[syzbot] [fs?] WARNING in copy_mnt_ns

[syzbot <syzbot+e0f8855a87443d6a2413@syzkaller.appspotmail.com>]

Hello,

syzbot found the following issue on:

HEAD commit:    50c19e20ed2e Merge tag 'nolibc-20250928-for-6.18-1' of git..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=126f605b980000
kernel config:  https://syzkaller.appspot.com/x/.config?x=8f1ac8502efee0ee
dashboard link: https://syzkaller.appspot.com/bug?extid=e0f8855a87443d6a2413
compiler:       Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1374b858580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=15602092580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/5fb1f87b20e9/disk-50c19e20.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/aebfd0341e80/vmlinux-50c19e20.xz
kernel image: https://storage.googleapis.com/syzbot-assets/11452a5eed6c/bzImage-50c19e20.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+e0f8855a87443d6a2413@syzkaller.appspotmail.com

------------[ cut here ]------------
ida_free called for id=1019 which is not allocated.
WARNING: CPU: 1 PID: 6112 at lib/idr.c:592 ida_free+0x280/0x310 lib/idr.c:592
Modules linked in:
CPU: 1 UID: 0 PID: 6112 Comm: syz.4.41 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/18/2025
RIP: 0010:ida_free+0x280/0x310 lib/idr.c:592
Code: 00 00 00 00 fc ff df 48 8b 5c 24 10 48 8b 7c 24 40 48 89 de e8 71 2c 0c 00 90 48 c7 c7 a0 7b 76 8c 44 89 fe e8 41 11 54 f6 90 <0f> 0b 90 90 eb 34 e8 85 a2 90 f6 49 bd 00 00 00 00 00 fc ff df eb
RSP: 0018:ffffc90003087b60 EFLAGS: 00010246

RAX: d54b8a9e76833a00 RBX: 0000000000000a02 RCX: ffff888026663c80
RDX: 0000000000000000 RSI: 0000000000000001 RDI: 0000000000000002
RBP: ffffc90003087c60 R08: ffff8880b8924293 R09: 1ffff11017124852
R10: dffffc0000000000 R11: ffffed1017124853 R12: 1ffff92000610f70
R13: dffffc0000000000 R14: ffff88801d6d9000 R15: 00000000000003fb
FS:  000055558bcc4500(0000) GS:ffff888126473000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007feaa252acf0 CR3: 000000005ee84000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 copy_mnt_ns+0x197/0x820 fs/namespace.c:4168
 create_new_namespaces+0xd1/0x720 kernel/nsproxy.c:78
 unshare_nsproxy_namespaces+0x11c/0x170 kernel/nsproxy.c:218
 ksys_unshare+0x4c8/0x8c0 kernel/fork.c:3132
 __do_sys_unshare kernel/fork.c:3203 [inline]
 __se_sys_unshare kernel/fork.c:3201 [inline]
 __x64_sys_unshare+0x38/0x50 kernel/fork.c:3201
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7feaa258eec9
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 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 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffd780455e8 EFLAGS: 00000246 ORIG_RAX: 0000000000000110
RAX: ffffffffffffffda RBX: 00007feaa27e5fa0 RCX: 00007feaa258eec9
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 000000002a020400
RBP: 00007ffd78045640 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000002
R13: 00007feaa27e5fa0 R14: 00007feaa27e5fa0 R15: 0000000000000001
 </TASK>


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