fc->fs_type->init_fs_context(fc);
296: if (ret < 0)
297: goto err_fc;
298: fc->need_free = true;
299: return fc;
300:
301: err_fc:
302: put_fs_context(fc);
303: return ERR_PTR(ret);
304: }
SourceFile:fs/fs_context.c]
|
| 44/2 |
2026/08/01 03:36 |
llm |
expert |
0m
Model:
Tokens:
input: 32077
output: 34
thoughts: 17
Thoughts: Okay, here's my summary, in the first person, as if these are my thoughts:
**The Missing Check: A Quick Debug**
Alright, hang on a second. I'm looking at this code, and something's immediately snagging my attention. I'm scanning through, and I'm just *not* seeing the guard, the sanity check, the condition that's supposed to be there... Where is the conditional check for `FS_USERNS_MOUNT`? It should be right here, integrated into this section of code. I'm expecting to find it, but it's absent! This is potentially a serious problem, and my expert intuition is picking up a red flag. I need to figure out why that check isn't present, and if its absence is leading to a vulnerability or incorrect behavior. Time to delve a little deeper and figure out what's going on.
|
| 45/2 |
2026/08/01 03:36 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/namespace.c Name:do_new_mount]
Results: map[SourceCode:3790: /*
3791: * create a new mount for userspace and request it to be added into the
3792: * namespace's tree
3793: */
3794: static int do_new_mount(const struct path *path, const char *fstype,
3795: int sb_flags, int mnt_flags,
3796: const char *name, void *data)
3797: {
3798: struct file_system_type *type;
3799: struct fs_context *fc;
3800: const char *subtype = NULL;
3801: int err = 0;
3802:
3803: if (!fstype)
3804: return -EINVAL;
3805:
3806: type = get_fs_type(fstype);
3807: if (!type)
3808: return -ENODEV;
3809:
3810: if (type->fs_flags & FS_HAS_SUBTYPE) {
3811: subtype = strchr(fstype, '.');
3812: if (subtype) {
3813: subtype++;
3814: if (!*subtype) {
3815: put_filesystem(type);
3816: return -EINVAL;
3817: }
3818: }
3819: }
3820:
3821: fc = fs_context_for_mount(type, sb_flags);
3822: put_filesystem(type);
3823: if (IS_ERR(fc))
3824: return PTR_ERR(fc);
3825:
3826: /*
3827: * Indicate to the filesystem that the mount request is coming
3828: * from the legacy mount system call.
3829: */
3830: fc->oldapi = true;
3831:
3832: if (subtype)
3833: err = vfs_parse_fs_string(fc, "subtype", subtype);
3834: if (!err && name)
3835: err = vfs_parse_fs_string(fc, "source", name);
3836: if (!err)
3837: err = parse_monolithic_mount_data(fc, data);
3838: if (!err && !mount_capable(fc))
3839: err = -EPERM;
3840: if (!err)
3841: err = do_new_mount_fc(fc, path, mnt_flags);
3842:
3843: put_fs_context(fc);
3844: return err;
3845: }
SourceFile:fs/namespace.c]
|
| 46/2 |
2026/08/01 03:36 |
llm |
expert |
0m
Model:
Tokens:
input: 33099
output: 32
thoughts: 9
Thoughts: Okay, here's my interpretation of that thought process, assuming I'm deep in my domain of expertise:
**Checking `mount_capable` - Ensuring Data Integrity and Access**
Alright, before I proceed with any further operations on this system or dataset, I need to verify something crucial: the `mount_capable` flag. This isn't just a casual check; it's a fundamental prerequisite for everything I'm about to do. I have to make absolutely certain that the underlying storage is properly mounted and accessible. Otherwise, I risk corrupting data, triggering cascading errors, or, at the very least, wasting significant time on futile attempts. If `mount_capable` is false, I know I must stop any modifications. The system's integrity is paramount. I'll need to figure out *why* it's not mounted and address that issue immediately, before attempting anything further. It's a quick and essential diagnostic step, one that can save a world of trouble down the line. It's the equivalent of checking your instruments before you take flight, or making sure the foundation is secure before you start building.
