[syzbot] [fs?] WARNING in put_mnt_ns

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 2666975a8905 Add linux-next specific files for 20251111
git tree: linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=16d38914580000
kernel config: https://syzkaller.appspot.com/x/.config?x=e82ba9dc816af74c
dashboard link: https://syzkaller.appspot.com/bug?extid=d3005b4273499e174172
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/26ac789d9bdd/disk-2666975a.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/fabfe7978a23/vmlinux-2666975a.xz
kernel image: https://storage.googleapis.com/syzbot-assets/82f010d50b37/bzImage-2666975a.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+d3005b...@syzkaller.appspotmail.com

------------[ cut here ]------------
WARNING: ./include/linux/ns_common.h:255 at __ns_ref_put include/linux/ns_common.h:255 [inline], CPU#0: syz.1.878/9177
WARNING: ./include/linux/ns_common.h:255 at put_mnt_ns+0x152/0x190 fs/namespace.c:6048, CPU#0: syz.1.878/9177
Modules linked in:
CPU: 0 UID: 0 PID: 9177 Comm: syz.1.878 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
RIP: 0010:__ns_ref_put include/linux/ns_common.h:255 [inline]
RIP: 0010:put_mnt_ns+0x152/0x190 fs/namespace.c:6048
Code: 79 00 00 bf 01 00 00 00 89 ee e8 99 15 7e ff 85 ed 7e 1f e8 50 11 7e ff 5b 41 5e 41 5f 5d c3 cc cc cc cc cc e8 3f 11 7e ff 90 <0f> 0b 90 e9 33 ff ff ff e8 31 11 7e ff 4c 89 f7 be 03 00 00 00 5b
RSP: 0018:ffffc900038bfaa8 EFLAGS: 00010293
RAX: ffffffff82434e41 RBX: ffff88807a516000 RCX: ffff88807ce9bd00
RDX: 0000000000000000 RSI: 00000000ffffffff RDI: 0000000000000000
RBP: 00000000ffffffff R08: ffff88807a5160bb R09: 1ffff1100f4a2c17
R10: dffffc0000000000 R11: ffffed100f4a2c18 R12: dffffc0000000000
R13: 0000000000000009 R14: ffff88807a5160b8 R15: dffffc0000000000
FS: 0000000000000000(0000) GS:ffff888125a82000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007ffc52318fb8 CR3: 0000000030cb8000 CR4: 00000000003526f0
Call Trace:
<TASK>
free_nsproxy+0x46/0x560 kernel/nsproxy.c:192
do_exit+0x6b8/0x2300 kernel/exit.c:969
do_group_exit+0x21c/0x2d0 kernel/exit.c:1111
get_signal+0x1285/0x1340 kernel/signal.c:3034
arch_do_signal_or_restart+0x9a/0x7a0 arch/x86/kernel/signal.c:337
__exit_to_user_mode_loop kernel/entry/common.c:41 [inline]
exit_to_user_mode_loop kernel/entry/common.c:75 [inline]
__exit_to_user_mode_prepare include/linux/irq-entry-common.h:226 [inline]
irqentry_exit_to_user_mode_prepare include/linux/irq-entry-common.h:270 [inline]
irqentry_exit_to_user_mode include/linux/irq-entry-common.h:339 [inline]
irqentry_exit+0x171/0x640 kernel/entry/common.c:196
exc_page_fault+0xab/0x100 arch/x86/mm/fault.c:1535
asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:618
RIP: 0033:0x7ff8b0654d64
Code: Unable to access opcode bytes at 0x7ff8b0654d3a.
RSP: 002b:00007ff8b15f2ff0 EFLAGS: 00010217
RAX: 0000000000000000 RBX: 00007ff8b09e6090 RCX: 00007ff8b078f6c9
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: 00007ff8b0811f91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007ff8b09e6128 R14: 00007ff8b09e6090 R15: 00007ffc5231c618
</TASK>


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