[moderation] [fs?] WARNING in do_loopback (4)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 7dff99b35460 Remove WARN_ALL_UNSEEDED_RANDOM kernel config..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=104e2006580000
kernel config: https://syzkaller.appspot.com/x/.config?x=70fe0401f305d8d4
dashboard link: https://syzkaller.appspot.com/bug?extid=424c14d1cd6ea2634867
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
CC: [bra...@kernel.org ja...@suse.cz linux-...@vger.kernel.org linux-...@vger.kernel.org vi...@zeniv.linux.org.uk]

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/f33ddab53df8/disk-7dff99b3.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/3c50676d2681/vmlinux-7dff99b3.xz
kernel image: https://storage.googleapis.com/syzbot-assets/bbae16d525e0/bzImage-7dff99b3.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+424c14...@syzkaller.appspotmail.com

------------[ cut here ]------------
DEBUG_LOCKS_WARN_ON(rt_mutex_owner(lock) != current)
WARNING: kernel/locking/rtmutex_common.h:182 at debug_rt_mutex_unlock kernel/locking/rtmutex_common.h:182 [inline], CPU#1: syz.0.1002/14208
WARNING: kernel/locking/rtmutex_common.h:182 at rt_mutex_slowunlock+0x6d9/0x8b0 kernel/locking/rtmutex.c:1419, CPU#1: syz.0.1002/14208
Modules linked in:
CPU: 1 UID: 0 PID: 14208 Comm: syz.0.1002 Not tainted syzkaller #0 PREEMPT_{RT,(full)}
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
RIP: 0010:debug_rt_mutex_unlock kernel/locking/rtmutex_common.h:182 [inline]
RIP: 0010:rt_mutex_slowunlock+0x6e0/0x8b0 kernel/locking/rtmutex.c:1419
Code: 6a 8f 48 c1 e8 03 42 0f b6 04 20 84 c0 0f 85 c0 01 00 00 83 3d 64 c4 46 04 00 75 13 48 8d 3d 77 d0 49 04 48 c7 c6 20 e8 4c 8b <67> 48 0f b9 3a 90 e9 2a fa ff ff 90 0f 0b 90 e9 4c fe ff ff be 02
RSP: 0018:ffffc90004f97c00 EFLAGS: 00010046
RAX: 0000000000000000 RBX: ffff88805f217770 RCX: 0000000000080000
RDX: ffffc9000eafa000 RSI: ffffffff8b4ce820 RDI: ffffffff8f6d4500
RBP: ffffc90004f97cf0 R08: 0000000000000000 R09: 0000000000000000
R10: dffffc0000000000 R11: fffffbfff1ed471d R12: dffffc0000000000
R13: 1ffff920009f2f88 R14: 0000000000000246 R15: ffff88805f217720
FS: 00007f9f4d3ad6c0(0000) GS:ffff888126443000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000055556f70aa68 CR3: 000000005be50000 CR4: 00000000003526f0
Call Trace:
<TASK>
inode_unlock include/linux/fs.h:1038 [inline]
__unlock_mount fs/namespace.c:2780 [inline]
unlock_mount fs/namespace.c:2790 [inline]
do_loopback+0x44b/0x6c0 fs/namespace.c:3017
do_mount fs/namespace.c:4159 [inline]
__do_sys_mount fs/namespace.c:4348 [inline]
__se_sys_mount+0x31d/0x420 fs/namespace.c:4325
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f9f4f17c629
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:00007f9f4d3ad028 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
RAX: ffffffffffffffda RBX: 00007f9f4f3f6090 RCX: 00007f9f4f17c629
RDX: 0000000000000000 RSI: 00002000000004c0 RDI: 0000200000000200
RBP: 00007f9f4f212b39 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000002125099 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f9f4f3f6128 R14: 00007f9f4f3f6090 R15: 00007ffc38f30298
</TASK>
----------------
Code disassembly (best guess):
0: 6a 8f push $0xffffffffffffff8f
2: 48 c1 e8 03 shr $0x3,%rax
6: 42 0f b6 04 20 movzbl (%rax,%r12,1),%eax
b: 84 c0 test %al,%al
d: 0f 85 c0 01 00 00 jne 0x1d3
13: 83 3d 64 c4 46 04 00 cmpl $0x0,0x446c464(%rip) # 0x446c47e
1a: 75 13 jne 0x2f
1c: 48 8d 3d 77 d0 49 04 lea 0x449d077(%rip),%rdi # 0x449d09a
23: 48 c7 c6 20 e8 4c 8b mov $0xffffffff8b4ce820,%rsi
* 2a: 67 48 0f b9 3a ud1 (%edx),%rdi <-- trapping instruction
2f: 90 nop
30: e9 2a fa ff ff jmp 0xfffffa5f
35: 90 nop
36: 0f 0b ud2
38: 90 nop
39: e9 4c fe ff ff jmp 0xfffffe8a
3e: be .byte 0xbe
3f: 02 .byte 0x2


---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzk...@googlegroups.com.

syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

If the report is already addressed, let syzbot know by replying with:
#syz fix: exact-commit-title

If you want to overwrite report's subsystems, reply with:
#syz set subsystems: new-subsystem
(See the list of subsystem names on the web dashboard)

If the report is a duplicate of another one, reply with:
#syz dup: exact-subject-of-another-report

If you want to undo deduplication, reply with:
#syz undup