[syzbot] [fs?] WARNING in free_mnt_ns

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

Hello,

syzbot found the following issue on:

HEAD commit:    449c2b302c8e Merge tag 'vfs-6.18-rc1.async' of git://git.k..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=15b43858580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=595e5938a1dd5b4e
dashboard link: https://syzkaller.appspot.com/bug?extid=7d23dc5cd4fa132fb9f3
compiler:       gcc (Debian 12.2.0-14+deb12u1) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=11c9ad04580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=160bf27c580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/ddc1ff1fc7e3/disk-449c2b30.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/92fc60e0e9d5/vmlinux-449c2b30.xz
kernel image: https://storage.googleapis.com/syzbot-assets/e50a03ce90e3/bzImage-449c2b30.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+7d23dc5cd4fa132fb9f3@syzkaller.appspotmail.com

------------[ cut here ]------------
ida_free called for id=1125 which is not allocated.
WARNING: CPU: 1 PID: 6109 at lib/idr.c:592 ida_free+0x1f9/0x2e0 lib/idr.c:592
Modules linked in:
CPU: 1 UID: 0 PID: 6109 Comm: syz.1.23 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/18/2025
RIP: 0010:ida_free+0x1f9/0x2e0 lib/idr.c:592
Code: 33 f6 41 83 fe 3e 76 72 e8 14 31 33 f6 48 8b 7c 24 28 4c 89 ee e8 57 65 0d 00 90 48 c7 c7 e0 17 16 8d 89 ee e8 78 13 f2 f5 90 <0f> 0b 90 90 e8 ee 30 33 f6 48 b8 00 00 00 00 00 fc ff df 48 01 c3
RSP: 0018:ffffc90003037980 EFLAGS: 00010286
RAX: 0000000000000000 RBX: 1ffff92000606f31 RCX: ffffffff817a02f8
RDX: ffff88802c8ebc00 RSI: ffffffff817a0305 RDI: 0000000000000001
RBP: 0000000000000465 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000001 R12: ffff8880791f7f00
R13: 0000000000000293 R14: 0000000000000065 R15: ffff8880791f7f08
FS:  0000000000000000(0000) GS:ffff8881247b3000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fe8d5f156c0 CR3: 000000007680c000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 free_mnt_ns+0xe0/0x110 fs/namespace.c:4096
 namespace_unlock+0x542/0x920 fs/namespace.c:1701
 put_mnt_ns fs/namespace.c:6135 [inline]
 put_mnt_ns+0xf5/0x120 fs/namespace.c:6126
 free_nsproxy+0x3a/0x400 kernel/nsproxy.c:188
 put_nsproxy include/linux/nsproxy.h:107 [inline]
 switch_task_namespaces+0xeb/0x100 kernel/nsproxy.c:241
 do_exit+0x86a/0x2bf0 kernel/exit.c:960
 do_group_exit+0xd3/0x2a0 kernel/exit.c:1102
 get_signal+0x2673/0x26d0 kernel/signal.c:3034
 arch_do_signal_or_restart+0x8f/0x790 arch/x86/kernel/signal.c:337
 exit_to_user_mode_loop+0x84/0x110 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+0x41c/0x4c0 arch/x86/entry/syscall_64.c:100
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f5cffd8eec9
Code: Unable to access opcode bytes at 0x7f5cffd8ee9f.
RSP: 002b:00007f5d00ba0038 EFLAGS: 00000246 ORIG_RAX: 0000000000000110
RAX: fffffffffffffff4 RBX: 00007f5cfffe5fa0 RCX: 00007f5cffd8eec9
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000020000
RBP: 00007f5cffe11f91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f5cfffe6038 R14: 00007f5cfffe5fa0 R15: 00007ffee87a9438
 </TASK>


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