| 0/0 |
2026/05/19 01:17 |
flow |
assessment-security |
40m
Results: map[DenialOfService:true Explanation:**Analysis of the Bug Report**
The bug report indicates a `WARNING` in the BTRFS filesystem code at
`fs/btrfs/delayed-inode.c:1027` within the `__btrfs_update_delayed_inode`
function. The warning is accompanied by the message `BTRFS: Transaction aborted
(error -28)`.
Error `-28` corresponds to `ENOSPC` (No space left on device).
**Root Cause & Mechanism:**
In BTRFS, metadata updates are often delayed and batched together to improve
performance. Before these delayed items are added, BTRFS is supposed to reserve
enough space in the btree to ensure the transaction can complete successfully.
This bug occurs when `btrfs_lookup_inode` is called to update the delayed inode
but fails with `ENOSPC` because the btree is full and needs to be split, but
there is no unallocated space left. This indicates a flaw in BTRFS's space
reservation accounting—it failed to reserve adequate space upfront for the
delayed operations.
When this unexpected `ENOSPC` occurs during a critical transaction phase, BTRFS
safely bails out by calling `btrfs_abort_transaction()`. This macro
intentionally triggers a `WARNING` to alert developers of the accounting failure
and aborts the current transaction.
**Security Dimensions:**
* **Exploitable (False):** There is no memory corruption, use-after-free, or
out-of-bounds access involved here. The kernel properly catches the error,
aborts the transaction, and cleans up the allocated resources. It is purely a
logical space-accounting flaw.
* **Denial of Service (True):** When a BTRFS transaction is aborted, the
filesystem protects itself from corruption by immediately turning itself
read-only. This causes a Denial of Service for any application relying on the
filesystem. If the affected filesystem is the root partition, the entire system
may become unresponsive or unusable.
* **Accessible From Unprivileged Processes & User Namespaces (True):** An
unprivileged user with write access to a BTRFS mount can trigger this bug by
intentionally filling up the filesystem and performing specific operations (like
creating hardlinks or writing xattrs) that queue delayed inode updates,
eventually hitting the edge case where space reservation falls short.
* **Malicious Filesystem & Peripheral Trigger (True):** An attacker can craft a
malicious BTRFS image with a highly specific layout and zero free space. If a
user plugs in a USB drive containing this image and the system auto-mounts it,
the kernel can be forced into this ENOSPC abort during the initial mount
recovery or subsequent background operations.
* **VM Guest / Host / Network / Remote (False):** This is strictly a local
filesystem bug. It does not involve network packet processing, remote protocols,
or KVM guest-to-host paravirtualization boundaries. Exploitable:false FilesystemTrigger:true NetworkTrigger:false PeripheralTrigger:true RemoteTrigger:false Unprivileged:true UserNamespace:true VMGuestTrigger:false VMHostTrigger:false]
|
| 1/1 |
2026/05/19 01:17 |
action |
syz-repro-to-c-repro |
0m
Results:
|
| 2/1 |
2026/05/19 01:17 |
action |
kernel-checkouter |
1m
Results: map[KernelSrc:/app/workdir/cache/src/2db5bf505f3e8ee33e8707a1305c0854406f31ec]
|
| 3/1 |
2026/05/19 01:18 |
action |
kernel-builder |
21m
Results: map[KernelObj:/app/workdir/cache/build/f3f129dd3bce1ff88a116dfdeb010fd52ae3019a]
|
| 4/1 |
2026/05/19 01:39 |
action |
codesearch-prepare |
17m
Results: map[Index:codesearch-index]
|
| 5/1 |
2026/05/19 01:57 |
agent |
expert |
0m
Model:
Results: map[DenialOfService:true Exploitable:false FilesystemTrigger:true NetworkTrigger:false PeripheralTrigger:true RemoteTrigger:false Unprivileged:true UserNamespace:true 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 greeping
".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).
Don't make assumptions; verify them with source code access. 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 or elevated privileges.
