[syzbot] [kernel?] general protection fault in remove_waiter

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 897d54018cc9 Merge tag 'fbdev-for-7.1-rc1-2' of git://git...
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=172c32ce580000
kernel config: https://syzkaller.appspot.com/x/.config?x=59da38148f3a3d24
dashboard link: https://syzkaller.appspot.com/bug?extid=78147abe6c524f183ee9
compiler: gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=10c7f273980000

Downloadable assets:
disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/d900f083ada3/non_bootable_disk-897d5401.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/20848bcda8fe/vmlinux-897d5401.xz
kernel image: https://storage.googleapis.com/syzbot-assets/2351a3d1bb73/bzImage-897d5401.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+78147a...@syzkaller.appspotmail.com

Oops: general protection fault, probably for non-canonical address 0xdffffc0000000151: 0000 [#1] SMP KASAN NOPTI
KASAN: null-ptr-deref in range [0x0000000000000a88-0x0000000000000a8f]
CPU: 2 UID: 0 PID: 6157 Comm: syz.3.37 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
RIP: 0010:kasan_byte_accessible+0x15/0x30 mm/kasan/generic.c:210
Code: 00 00 0f 1f 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 0f 1f 40 d6 48 b8 00 00 00 00 00 fc ff df 48 c1 ef 03 48 01 c7 <0f> b6 07 3c 07 0f 96 c0 c3 cc cc cc cc 66 66 2e 0f 1f 84 00 00 00
RSP: 0018:ffffc90003c4f9b0 EFLAGS: 00010082
RAX: dffffc0000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffffffff8b8c8c8e RDI: dffffc0000000151
RBP: 0000000000000a88 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000001 R12: ffffffff8b8c8c8e
R13: 0000000000000001 R14: 0000000000000000 R15: 0000000000000000
FS: 00007f1de46966c0(0000) GS:ffff8880d6579000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f1de4633d58 CR3: 00000000479b7000 CR4: 0000000000352ef0
Call Trace:
<TASK>
__kasan_check_byte+0x13/0x50 mm/kasan/common.c:573
kasan_check_byte include/linux/kasan.h:402 [inline]
lock_acquire kernel/locking/lockdep.c:5842 [inline]
lock_acquire+0x12a/0x370 kernel/locking/lockdep.c:5825
__raw_spin_lock include/linux/spinlock_api_smp.h:158 [inline]
_raw_spin_lock+0x2e/0x40 kernel/locking/spinlock.c:158
class_raw_spinlock_constructor include/linux/spinlock.h:535 [inline]
remove_waiter+0x159/0x1200 kernel/locking/rtmutex.c:1561
rt_mutex_start_proxy_lock+0x103/0x120 kernel/locking/rtmutex_api.c:369
futex_requeue+0x10e4/0x20d0 kernel/futex/requeue.c:643
do_futex+0x2af/0x350 kernel/futex/syscalls.c:156
__do_sys_futex kernel/futex/syscalls.c:207 [inline]
__se_sys_futex kernel/futex/syscalls.c:188 [inline]
__x64_sys_futex+0x34f/0x4d0 kernel/futex/syscalls.c:188
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x10b/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f1de379cdd9
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:00007f1de4696028 EFLAGS: 00000246 ORIG_RAX: 00000000000000ca
RAX: ffffffffffffffda RBX: 00007f1de3a15fa0 RCX: 00007f1de379cdd9
RDX: 0000000000000001 RSI: 000000000000000c RDI: 000020000000cffc
RBP: 00007f1de3832d69 R08: 0000200000048000 R09: 0000000000000000
R10: 0000200000000040 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f1de3a16038 R14: 00007f1de3a15fa0 R15: 00007ffea9ca8968
</TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:kasan_byte_accessible+0x15/0x30 mm/kasan/generic.c:210
Code: 00 00 0f 1f 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 0f 1f 40 d6 48 b8 00 00 00 00 00 fc ff df 48 c1 ef 03 48 01 c7 <0f> b6 07 3c 07 0f 96 c0 c3 cc cc cc cc 66 66 2e 0f 1f 84 00 00 00
RSP: 0018:ffffc90003c4f9b0 EFLAGS: 00010082
RAX: dffffc0000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffffffff8b8c8c8e RDI: dffffc0000000151
RBP: 0000000000000a88 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000001 R12: ffffffff8b8c8c8e
R13: 0000000000000001 R14: 0000000000000000 R15: 0000000000000000
FS: 00007f1de46966c0(0000) GS:ffff8880d6579000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f1de4633d58 CR3: 00000000479b7000 CR4: 0000000000352ef0
----------------
Code disassembly (best guess):
0: 00 00 add %al,(%rax)
2: 0f 1f 00 nopl (%rax)
5: 90 nop
6: 90 nop
7: 90 nop
8: 90 nop
9: 90 nop
a: 90 nop
b: 90 nop
c: 90 nop
d: 90 nop
e: 90 nop
f: 90 nop
10: 90 nop
11: 90 nop
12: 90 nop
13: 90 nop
14: 90 nop
15: 0f 1f 40 d6 nopl -0x2a(%rax)
19: 48 b8 00 00 00 00 00 movabs $0xdffffc0000000000,%rax
20: fc ff df
23: 48 c1 ef 03 shr $0x3,%rdi
27: 48 01 c7 add %rax,%rdi
* 2a: 0f b6 07 movzbl (%rdi),%eax <-- trapping instruction
2d: 3c 07 cmp $0x7,%al
2f: 0f 96 c0 setbe %al
32: c3 ret
33: cc int3
34: cc int3
35: cc int3
36: cc int3
37: 66 data16
38: 66 data16
39: 2e cs
3a: 0f .byte 0xf
3b: 1f (bad)
3c: 84 00 test %al,(%rax)


---
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[Hillf Danton]

> Date: Tue, 28 Apr 2026 13:12:28 -0700 [thread overview]

> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit: 897d54018cc9 Merge tag 'fbdev-for-7.1-rc1-2' of git://git...
> git tree: upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=172c32ce580000
> kernel config: https://syzkaller.appspot.com/x/.config?x=59da38148f3a3d24
> dashboard link: https://syzkaller.appspot.com/bug?extid=78147abe6c524f183ee9
> compiler: gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=10c7f273980000

#syz test

--- x/kernel/locking/rtmutex.c
+++ y/kernel/locking/rtmutex.c
@@ -1558,6 +1558,8 @@ static void __sched remove_waiter(struct

lockdep_assert_held(&lock->wait_lock);

+ if (!waiter_task)
+ return;
scoped_guard(raw_spinlock, &waiter_task->pi_lock) {
rt_mutex_dequeue(lock, waiter);
waiter_task->pi_blocked_on = NULL;
--

[syzbot]

Hello,

syzbot has tested the proposed patch and the reproducer did not trigger any issue:

Reported-by: syzbot+78147a...@syzkaller.appspotmail.com
Tested-by: syzbot+78147a...@syzkaller.appspotmail.com

Tested on:

commit: dca922e0 Merge tag 'xsa48x-7.1-tag' of git://git.kerne..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=17916b6c580000

kernel config: https://syzkaller.appspot.com/x/.config?x=59da38148f3a3d24
dashboard link: https://syzkaller.appspot.com/bug?extid=78147abe6c524f183ee9
compiler: gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44

patch: https://syzkaller.appspot.com/x/patch.diff?x=17650a73980000

Note: testing is done by a robot and is best-effort only.