Re: [syzbot] [fs?] WARNING in free_mnt_ns

[syzbot <syzbot+7d23dc5cd4fa132fb9f3@syzkaller.appspotmail.com>]

Hello,

syzbot has tested the proposed patch but the reproducer is still triggering an issue:
WARNING: refcount bug in free_nsproxy

R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001
R13: 00007fa688de6038 R14: 00007fa688de5fa0 R15: 00007fffb55a1f68
 </TASK>
------------[ cut here ]------------
refcount_t: underflow; use-after-free.
WARNING: CPU: 1 PID: 6516 at lib/refcount.c:28 refcount_warn_saturate+0x11a/0x1d0 lib/refcount.c:28
Modules linked in:
CPU: 1 UID: 0 PID: 6516 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/18/2025
RIP: 0010:refcount_warn_saturate+0x11a/0x1d0 lib/refcount.c:28
Code: c0 14 9e 8b e8 87 7b fd fc 90 0f 0b 90 90 eb d7 e8 0b 0e 3a fd c6 05 f4 66 c2 0a 01 90 48 c7 c7 20 15 9e 8b e8 67 7b fd fc 90 <0f> 0b 90 90 eb b7 e8 eb 0d 3a fd c6 05 d1 66 c2 0a 01 90 48 c7 c7
RSP: 0018:ffffc90003d2fa98 EFLAGS: 00010246
RAX: b510e5eecce46500 RBX: 0000000000000003 RCX: ffff88802e82bc80
RDX: 0000000000000000 RSI: 0000000000000001 RDI: 0000000000000002
RBP: ffffc90003d2fc30 R08: 0000000000000003 R09: 0000000000000004
R10: dffffc0000000000 R11: fffffbfff1b7a138 R12: dffffc0000000000
R13: 0000000000000000 R14: ffff88807dbeca1c R15: 0000000000000000
FS:  0000000000000000(0000) GS:ffff888126475000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000001b2f663fff CR3: 000000002fe8a000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 free_nsproxy+0x3e/0x350 kernel/nsproxy.c:188
 do_exit+0x6b0/0x2300 kernel/exit.c:960
 do_group_exit+0x21c/0x2d0 kernel/exit.c:1102
 get_signal+0x1285/0x1340 kernel/signal.c:3034
 arch_do_signal_or_restart+0xa0/0x790 arch/x86/kernel/signal.c:337
 exit_to_user_mode_loop+0x72/0x110 kernel/entry/common.c:40
 exit_to_user_mode_prepare include/linux/irq-entry-common.h:225 [inline]
 syscall_exit_to_user_mode_work include/linux/entry-common.h:175 [inline]
 syscall_exit_to_user_mode include/linux/entry-common.h:210 [inline]
 do_syscall_64+0x2bd/0x3b0 arch/x86/entry/syscall_64.c:100
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fa688b8eec9
Code: Unable to access opcode bytes at 0x7fa688b8ee9f.
RSP: 002b:00007fa689a0a0e8 EFLAGS: 00000246 ORIG_RAX: 00000000000000ca
RAX: fffffffffffffe00 RBX: 00007fa688de5fa8 RCX: 00007fa688b8eec9
RDX: 0000000000000000 RSI: 0000000000000080 RDI: 00007fa688de5fa8
RBP: 00007fa688de5fa0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fa688de6038 R14: 00007fffb55a1e80 R15: 00007fffb55a1f68
 </TASK>


Tested on:

commit:         080ffb4b Merge tag 'i3c/for-6.18' of git://git.kernel...
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=179785cd980000
kernel config:  https://syzkaller.appspot.com/x/.config?x=5a792dde4ff127ac
dashboard link: https://syzkaller.appspot.com/bug?extid=7d23dc5cd4fa132fb9f3
compiler:       Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
patch:          https://syzkaller.appspot.com/x/patch.diff?x=11bb85cd980000