[syzbot] [fs?] possible deadlock in put_mnt_ns

[syzbot] [fs?] possible deadlock in put_mnt_ns

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    0f853ca2a798 Add linux-next specific files for 20260113
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=1386f99a580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=789b351b8a4cafe1
dashboard link: https://syzkaller.appspot.com/bug?extid=55fd613012e606d75d45
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=1482859a580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=13402052580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/9c7778b6d6be/disk-0f853ca2.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/ea278b0d6aff/vmlinux-0f853ca2.xz
kernel image: https://storage.googleapis.com/syzbot-assets/60b94236c918/bzImage-0f853ca2.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+55fd613012e606d75d45@syzkaller.appspotmail.com

R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001
R13: 00007fca229e5fa0 R14: 00007fca229e5fa0 R15: 0000000000000003
 </TASK>
============================================
WARNING: possible recursive locking detected
syzkaller #0 Not tainted
--------------------------------------------
syz.0.23/6026 is trying to acquire lock:
ffffffff8e49efd0 (namespace_sem){++++}-{4:4}, at: namespace_lock fs/namespace.c:1730 [inline]
ffffffff8e49efd0 (namespace_sem){++++}-{4:4}, at: class_namespace_excl_constructor fs/namespace.c:101 [inline]
ffffffff8e49efd0 (namespace_sem){++++}-{4:4}, at: put_mnt_ns+0x170/0x2f0 fs/namespace.c:6241

but task is already holding lock:
ffffffff8e49efd0 (namespace_sem){++++}-{4:4}, at: namespace_lock fs/namespace.c:1730 [inline]
ffffffff8e49efd0 (namespace_sem){++++}-{4:4}, at: lock_mount_exact+0x89/0x630 fs/namespace.c:3852

other info that might help us debug this:
 Possible unsafe locking scenario:

       CPU0
       ----
  lock(namespace_sem);
  lock(namespace_sem);

 *** DEADLOCK ***

 May be due to missing lock nesting notation

2 locks held by syz.0.23/6026:
 #0: ffff888149040148 (&sb->s_type->i_mutex_key){++++}-{4:4}, at: inode_lock include/linux/fs.h:1028 [inline]
 #0: ffff888149040148 (&sb->s_type->i_mutex_key){++++}-{4:4}, at: lock_mount_exact+0x7d/0x630 fs/namespace.c:3851
 #1: ffffffff8e49efd0 (namespace_sem){++++}-{4:4}, at: namespace_lock fs/namespace.c:1730 [inline]
 #1: ffffffff8e49efd0 (namespace_sem){++++}-{4:4}, at: lock_mount_exact+0x89/0x630 fs/namespace.c:3852

stack backtrace:
CPU: 1 UID: 0 PID: 6026 Comm: syz.0.23 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
Call Trace:
 <TASK>
 dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
 print_deadlock_bug+0x279/0x290 kernel/locking/lockdep.c:3041
 check_deadlock kernel/locking/lockdep.c:3093 [inline]
 validate_chain kernel/locking/lockdep.c:3895 [inline]
 __lock_acquire+0x2540/0x2cf0 kernel/locking/lockdep.c:5237
 lock_acquire+0x107/0x340 kernel/locking/lockdep.c:5868
 down_write+0x96/0x1f0 kernel/locking/rwsem.c:1590
 namespace_lock fs/namespace.c:1730 [inline]
 class_namespace_excl_constructor fs/namespace.c:101 [inline]
 put_mnt_ns+0x170/0x2f0 fs/namespace.c:6241
 prepare_anon_dentry fs/libfs.c:2167 [inline]
 path_from_stashed+0x40a/0x5c0 fs/libfs.c:2252
 open_namespace_file+0x8f/0x130 fs/nsfs.c:108
 open_new_namespace fs/namespace.c:3174 [inline]
 vfs_open_tree+0xa9d/0xf50 fs/namespace.c:3220
 __do_sys_open_tree fs/namespace.c:3230 [inline]
 __se_sys_open_tree fs/namespace.c:3228 [inline]
 __x64_sys_open_tree+0x96/0x110 fs/namespace.c:3228
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xec/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fca2278f749
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:00007ffc7a04e728 EFLAGS: 00000246 ORIG_RAX: 00000000000001ac
RAX: ffffffffffffffda RBX: 00007fca229e5fa0 RCX: 00007fca2278f749
RDX: 0000000000001802 RSI: 0000200000000000 RDI: ffffffffffffffff
RBP: 00007ffc7a04e780 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001
R13: 00007fca229e5fa0 R14: 00007fca229e5fa0 R15: 0000000000000003
 </TASK>


---
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Re: [syzbot] [fs?] possible deadlock in put_mnt_ns

[Christian Brauner]