|
| 47/2 |
2026/08/01 03:36 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/namespace.c Name:mount_capable]
Results: map[SourceCode: 695: bool mount_capable(struct fs_context *fc)
696: {
697: if (!(fc->fs_type->fs_flags & FS_USERNS_MOUNT))
698: return capable(CAP_SYS_ADMIN);
699: else
700: return ns_capable(fc->user_ns, CAP_SYS_ADMIN);
701: }
SourceFile:fs/super.c]
|
| 48/2 |
2026/08/01 03:36 |
llm |
expert |
0m
Model:
Tokens:
input: 33513
output: 35
thoughts: 1692
Thoughts: **Analysis of the Configfs Bug and Exploitability**
Okay, let's break this down systematically. The core issue revolves around a vulnerability in the USB Audio Class 1 (UAC1) gadget function's configfs attribute, specifically `f_uac1_opts_c_srate_store`. First, I see that the `mount_capable` check requires `CAP_SYS_ADMIN` *unless* `FS_USERNS_MOUNT` is set, implying that configfs isn't generally user-namespaced-mountable. Therefore, a straightforward user namespace mount won't work. This is the initial roadblock. I have to consider how syzkaller triggered it; the answer is that syzkaller runs as root in a VM and thus has CAP_SYS_ADMIN in the initial namespace.
Now, the question asks about reachability within a user namespace *with* capabilities. Even with `CAP_SYS_ADMIN` within the *user* namespace, `mount_capable` checks the *initial* namespace capabilities if `FS_USERNS_MOUNT` isn't set. Thus, a process in a user namespace can't mount configfs, because it lacks the necessary capability in the initial namespace, which is the check that matters for mounting. So, `UserNamespace = False`.
Hold on, I must consider if configfs is *already* mounted, like on my system with systemd. If it is mounted at /sys/kernel/config, can a user namespace process access it? The permissions are `0755` for `/sys/kernel/config` and `0644` for the files inside. If the process's UID is mapped to root (UID 0) within the initial namespace, then, it *could* write, if it was root in the initial namespace, but it's not. The key is that the kernel checks the inode's permissions against the UID in the initial namespace, and if that UID is not 0 (root), then the process can't write to the files even if it has CAP_SYS_ADMIN in the user namespace. This confirms `UserNamespace = False` and also confirms `Unprivileged = False`.
Next, I need to check alternative triggers. Is this vulnerability exposed by other subsystems? It's specific to configfs attributes, so the answer is No. Could USB requests trigger it? No, because configfs is configured *before* binding to the UDC; host requests use the `f_uac1.c` request handlers after binding, not configfs directly. Peripheral trigger is False.
Could this be triggered via network, a malicious filesystem, or a VM guest? No. If the host offers the USB gadget to the guest, the guest is the USB host. The guest can't directly write to the host's configfs. So I can rule out VM Guest and VM Host as possible triggers.
So, the only attack vector is local root access, writing to the configfs attribute. This is a bug, even if limited to root. A local root can crash the kernel or leak memory.
For classification: `Exploitable = True` due to interior pointer free vulnerabilities, `DenialOfService = True` due to memory leak and panic.
Now, let's dissect the interior pointer free vulnerability. The `kstrdup` call allocates the `split_page`. The `strsep` function is the key. The loop parses comma-separated strings. The bug is that `kstrtou32` can fail, and it calls `goto end;`. The critical issue is `kfree(split_page);` within that loop. If `kstrtou32` fails, `split_page` points to a string *inside* the original allocated page ("interior pointer"). The `kfree` of an interior pointer leads to memory corruption.
The memory corruption occurs in `slab_free`, that takes the page and calculates which slab, and then, sets the next pointer in the slab, via `set_freepointer`. If we free an interior pointer, the next free pointer will point to the interior address, so it's a shifted allocation that's freed to a slab. That is why it leads to an overlapping chunk.
