WARNING in rfkill_dev_uevent

Status: upstream: reported C repro on 2026/10/31 08:52
Subsystems: wireless
Labels: prio:low
[Documentation on labels]
Reported-by: syzbot+56f5119a6b206041a6b7@syzkaller.appspotmail.com
First crash: 4d03h, last: 1d18h
Cause bisection: failed (error log, bisect log)
  
āœØ AI-generated root cause summary:

In mac80211_hwsim and cfg80211, the netlink attribute HWSIM_ATTR_RADIO_NAME is accepted without a length limit, and cfg80211 does not bound requested_name when setting the device and rfkill names. When a user creates a virtual radio with an excessively long name via netlink, rfkill registration triggers a uevent where RFKILL_NAME exceeds the fixed 2048-byte UEVENT_BUFFER_SIZE in add_uevent_var(), triggering a WARN. In production kernels, add_uevent_var() safely returns -ENOMEM and drops the uevent without memory corruption or system crashes.

▶ ▼ āœØ Full AI root cause analysis
Based on kernel commit 195a3519ff27:

Root Cause Analysis

1. Components and Files Involved

  • drivers/net/wireless/virtual/mac80211_hwsim_main.c: In hwsim_genl_policy, [HWSIM_ATTR_RADIO_NAME] = { .type = NLA_STRING } is defined without .len bounds. In hwsim_new_radio_nl(), the attribute is duplicated with kstrndup() without validating or capping its length before passing it to mac80211_hwsim_new_radio().
  • net/wireless/core.c: wiphy_new_nm() accepts requested_name without length restriction, assigns it as the device name via dev_set_name(&rdev->wiphy.dev, "%s", requested_name), and subsequently forwards dev_name(&rdev->wiphy.dev) to rfkill_alloc().
  • net/rfkill/core.c: rfkill_alloc() stores the wiphy name in rfkill->name. When rfkill_register() is invoked during wiphy_register(), device_add() triggers a kobject_uevent() with action KOBJ_ADD.
  • lib/kobject_uevent.c: During the KOBJ_ADD event dispatch, rfkill_dev_uevent() formats environment variables including "RFKILL_NAME=%s" into the fixed-size 2048-byte buffer (env->buf of struct kobj_uevent_env). add_uevent_var() checks if the formatted variable exceeds available buffer space.

2. Trigger Mechanism & Lifecycle Violation

  1. A userspace process sends a Generic Netlink command HWSIM_CMD_NEW_RADIO with an oversized HWSIM_ATTR_RADIO_NAME attribute (several kilobytes long).
  2. hwsim_new_radio_nl() accepts the name and calls mac80211_hwsim_new_radio(), which calls ieee80211_alloc_hw_nm() -> wiphy_new_nm().
  3. In net/mac80211/main.c, ieee80211_register_hw() invokes alloc_ordered_workqueue("%s", 0, wiphy_name(local->hw.wiphy)) which emits: 
    workqueue: name exceeds WQ_NAME_LEN. Truncating to: ...
  4. ieee80211_register_hw() then calls wiphy_register(), which calls rfkill_register(rdev->wiphy.rfkill) -> device_add(&rfkill->dev).
  5. device_add() calls kobject_uevent(&dev->kobj, KOBJ_ADD) -> kobject_uevent_env() -> rfkill_dev_uevent().
  6. rfkill_dev_uevent() calls add_uevent_var(env, "RFKILL_NAME=%s", rfkill->name).
  7. Because rfkill->name exceeds the remaining capacity of the 2048-byte UEVENT_BUFFER_SIZE, vsnprintf() returns a length greater than or equal to sizeof(env->buf) - env->buflen.
  8. add_uevent_var() detects this buffer overflow condition, hits line 688 (WARN(1, KERN_ERR "add_uevent_var: buffer size too small\n");), and returns -ENOMEM.