On Thu, Jan 15, 2026 at 01:50:24AM -0800, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    0f853ca2a798 Add linux-next specific files for 20260113
> git tree:       linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=1386f99a580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=789b351b8a4cafe1
> dashboard link: https://syzkaller.appspot.com/bug?extid=55fd613012e606d75d45
> 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=1482859a580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=13402052580000
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/9c7778b6d6be/disk-0f853ca2.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/ea278b0d6aff/vmlinux-0f853ca2.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/60b94236c918/bzImage-0f853ca2.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+55fd613012e606d75d45@syzkaller.appspotmail.com
> 
> R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001
> R13: 00007fca229e5fa0 R14: 00007fca229e5fa0 R15: 0000000000000003
>  </TASK>
> ============================================
> WARNING: possible recursive locking detected
> syzkaller #0 Not tainted
> --------------------------------------------
> syz.0.23/6026 is trying to acquire lock:
> ffffffff8e49efd0 (namespace_sem){++++}-{4:4}, at: namespace_lock fs/namespace.c:1730 [inline]
> ffffffff8e49efd0 (namespace_sem){++++}-{4:4}, at: class_namespace_excl_constructor fs/namespace.c:101 [inline]
> ffffffff8e49efd0 (namespace_sem){++++}-{4:4}, at: put_mnt_ns+0x170/0x2f0 fs/namespace.c:6241
> 
> but task is already holding lock:
> ffffffff8e49efd0 (namespace_sem){++++}-{4:4}, at: namespace_lock fs/namespace.c:1730 [inline]
> ffffffff8e49efd0 (namespace_sem){++++}-{4:4}, at: lock_mount_exact+0x89/0x630 fs/namespace.c:3852
> 
> other info that might help us debug this:
>  Possible unsafe locking scenario:
> 
>        CPU0
>        ----
>   lock(namespace_sem);
>   lock(namespace_sem);

Groan, simple fix. opening nsfs fd must be done outside of the lock as
it would drop the mntns reference. Fixing.

Re: [syzbot] [fs?] possible deadlock in put_mnt_ns

[Christian Brauner]

On Fri, Jan 16, 2026 at 11:40:06AM +0100, Christian Brauner wrote:
> On Thu, Jan 15, 2026 at 01:50:24AM -0800, syzbot wrote:
> > Hello,
> > 
> > syzbot found the following issue on:
> > 
> > HEAD commit:    0f853ca2a798 Add linux-next specific files for 20260113
> > git tree:       linux-next
> > console output: https://syzkaller.appspot.com/x/log.txt?x=1386f99a580000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=789b351b8a4cafe1
> > dashboard link: https://syzkaller.appspot.com/bug?extid=55fd613012e606d75d45
> > 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=1482859a580000
> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=13402052580000
> > 
> > Downloadable assets:
> > disk image: https://storage.googleapis.com/syzbot-assets/9c7778b6d6be/disk-0f853ca2.raw.xz
> > vmlinux: https://storage.googleapis.com/syzbot-assets/ea278b0d6aff/vmlinux-0f853ca2.xz
> > kernel image: https://storage.googleapis.com/syzbot-assets/60b94236c918/bzImage-0f853ca2.xz
> > 
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+55fd613012e606d75d45@syzkaller.appspotmail.com
> > 
> > R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001
> > R13: 00007fca229e5fa0 R14: 00007fca229e5fa0 R15: 0000000000000003
> >  </TASK>
> > ============================================
> > WARNING: possible recursive locking detected
> > syzkaller #0 Not tainted
> > --------------------------------------------
> > syz.0.23/6026 is trying to acquire lock:
> > ffffffff8e49efd0 (namespace_sem){++++}-{4:4}, at: namespace_lock fs/namespace.c:1730 [inline]
> > ffffffff8e49efd0 (namespace_sem){++++}-{4:4}, at: class_namespace_excl_constructor fs/namespace.c:101 [inline]
> > ffffffff8e49efd0 (namespace_sem){++++}-{4:4}, at: put_mnt_ns+0x170/0x2f0 fs/namespace.c:6241
> > 
> > but task is already holding lock:
> > ffffffff8e49efd0 (namespace_sem){++++}-{4:4}, at: namespace_lock fs/namespace.c:1730 [inline]
> > ffffffff8e49efd0 (namespace_sem){++++}-{4:4}, at: lock_mount_exact+0x89/0x630 fs/namespace.c:3852
> > 
> > other info that might help us debug this:
> >  Possible unsafe locking scenario:
> > 
> >        CPU0
> >        ----
> >   lock(namespace_sem);
> >   lock(namespace_sem);
> 
> Groan, simple fix. opening nsfs fd must be done outside of the lock as
> it would drop the mntns reference. Fixing.

#syz test: https://github.com/brauner/linux.git vfs-7.0.namespace

Re: [syzbot] [fs?] possible deadlock in put_mnt_ns

[syzbot]

Hello,

syzbot has tested the proposed patch and the reproducer did not trigger any issue:

Reported-by: syzbot+55fd613012e606d75d45@syzkaller.appspotmail.com
Tested-by: syzbot+55fd613012e606d75d45@syzkaller.appspotmail.com

Tested on:

commit:         e64f65a7 FOLDME
git tree:       https://github.com/brauner/linux.git vfs-7.0.namespace
console output: https://syzkaller.appspot.com/x/log.txt?x=15fac39a580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=dac5329a4190a189
dashboard link: https://syzkaller.appspot.com/bug?extid=55fd613012e606d75d45
compiler:       Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8

Note: no patches were applied.
Note: testing is done by a robot and is best-effort only.