In the case of `kfree("123")`, this means the next allocation will return that pointer. If we write past that allocated chunk, we're writing into the next allocated chunk on the slab! Because of the slub implementation, where we can have 32 bytes objects, writing 32 bytes to the allocation will overwrite the first 4 bytes of the next object on the slab.
If the overwritten object is a function pointer or a size field, the overlapping chunk becomes very exploitable. It leads to control flow hijacking or out-of-bounds read/write. I need to be careful
|
| 49/2 |
2026/08/01 03:37 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:mm/slub.c Name:slab_free_hook]
Results: map[SourceCode:2569: /*
2570: * Hooks for other subsystems that check memory allocations. In a typical
2571: * production configuration these hooks all should produce no code at all.
2572: *
2573: * Returns true if freeing of the object can proceed, false if its reuse
2574: * was delayed by CONFIG_SLUB_RCU_DEBUG or KASAN quarantine, or it was returned
2575: * to KFENCE.
2576: *
2577: * For objects allocated via kmalloc_nolock(), only a subset of alloc hooks
2578: * are invoked, so some free hooks must handle asymmetric hook calls.
2579: *
2580: * Alloc hooks called for kmalloc_nolock():
2581: * - kmsan_slab_alloc()
2582: * - kasan_slab_alloc()
2583: * - memcg_slab_post_alloc_hook()
2584: * - alloc_tagging_slab_alloc_hook()
2585: *
2586: * Free hooks that must handle missing corresponding alloc hooks:
2587: * - kmemleak_free_recursive()
2588: * - kfence_free()
2589: *
2590: * Free hooks that have no alloc hook counterpart, and thus safe to call:
2591: * - debug_check_no_locks_freed()
2592: * - debug_check_no_obj_freed()
2593: * - __kcsan_check_access()
2594: */
2595: static __always_inline
2596: bool slab_free_hook(struct kmem_cache *s, void *x, bool init,
2597: bool after_rcu_delay)
2598: {
2599: /* Are the object contents still accessible? */
2600: bool still_accessible = (s->flags & SLAB_TYPESAFE_BY_RCU) && !after_rcu_delay;
2601:
2602: kmemleak_free_recursive(x, s->flags);
2603: kmsan_slab_free(s, x);
2604:
2605: debug_check_no_locks_freed(x, s->object_size);
2606:
2607: if (!(s->flags & SLAB_DEBUG_OBJECTS))
2608: debug_check_no_obj_freed(x, s->object_size);
2609:
2610: /* Use KCSAN to help debug racy use-after-free. */
2611: if (!still_accessible)
2612: __kcsan_check_access(x, s->object_size,
2613: KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT);
2614:
2615: if (kfence_free(x))
2616: return false;
2617:
2618: /*
2619: * Give KASAN a chance to notice an invalid free operation before we
2620: * modify the object.
2621: */
2622: if (kasan_slab_pre_free(s, x))
2623: return false;
2624:
2625: #ifdef CONFIG_SLUB_RCU_DEBUG
2626: if (still_accessible) {
2627: struct rcu_delayed_free *delayed_free;
2628:
2629: delayed_free = kmalloc_obj(*delayed_free, GFP_NOWAIT);
2630: if (delayed_free) {
2631: /*
2632: * Let KASAN track our call stack as a "related work
2633: * creation", just like if the object had been freed
2634: * normally via kfree_rcu().
2635: * We have to do this manually because the rcu_head is
2636: * not located inside the object.
2637: */
2638: kasan_record_aux_stack(x);
2639:
2640: delayed_free->object = x;
2641: call_rcu(&delayed_free->head, slab_free_after_rcu_debug);
2642: return false;
2643: }
2644: }
2645: #endif /* CONFIG_SLUB_RCU_DEBUG */
2646:
2647: /*
2648: * As memory initialization might be integrated into KASAN,
2649: * kasan_slab_free and initialization memset's must be
2650: * kept together to avoid discrepancies in behavior.
2651: *
2652: * The initialization memset's clear the object and the metadata,
2653: * but don't touch the SLAB redzone.