3. Runtime Consequences in Production Kernels

  • Memory Corruption (false): add_uevent_var() bounds formatting using vsnprintf() against sizeof(env->buf) - env->buflen and checks for truncation before advancing env->buflen or updating pointers. There is no out-of-bounds write, use-after-free, or corrupted memory state.
  • Information Leak ("none"): No uninitialized or kernel memory is leaked to userspace.
  • Denial of Service (false): In a production kernel without CONFIG_PANIC_ON_WARN, the warning does not halt execution. add_uevent_var() returns -ENOMEM, which causes kobject_uevent_env() to discard the uevent buffer and cleanly return -ENOMEM. The return code of kobject_uevent() is ignored in device_add(), meaning registration finishes normally and no hang, panic, or unhandled leak occurs.
  • Networking Candidate (true): The bug is triggered via netlink into mac80211_hwsim, cfg80211, and rfkill.
  • KVM Candidate (false): Virtualization / KVM subsystems are not involved.
▶ ▼ āœØ AI Jobs (1)
ID Workflow Result Correct Bug Created Started Finished Revision
e325b90e assessment-security DenialOfService: āŒ Exploitable: āŒ FilesystemTrigger: āŒ FuzzingArtifact: āŒ MemoryCorruption: āŒ NetworkNeedsSetup: āŒ NetworkTrigger: āŒ PeripheralTrigger: āŒ RemoteTrigger: āŒ Unprivileged: āŒ UserNamespace: āœ… VMGuestTrigger: āŒ VMHostTrigger: āŒ ā“ WARNING in rfkill_dev_uevent 2026/10/28 00:02 2026/10/28 00:02 2026/10/28 01:03 96923c60
▶ ▼ Discussions (1)
Title Replies (including bot) Last reply
[syzbot] [wireless?] WARNING in rfkill_dev_uevent 0 (1) 2026/10/31 08:52

Sample crash report:
workqueue
workqueue: name exceeds WQ_NAME_LEN. Truncating to: ’„śeč45µ8) r1 += -8
     15: .
------------[ cut here ]------------
add_uevent_var: buffer size too small
WARNING: lib/kobject_uevent.c:688 at add_uevent_var+0x2db/0x460 lib/kobject_uevent.c:688, CPU#0: syz-executor353/5646
Modules linked in:
CPU: 0 UID: 0 PID: 5646 Comm: syz-executor353 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/07/2026
RIP: 0010:add_uevent_var+0x2db/0x460 lib/kobject_uevent.c:688
Code: 48 8b 44 24 40 42 0f b6 04 38 84 c0 48 8b 5c 24 18 0f 85 73 01 00 00 44 01 33 31 c0 eb 20 e8 0c 1c dd f5 48 8d 3d 35 df a3 05 <67> 48 0f b9 3a b8 f4 ff ff ff 49 bf 00 00 00 00 00 fc ff df 66 43
RSP: 0018:ffffc90003996a00 EFLAGS: 00010293
RAX: ffffffff8beb4a14 RBX: 1ffff1100efa0443 RCX: ffff88807f68be80
RDX: 0000000000000000 RSI: 0000000000001770 RDI: ffffffff918f2950
RBP: ffffc90003996b38 R08: ffff88807f68be80 R09: 0000000000000008
R10: 0000000000000007 R11: 0000000000000000 R12: 0000000000001770
R13: 1ffff92000732d50 R14: 000000000000079e R15: 0000000000000062
FS:  000055556e425400(0000) GS:ffff888123a6b000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000000000069e000 CR3: 00000000713cc000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 rfkill_dev_uevent+0x2f/0x1f0 net/rfkill/core.c:875
 dev_uevent+0x4ab/0x870 drivers/base/core.c:2774
 kobject_uevent_env+0x475/0x9e0 lib/kobject_uevent.c:576
 device_add+0x544/0xb80 drivers/base/core.c:3742
 rfkill_register+0x170/0x9b0 net/rfkill/core.c:1100
 wiphy_register+0x2a5f/0x3290 net/wireless/core.c:1310
 ieee80211_register_hw+0x3d72/0x4b30 net/mac80211/main.c:1634
 mac80211_hwsim_new_radio+0x32f1/0x5990 drivers/net/wireless/virtual/mac80211_hwsim_main.c:6186
 hwsim_new_radio_nl+0xd8b/0xf90 drivers/net/wireless/virtual/mac80211_hwsim_main.c:7062
 genl_family_rcv_msg_doit+0x233/0x340 net/netlink/genetlink.c:1114
 genl_family_rcv_msg net/netlink/genetlink.c:1194 [inline]
 genl_rcv_msg+0x614/0x7a0 net/netlink/genetlink.c:1209
 netlink_rcv_skb+0x226/0x4a0 net/netlink/af_netlink.c:2580
 genl_rcv+0x28/0x40 net/netlink/genetlink.c:1218
 netlink_unicast_kernel net/netlink/af_netlink.c:1335 [inline]
 netlink_unicast+0x8ef/0xa80 net/netlink/af_netlink.c:1361
 netlink_sendmsg+0x7f0/0xb20 net/netlink/af_netlink.c:1920
 sock_sendmsg_nosec+0x14e/0x190 net/socket.c:800
 __sock_sendmsg net/socket.c:818 [inline]
 ____sys_sendmsg+0x54e/0x850 net/socket.c:2715
 ___sys_sendmsg+0x2a5/0x360 net/socket.c:2769
 __sys_sendmsg net/socket.c:2801 [inline]
 __do_sys_sendmsg net/socket.c:2806 [inline]
 __se_sys_sendmsg net/socket.c:2804 [inline]
 __x64_sys_sendmsg+0x1b1/0x290 net/socket.c:2804
 do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
 do_syscall_64+0x166/0x520 arch/x86/entry/syscall_64.c:84
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fbc181e2649
Code: c0 79 93 eb d5 48 8d 7c 1d 00 eb 99 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 d8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffc9f9d5c88 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007fbc181e2649
RDX: 0000000000000084 RSI: 0000200000000a00 RDI: 0000000000000004
RBP: 0000000000000000 R08: 0000000000000006 R09: 0000000000000006
R10: 0000000000000006 R11: 0000000000000246 R12: 0000000000000000
R13: 0000000000000000 R14: 00007fbc18275c40 R15: 0000000000000002
 </TASK>
----------------
Code disassembly (best guess):
   0:	48 8b 44 24 40       	mov    0x40(%rsp),%rax
   5:	42 0f b6 04 38       	movzbl (%rax,%r15,1),%eax
   a:	84 c0                	test   %al,%al
   c:	48 8b 5c 24 18       	mov    0x18(%rsp),%rbx
  11:	0f 85 73 01 00 00    	jne    0x18a
  17:	44 01 33             	add    %r14d,(%rbx)
  1a:	31 c0                	xor    %eax,%eax
  1c:	eb 20                	jmp    0x3e
  1e:	e8 0c 1c dd f5       	call   0xf5dd1c2f
  23:	48 8d 3d 35 df a3 05 	lea    0x5a3df35(%rip),%rdi        # 0x5a3df5f
* 2a:	67 48 0f b9 3a       	ud1    (%edx),%rdi <-- trapping instruction
  2f:	b8 f4 ff ff ff       	mov    $0xfffffff4,%eax
  34:	49 bf 00 00 00 00 00 	movabs $0xdffffc0000000000,%r15
  3b:	fc ff df
  3e:	66                   	data16
  3f:	43                   	rex.XB