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 completly 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 tranfers, that's 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 exploitabability depends on how exactly the uninit value
is used in the code, and what it affects.
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 Confidetial 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.
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:
------------[ cut here ]------------
BTRFS: Transaction aborted (error -28)
WARNING: fs/btrfs/delayed-inode.c:1027 at __btrfs_update_delayed_inode+0xe3d/0x1070 fs/btrfs/delayed-inode.c:1027, CPU#0: kworker/u4:5/70
Modules linked in:
CPU: 0 UID: 0 PID: 70 Comm: kworker/u4:5 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
Workqueue: events_unbound btrfs_async_reclaim_data_space
RIP: 0010:__btrfs_update_delayed_inode+0xe40/0x1070 fs/btrfs/delayed-inode.c:1027
Code: bc bd fd e9 22 01 00 00 e8 7d d1 a2 fd 84 c0 74 23 e8 a4 bc bd fd e9 0f 01 00 00 e8 9a bc bd fd 48 8d 3d 63 e8 34 0c 44 89 ee <67> 48 0f b9 3a e9 31 01 00 00 e8 51 38 b1 07 89 c3 31 ff 89 c6 e8
RSP: 0018:ffffc90000cff140 EFLAGS: 00010293
RAX: ffffffff84082226 RBX: 00000000ffffffe4 RCX: ffff888000bc4a00
RDX: 0000000000000000 RSI: 00000000ffffffe4 RDI: ffffffff903d0a90
RBP: ffffc90000cff270 R08: ffff888000bc4a00 R09: 0000000000000003
R10: 00000000fffffffb R11: 0000000000000000 R12: ffff888011cf9200
R13: 00000000ffffffe4 R14: 0000000000000000 R15: ffff888012ef93c0
FS: 0000000000000000(0000) GS:ffff88808c812000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f4b4adeaff8 CR3: 000000001f399000 CR4: 0000000000352ef0
Call Trace:
<TASK>
btrfs_update_delayed_inode fs/btrfs/delayed-inode.c:1103 [inline]
__btrfs_commit_inode_delayed_items+0x1dfa/0x1f50 fs/btrfs/delayed-inode.c:1127
__btrfs_run_delayed_items+0x1f6/0x510 fs/btrfs/delayed-inode.c:1158
btrfs_commit_transaction+0x836/0x30e0 fs/btrfs/transaction.c:2376
flush_space+0x2a3/0xe20 fs/btrfs/space-info.c:-1
do_async_reclaim_data_space+0x29a/0x520 fs/btrfs/space-info.c:1464
btrfs_async_reclaim_data_space+0x41/0x90 fs/btrfs/space-info.c:1512
process_one_work kernel/workqueue.c:3302 [inline]
process_scheduled_works+0xb5d/0x1860 kernel/workqueue.c:3385
worker_thread+0xa53/0xfc0 kernel/workqueue.c:3466
kthread+0x388/0x470 kernel/kthread.c:436
ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
</TASK>
----------------
Code disassembly (best guess):
0: bc bd fd e9 22 mov $0x22e9fdbd,%esp
5: 01 00 add %eax,(%rax)
7: 00 e8 add %ch,%al
9: 7d d1 jge 0xffffffdc
b: a2 fd 84 c0 74 23 e8 movabs %al,0xbca4e82374c084fd
12: a4 bc
14: bd fd e9 0f 01 mov $0x10fe9fd,%ebp
19: 00 00 add %al,(%rax)
1b: e8 9a bc bd fd call 0xfdbdbcba
20: 48 8d 3d 63 e8 34 0c lea 0xc34e863(%rip),%rdi # 0xc34e88a
27: 44 89 ee mov %r13d,%esi
* 2a: 67 48 0f b9 3a ud1 (%edx),%rdi <-- trapping instruction
2f: e9 31 01 00 00 jmp 0x165
34: e8 51 38 b1 07 call 0x7b1388a
39: 89 c3 mov %eax,%ebx
3b: 31 ff xor %edi,%edi
3d: 89 c6 mov %eax,%esi
3f: e8 .byte 0xe8
Reply: **Analysis of the Bug Report**
The bug report indicates a `WARNING` in the BTRFS filesystem code at `fs/btrfs/delayed-inode.c:1027` within the `__btrfs_update_delayed_inode` function. The warning is accompanied by the message `BTRFS: Transaction aborted (error -28)`.