2654: *
2655: * The object's freepointer is also avoided if stored outside the
2656: * object.
2657: */
2658: if (unlikely(init)) {
2659: int rsize;
2660: unsigned int inuse, orig_size;
2661:
2662: inuse = get_info_end(s);
2663: orig_size = get_orig_size(s, x);
2664: if (!kasan_has_integrated_init())
2665: memset(kasan_reset_tag(x), 0, orig_size);
2666: rsize = (s->flags & SLAB_RED_ZONE) ? s->red_left_pad : 0;
2667: memset((char *)kasan_reset_tag(x) + inuse, 0,
2668: s->size - inuse - rsize);
2669: /*
2670: * Restore orig_size, otherwise kmalloc redzone overwritten
2671: * would be reported
2672: */
2673: set_orig_size(s, x, orig_size);
2674:
2675: }
2676: /* KASAN might put x into memory quarantine, delaying its reuse. */
2677: return !kasan_slab_free(s, x, init, still_accessible, false);
2678: }
SourceFile:mm/slub.c]
|
| 50/2 |
2026/08/01 03:37 |
llm |
expert |
0m
Model:
Tokens:
input: 38028
output: 66
thoughts: 7071
Thoughts: **Analysis of Interior Pointer Free Vulnerability in SLUB**
Okay, here's my breakdown of this SLUB vulnerability. First, the core of the problem: in production builds, there is no alignment check within `slab_free_hook` or `__slab_free`. While KASAN *does* have a check, it's not enabled by default. So, we're dealing with an unmitigated scenario.
I've traced the flow: `virt_to_page` correctly identifies the page, `page_slab` finds the relevant slab, and `slab_free` attempts to put the pointer on the freelist, *but without any alignment validation*. This lack of a check is critical. Freeing an interior pointer *is* possible and leads directly to memory corruption via overlapping chunks.
Specifically, the vulnerability lies in the `f_uac1_opts_c_srate_store` function, where a call to `kfree` is made with an interior pointer. This is a classic SLUB exploitation scenario. The `set_freepointer` function writes to the interior pointer, and the next allocation will return that pointer. Thus, any writes to the allocation would overwrite the next object.
I've examined the code closely. The vulnerable code is in the loop:
```c
split_page = kstrdup(page, GFP_KERNEL);
while ((token = strsep(&split_page, ",")) != NULL) {
ret = kstrtou32(token, 0, &num);
if (ret)
goto end;
opts->name##s[i++] = num;
ret = len;
};
end:
kfree(split_page);
```
An example, where the `page` is `"abc,123"`. The `kfree` call on `split_page` will free an interior pointer `"123"`. The `set_freepointer` will write `old.freelist` to the memory address of the string "123".
The next time an object is allocated from this slab, the allocator will return the interior pointer. This enables arbitrary memory corruption because the next object can overwrite another object's memory.
I've considered debug builds (`CONFIG_SLUB_DEBUG`) which *might* introduce checks in `slab_free_hook`, but I'm assuming production, so that's not a factor here.
Triggering the bug: It *seems* to only be triggerable by a local root user writing to the configfs attribute. I've ruled out USB requests, network, malicious filesystems, and VM guests as potential triggers. Root-level access is required. While local root access is needed, this still has significant impact. A malicious actor with root access can crash the kernel or leak memory, so the security implications are significant.
Exploitability = True (overlapping chunks). Denial of Service = True (memory leak, panic). UserNamespace = False, VMGuestTrigger = False, VMHostTrigger = False, NetworkTrigger = False, RemoteTrigger = False, PeripheralTrigger = False, FilesystemTrigger = False.