Crashes (6):
Time Kernel Commit Syzkaller Config Log Report Syz repro C repro VM info Assets (help?) Manager Title
2026/10/29 18:11 net dda51b737216 b1d00f76 .config strace log report syz / log C [disk image] [vmlinux] [kernel image] ci-upstream-net-this-kasan-gce WARNING in rfkill_dev_uevent
2026/10/27 11:07 bpf 195a3519ff27 168cb7a2 .config strace log report syz / log C [disk image] [vmlinux] [kernel image] ci-upstream-bpf-kasan-gce WARNING in rfkill_dev_uevent
2026/10/27 19:15 net-next 734bef548a96 96923c60 .config strace log report syz / log C [disk image] [vmlinux] [kernel image] ci-upstream-net-kasan-gce WARNING in rfkill_dev_uevent
2026/10/27 11:48 bpf-next 4169c9c90532 168cb7a2 .config strace log report syz / log C [disk image] [vmlinux] [kernel image] ci-upstream-bpf-next-kasan-gce WARNING in rfkill_dev_uevent
2026/10/27 10:18 bpf-next 4169c9c90532 168cb7a2 .config strace log report syz / log C [disk image] [vmlinux] [kernel image] ci-upstream-bpf-next-kasan-gce WARNING in rfkill_dev_uevent
2026/10/27 08:46 bpf-next 4169c9c90532 168cb7a2 .config console log report info [disk image] [vmlinux] [kernel image] ci-upstream-bpf-next-kasan-gce WARNING in rfkill_dev_uevent
* Struck through repros no longer work on HEAD.