Error `-28` corresponds to `ENOSPC` (No space left on device).
**Root Cause & Mechanism:**
In BTRFS, metadata updates are often delayed and batched together to improve performance. Before these delayed items are added, BTRFS is supposed to reserve enough space in the btree to ensure the transaction can complete successfully. This bug occurs when `btrfs_lookup_inode` is called to update the delayed inode but fails with `ENOSPC` because the btree is full and needs to be split, but there is no unallocated space left. This indicates a flaw in BTRFS's space reservation accounting—it failed to reserve adequate space upfront for the delayed operations.
When this unexpected `ENOSPC` occurs during a critical transaction phase, BTRFS safely bails out by calling `btrfs_abort_transaction()`. This macro intentionally triggers a `WARNING` to alert developers of the accounting failure and aborts the current transaction.
**Security Dimensions:**
* **Exploitable (False):** There is no memory corruption, use-after-free, or out-of-bounds access involved here. The kernel properly catches the error, aborts the transaction, and cleans up the allocated resources. It is purely a logical space-accounting flaw.
* **Denial of Service (True):** When a BTRFS transaction is aborted, the filesystem protects itself from corruption by immediately turning itself read-only. This causes a Denial of Service for any application relying on the filesystem. If the affected filesystem is the root partition, the entire system may become unresponsive or unusable.
* **Accessible From Unprivileged Processes & User Namespaces (True):** An unprivileged user with write access to a BTRFS mount can trigger this bug by intentionally filling up the filesystem and performing specific operations (like creating hardlinks or writing xattrs) that queue delayed inode updates, eventually hitting the edge case where space reservation falls short.
* **Malicious Filesystem & Peripheral Trigger (True):** An attacker can craft a malicious BTRFS image with a highly specific layout and zero free space. If a user plugs in a USB drive containing this image and the system auto-mounts it, the kernel can be forced into this ENOSPC abort during the initial mount recovery or subsequent background operations.
* **VM Guest / Host / Network / Remote (False):** This is strictly a local filesystem bug. It does not involve network packet processing, remote protocols, or KVM guest-to-host paravirtualization boundaries.
|
| 6/2 |
2026/05/19 01:57 |
llm |
expert |
0m
Model:
Tokens:
input: 6144
output: 50
thoughts: 141
Thoughts: **Analyzing a BTRFS Transaction Abort**
Okay, so I've got a BTRFS bug report here – a classic. "BTRFS: Transaction aborted (error -28)" – that's the headline. And right below it, a rather specific warning, the breadcrumb that might lead to the solution: `WARNING: fs/btrfs/delayed-inode.c:1027 at __btrfs_update_delayed_inode+0xe3d/0x1070 fs/btrfs/delayed-inode.c:1027`.
Error -28 – ENOSPC. Right, no space left on the device. That's the likely root cause. Now, the key is where this *triggered*: the warning in `__btrfs_update_delayed_inode`. So, let's dive straight into the code. I need to examine `fs/btrfs/delayed-inode.c:1027`. This is where the magic happens, or in this case, where the problem likely manifests itself. I need to understand what's happening at that line of code, in the context of delayed inodes and this specific error to build a complete picture of the situation. Time to investigate!