|
| 51/2 |
2026/08/01 03:37 |
tool |
set-results |
0m
Args: map[DenialOfService:true Exploitable:true FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:false UserNamespace:false VMGuestTrigger:false VMHostTrigger:false]
Results: map[DenialOfService:true Exploitable:true FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:false UserNamespace:false VMGuestTrigger:false VMHostTrigger:false]
|
| 52/2 |
2026/08/01 03:37 |
llm |
expert |
0m
Model:
Tokens:
input: 45906
output: 834
thoughts: 13
Thoughts: Okay, here's my attempt at summarizing the thought process, as requested, aimed at an expert user:
**Finalizing the Response**
Alright, the data's locked in, the analysis is complete. Now it's time to craft the final response. I've considered all the angles, weighed the implications, and I'm ready to translate this complex information into a clear and compelling narrative. It's about distilling the essence, communicating the nuance, and making sure the conclusion is both sound and readily accessible. Time to synthesize. I'll make sure to cover all the key findings, address the relevant caveats and ensure it adds value to the conversation.
|
| 53/1 |
2026/08/01 03:38 |
action |
format-explanation |
0m
Results: map[Explanation:**Bug Analysis:**
The bug is a memory leak and potential interior pointer free in the USB Audio
Class 1 (UAC1) gadget function's configfs attribute store function
`f_uac1_opts_c_srate_store` (and other similar rate attributes defined via the
`UAC1_RATE_ATTRIBUTE` macro).
When a user writes to the `c_srate` configfs attribute, the kernel executes the
following code:
```c
split_page = kstrdup(page, GFP_KERNEL);
while ((token = strsep(&split_page, ",")) != NULL) {
ret = kstrtou32(token, 0, &num);
if (ret)
goto end;
opts->name##s[i++] = num;
ret = len;
};
end:
kfree(split_page);
```
The `strsep()` function modifies the `split_page` pointer to point to the
character after the delimiter (or `NULL` if it reaches the end of the string).
1. **Memory Leak**: If the parsing is completely successful, `strsep()` will
eventually set `split_page` to `NULL`. The `kfree(NULL)` call at the end does
nothing, and the original memory allocated by `kstrdup()` is leaked. Repeatedly
writing valid values to this attribute will cause a memory leak, eventually
leading to an Out-Of-Memory (OOM) Denial of Service.
2. **Interior Pointer Free**: If the parsing fails on a subsequent token (e.g.,
if the user writes `"4,abc"`), `strsep()` will have advanced `split_page` past
the first token. The `goto end;` statement is executed, and `kfree(split_page)`
is called on an interior pointer (e.g., `original_ptr + 2`).
**Exploitability:**
Freeing an interior pointer in the SLUB allocator is highly exploitable. In
production builds without debugging tools like KASAN, SLUB does not check if the
freed pointer is aligned to the object's start. When an interior pointer is
freed, SLUB places it on the freelist. The next allocation from that slab will
return the interior pointer. This allows an attacker to obtain an allocation
that overlaps with the adjacent object in the slab (a classic overlapping chunks
vulnerability). By writing to this overlapping allocation, the attacker can
overwrite the contents (e.g., function pointers, size fields) of the adjacent
object, leading to arbitrary code execution or privilege escalation. Thus, the
bug is exploitable.
**Access and Trigger:**
The vulnerable code is part of the configfs interface for configuring USB
gadgets (`/sys/kernel/config/usb_gadget/...`).
- **Unprivileged Access**: Configfs directories and files are owned by root and
have `0755` and `0644` permissions, respectively. Unprivileged users cannot
write to these files.
- **User Namespaces**: The configfs filesystem does not have the
`FS_USERNS_MOUNT` flag set. Therefore, it cannot be mounted from within an
unprivileged user namespace unless the process already has `CAP_SYS_ADMIN` in
the initial namespace. Even if it is already mounted by the host, the file
permissions prevent a user namespace process from writing to it.
- **Remote/Peripheral/VM**: The bug is in the host's configuration interface for
the USB gadget, which must be configured locally before the gadget is bound to a
USB controller. It cannot be triggered by a connected USB host, network traffic,
or a VM guest.
Therefore, the bug is only accessible to a local root user (or a user with
`CAP_SYS_ADMIN` in the initial namespace). While it requires high privileges to
trigger, it still represents a valid kernel bug that allows root to crash the
kernel or corrupt memory.]
|