CVE-2026-31551: wifi: mac80211: Fix static_branch_dec() underflow for aql_disable.
In the Linux kernel, the following vulnerability has been resolved:
wifi: mac80211: Fix staticbranchdec() underflow for aqldisable.
syzbot reported staticbranchdec() underflow in aqlenablewrite(). [0]
The problem is that aqlenablewrite() does not serialise concurrent write()s to the debugfs.
aqlenablewrite() checks statickeyfalse(&aqldisable.key) and later calls staticbranchinc() or staticbranchdec(), but the state may change between the two calls.
aqldisable does not need to track inc/dec.
Let's use staticbranchenable() and staticbranchdisable().
[0]: val == 0 WARNING: kernel/jumplabel.c:311 at statickeyslowdeccpuslocked.part.0+0x107/0x120 kernel/jumplabel.c:311, CPU#0: syz.1.3155/20288 Modules linked in: CPU: 0 UID: 0 PID: 20288 Comm: syz.1.3155 Tainted: G U L syzkaller #0 PREEMPT(full) Tainted: [U]=USER, [L]=SOFTLOCKUP Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/24/2026 RIP: 0010:statickeyslowdeccpuslocked.part.0+0x107/0x120 kernel/jumplabel.c:311 Code: f2 c9 ff 5b 5d c3 cc cc cc cc e8 54 f2 c9 ff 48 89 df e8 ac f9 ff ff eb ad e8 45 f2 c9 ff 90 0f 0b 90 eb a2 e8 3a f2 c9 ff 90 <0f> 0b 90 eb 97 48 89 df e8 5c 4b 33 00 e9 36 ff ff ff 0f 1f 80 00 RSP: 0018:ffffc9000b9f7c10 EFLAGS: 00010293 RAX: 0000000000000000 RBX: ffffffff9b3e5d40 RCX: ffffffff823c57b4 RDX: ffff8880285a0000 RSI: ffffffff823c5846 RDI: ffff8880285a0000 RBP: 0000000000000000 R08: 0000000000000005 R09: 0000000000000000 R10: 0000000000000000 R11: 0000000000000000 R12: 000000000000000a R13: 1ffff9200173ef88 R14: 0000000000000001 R15: ffffc9000b9f7e98 FS: 00007f530dd726c0(0000) GS:ffff8881245e3000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000200000001140 CR3: 000000007cc4a000 CR4: 00000000003526f0 Call Trace: <TASK> statickeyslowdeccpuslocked kernel/jumplabel.c:297 [inline] statickeyslowdec kernel/jumplabel.c:321 [inline] statickeyslowdec+0x7c/0xc0 kernel/jumplabel.c:336 aqlenablewrite+0x2b2/0x310 net/mac80211/debugfs.c:343 shortproxywrite+0x133/0x1a0 fs/debugfs/file.c:383 vfswrite+0x2aa/0x1070 fs/readwrite.c:684 ksyspwrite64 fs/readwrite.c:793 [inline] dosyspwrite64 fs/readwrite.c:801 [inline] sesyspwrite64 fs/readwrite.c:798 [inline] x64syspwrite64+0x1eb/0x250 fs/readwrite.c:798 dosyscallx64 arch/x86/entry/syscall64.c:63 [inline] dosyscall64+0xc9/0xf80 arch/x86/entry/syscall64.c:94 entrySYSCALL64afterhwframe+0x77/0x7f RIP: 0033:0x7f530cf9aeb9 Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 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 e8 ff ff ff f7 d8 64 89 01 48 RSP: 002b:00007f530dd72028 EFLAGS: 00000246 ORIGRAX: 0000000000000012 RAX: ffffffffffffffda RBX: 00007f530d215fa0 RCX: 00007f530cf9aeb9 RDX: 0000000000000003 RSI: 0000000000000000 RDI: 0000000000000010 RBP: 00007f530d008c1f R08: 0000000000000000 R09: 0000000000000000 R10: 4200000000000005 R11: 0000000000000246 R12: 0000000000000000 R13: 00007f530d216038 R14: 00007f530d215fa0 R15: 00007ffde89fb978 </TASK>