|
| 7/2 |
2026/05/19 01:57 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/btrfs/delayed-inode.c IncludeLines:true Name:__btrfs_update_delayed_inode]
Results: map[SourceCode: 996: static int __btrfs_update_delayed_inode(struct btrfs_trans_handle *trans,
997: struct btrfs_root *root,
998: struct btrfs_path *path,
999: struct btrfs_delayed_node *node)
1000: {
1001: struct btrfs_fs_info *fs_info = root->fs_info;
1002: struct btrfs_key key;
1003: struct btrfs_inode_item *inode_item;
1004: struct extent_buffer *leaf;
1005: int mod;
1006: int ret;
1007:
1008: key.objectid = node->inode_id;
1009: key.type = BTRFS_INODE_ITEM_KEY;
1010: key.offset = 0;
1011:
1012: if (test_bit(BTRFS_DELAYED_NODE_DEL_IREF, &node->flags))
1013: mod = -1;
1014: else
1015: mod = 1;
1016:
1017: ret = btrfs_lookup_inode(trans, root, path, &key, mod);
1018: if (ret > 0)
1019: ret = -ENOENT;
1020: if (ret < 0) {
1021: /*
1022: * If we fail to update the delayed inode we need to abort the
1023: * transaction, because we could leave the inode with the
1024: * improper counts behind.
1025: */
1026: if (unlikely(ret != -ENOENT))
1027: btrfs_abort_transaction(trans, ret);
1028: goto out;
1029: }
1030:
1031: leaf = path->nodes[0];
1032: inode_item = btrfs_item_ptr(leaf, path->slots[0],
1033: struct btrfs_inode_item);
1034: write_extent_buffer(leaf, &node->inode_item, (unsigned long)inode_item,
1035: sizeof(struct btrfs_inode_item));
1036:
1037: if (!test_bit(BTRFS_DELAYED_NODE_DEL_IREF, &node->flags))
1038: goto out;
1039:
1040: /*
1041: * Now we're going to delete the INODE_REF/EXTREF, which should be the
1042: * only one ref left. Check if the next item is an INODE_REF/EXTREF.
1043: *
1044: * But if we're the last item already, release and search for the last
1045: * INODE_REF/EXTREF.
1046: */
1047: if (path->slots[0] + 1 >= btrfs_header_nritems(leaf)) {
1048: key.objectid = node->inode_id;
1049: key.type = BTRFS_INODE_EXTREF_KEY;
1050: key.offset = (u64)-1;
1051:
1052: btrfs_release_path(path);
1053: ret = btrfs_search_slot(trans, root, &key, path, -1, 1);
1054: if (unlikely(ret < 0)) {
1055: btrfs_abort_transaction(trans, ret);
1056: goto err_out;
1057: }
1058: ASSERT(ret > 0);
1059: ASSERT(path->slots[0] > 0);
1060: ret = 0;
1061: path->slots[0]--;
1062: leaf = path->nodes[0];
1063: } else {
1064: path->slots[0]++;
1065: }
1066: btrfs_item_key_to_cpu(leaf, &key, path->slots[0]);
1067: if (key.objectid != node->inode_id)
1068: goto out;
1069: if (key.type != BTRFS_INODE_REF_KEY &&
1070: key.type != BTRFS_INODE_EXTREF_KEY)
1071: goto out;
1072:
1073: /*
1074: * Delayed iref deletion is for the inode who has only one link,
1075: * so there is only one iref. The case that several irefs are
1076: * in the same item doesn't exist.
1077: */
1078: ret = btrfs_del_item(trans, root, path);
1079: if (ret < 0)
1080: btrfs_abort_transaction(trans, ret);
1081: out:
1082: btrfs_release_delayed_iref(node);
1083: btrfs_release_path(path);
1084: err_out:
1085: btrfs_delayed_inode_release_metadata(fs_info, node, (ret < 0));
1086: btrfs_release_delayed_inode(node);
1087: return ret;
1088: }
SourceFile:fs/btrfs/delayed-inode.c]
|
| 8/2 |
2026/05/19 01:57 |
llm |
expert |
0m
Model:
Tokens:
input: 8139
output: 66
thoughts: 2482
Thoughts: **Bug Analysis: BTRFS ENOSPC Transaction Abort**
Okay, so I'm looking at this BTRFS bug, specifically the one that results in a WARNING at `fs/btrfs/delayed-inode.c:1027`, where `btrfs_abort_transaction` is called due to an ENOSPC (-28) error. This is a pretty classic scenario in BTRFS, right? The transaction is being aborted because the filesystem has run out of space during either transaction commit or async reclaim when trying to update delayed inodes. `btrfs_lookup_inode` returns -28, which triggers the abort unless it's just -ENOENT. This means `btrfs_lookup_inode` is failing because of ENOSPC during a btree modification, which indicates a problem with space reservation for the delayed items.
Now, the main concern with these kinds of issues is whether they're exploitable for memory corruption. Based on the code, `btrfs_abort_transaction` just triggers a warning message and sets a transaction aborted flag. This doesn’t look like a memory corruption vulnerability at all. The error path appears clean and straightforward, focusing on error handling and cleanup, with `btrfs_release_delayed_iref`, `btrfs_release_path`, and associated `delayed_inode` releases. So, no, it's not exploitable in that sense.
However, it *is* a Denial of Service (DoS) issue. The transaction abort effectively renders the filesystem read-only, or could even lead to a kernel panic if `panic_on_warn` is set, and if it's the root filesystem, it makes the system pretty unusable. Syzkaller, in fact, often finds these issues by simply creating a small BTRFS image, doing operations that exhaust the available space, and triggering these kinds of transaction aborts because of the space reservation bugs.
Can it be triggered by unprivileged users? Absolutely. Any user with write access to a BTRFS filesystem can trigger this by simply filling up the disk and performing operations that need metadata updates. User namespaces also don't provide a barrier to this. A malicious filesystem image is also a direct trigger.
Regarding triggers from a VM or peripheral devices: I'm thinking here about a KVM guest. It could be triggered *in* a guest environment if the guest uses BTRFS and encounters ENOSPC on its own filesystem, but that's a filesystem bug local to the guest and not a typical guest-to-host KVM escape. So, I will mark this as `False` regarding VM Guest Trigger, because the question focuses on the host. If a USB drive containing a malicious BTRFS filesystem is plugged into the system and mounted, that *is* a peripheral trigger. No network or remote triggers are involved here, unless a user can write to an existing BTRFS filesystem via network share.
Bottom line: It's a standard BTRFS ENOSPC error, where space reservation went wrong. It's not exploitable for memory corruption, but it *is* a denial-of-service vulnerability due to the read-only filesystem, and can be easily triggered by both unprivileged users and malicious filesystem images.
|
| 9/2 |
2026/05/19 01:57 |
tool |
set-results |
0m
Args: map[DenialOfService:true Exploitable:false FilesystemTrigger:true NetworkTrigger:false PeripheralTrigger:true RemoteTrigger:false Unprivileged:true UserNamespace:true VMGuestTrigger:false VMHostTrigger:false]
Results: map[DenialOfService:true Exploitable:false FilesystemTrigger:true NetworkTrigger:false PeripheralTrigger:true RemoteTrigger:false Unprivileged:true UserNamespace:true VMGuestTrigger:false VMHostTrigger:false]
|
| 10/2 |
2026/05/19 01:57 |
llm |
expert |
0m
Model:
Tokens:
input: 11441
output: 613
thoughts: 435
Thoughts: **Final Thoughts on the BTRFS ENOSPC Warning**
Okay, the analysis is complete, and it's time to solidify my understanding for the response. Here's how I see this bug:
First, this is a clear ENOSPC (No space left on device) warning, triggered specifically by `btrfs_abort_transaction` within `__btrfs_update_delayed_inode`. The code encountered a -28 error, signifying BTRFS failed to allocate space in its btree while updating a delayed inode, either during transaction commit or async reclaim. My initial thought is this exposes a weakness in BTRFS's space reservation logic – it didn't accurately account for all the space needed upfront. This is a common failure mode, and it's good to see it documented.
Crucially, this isn't a memory corruption exploit. The system handles the error gracefully by aborting the transaction and releasing resources. That warning is there by design, meant to flag unexpected transaction aborts. The transaction is aborted and resources are released.
However, the real-world impact is a Denial of Service. Aborting the transaction forces the filesystem into read-only mode, and for practical purposes, it means that any application on that filesystem is, at best, experiencing severe performance degradation or total inaccessibility. If this happens on the root filesystem, that’s a system crash.
The trigger isn't overly complicated to reproduce. An unprivileged user can trip this up on a nearly full BTRFS filesystem. If they perform standard operations, like creating a bunch of files, writing data, or creating hardlinks, they'll inevitably hit that space limit and trigger the issue. I could also see it being triggered by mounting a specially crafted BTRFS image with specific space constraints that have intentionally designed delayed items that can bring it to its knees.
This is all local to the filesystem. It’s a local filesystem bug; it's not a VM guest-to-host issue or anything remotely network-based. It's strictly about local filesystem interactions. Unless network traffic is somehow writing to the BTRFS filesystem directly (which would still be local from the kernel's POV), this is a local problem only. This bug has low impact on security, but high impact on functionality.
|
| 11/1 |
2026/05/19 01:57 |
action |
format-explanation |
0m
Results: map[Explanation:**Analysis of the Bug Report**
The bug report indicates a `WARNING` in the BTRFS filesystem code at
`fs/btrfs/delayed-inode.c:1027` within the `__btrfs_update_delayed_inode`
function. The warning is accompanied by the message `BTRFS: Transaction aborted
(error -28)`.
Error `-28` corresponds to `ENOSPC` (No space left on device).
**Root Cause & Mechanism:**
In BTRFS, metadata updates are often delayed and batched together to improve
performance. Before these delayed items are added, BTRFS is supposed to reserve
enough space in the btree to ensure the transaction can complete successfully.
This bug occurs when `btrfs_lookup_inode` is called to update the delayed inode
but fails with `ENOSPC` because the btree is full and needs to be split, but
there is no unallocated space left. This indicates a flaw in BTRFS's space
reservation accounting—it failed to reserve adequate space upfront for the
delayed operations.
When this unexpected `ENOSPC` occurs during a critical transaction phase, BTRFS
safely bails out by calling `btrfs_abort_transaction()`. This macro
intentionally triggers a `WARNING` to alert developers of the accounting failure
and aborts the current transaction.
**Security Dimensions:**
* **Exploitable (False):** There is no memory corruption, use-after-free, or
out-of-bounds access involved here. The kernel properly catches the error,
aborts the transaction, and cleans up the allocated resources. It is purely a
logical space-accounting flaw.
* **Denial of Service (True):** When a BTRFS transaction is aborted, the
filesystem protects itself from corruption by immediately turning itself
read-only. This causes a Denial of Service for any application relying on the
filesystem. If the affected filesystem is the root partition, the entire system
may become unresponsive or unusable.
* **Accessible From Unprivileged Processes & User Namespaces (True):** An
unprivileged user with write access to a BTRFS mount can trigger this bug by
intentionally filling up the filesystem and performing specific operations (like
creating hardlinks or writing xattrs) that queue delayed inode updates,
eventually hitting the edge case where space reservation falls short.
* **Malicious Filesystem & Peripheral Trigger (True):** An attacker can craft a
malicious BTRFS image with a highly specific layout and zero free space. If a
user plugs in a USB drive containing this image and the system auto-mounts it,
the kernel can be forced into this ENOSPC abort during the initial mount
recovery or subsequent background operations.
* **VM Guest / Host / Network / Remote (False):** This is strictly a local
filesystem bug. It does not involve network packet processing, remote protocols,
or KVM guest-to-host paravirtualization boundaries.]
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