[syzbot] [fs?] WARNING: bad unlock balance in namespace_unlock

[syzbot] [fs?] WARNING: bad unlock balance in namespace_unlock

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    ce7f1a983b07 Add linux-next specific files for 20250923
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=151b8d34580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=91ae0b9529ab8226
dashboard link: https://syzkaller.appspot.com/bug?extid=0d671007a95cd2835e05
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=131b8d34580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=16a194e2580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/3f1b65edb63f/disk-ce7f1a98.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/b132cb8d99cd/vmlinux-ce7f1a98.xz
kernel image: https://storage.googleapis.com/syzbot-assets/80f316094043/bzImage-ce7f1a98.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+0d671007a95cd2835e05@syzkaller.appspotmail.com

=====================================
WARNING: bad unlock balance detected!
syzkaller #0 Not tainted
-------------------------------------
syz.3.25/6203 is trying to release lock (namespace_sem) at:
[<ffffffff82401096>] namespace_unlock+0x486/0x760 fs/namespace.c:1705
but there are no more locks to release!

other info that might help us debug this:
no locks held by syz.3.25/6203.

stack backtrace:
CPU: 0 UID: 0 PID: 6203 Comm: syz.3.25 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/18/2025
Call Trace:
 <TASK>
 dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120
 print_unlock_imbalance_bug+0xdc/0xf0 kernel/locking/lockdep.c:5298
 __lock_release kernel/locking/lockdep.c:5527 [inline]
 lock_release+0x212/0x3e0 kernel/locking/lockdep.c:5889
 up_write+0x2d/0x420 kernel/locking/rwsem.c:1642
 namespace_unlock+0x486/0x760 fs/namespace.c:1705
 class_namespace_excl_destructor fs/namespace.c:96 [inline]
 copy_mnt_ns+0x6e5/0x880 fs/namespace.c:4176
 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:3198
 __do_sys_unshare kernel/fork.c:3269 [inline]
 __se_sys_unshare kernel/fork.c:3267 [inline]
 __x64_sys_unshare+0x38/0x50 kernel/fork.c:3267
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xfa/0xfa0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f7cd618eec9
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:00007f7cd6feb038 EFLAGS: 00000246 ORIG_RAX: 0000000000000110
RAX: ffffffffffffffda RBX: 00007f7cd63e6180 RCX: 00007f7cd618eec9
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000040020000
RBP: 00007f7cd6211f91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f7cd63e6218 R14: 00007f7cd63e6180 R15: 00007ffc3db53b78
 </TASK>
------------[ cut here ]------------
DEBUG_RWSEMS_WARN_ON((rwsem_owner(sem) != current) && !rwsem_test_oflags(sem, RWSEM_NONSPINNABLE)): count = 0x3, magic = 0xffffffff8e48df00, owner = 0xffff88803389dac0, curr 0xffff88803408dac0, list not empty
WARNING: kernel/locking/rwsem.c:1381 at __up_write kernel/locking/rwsem.c:1380 [inline], CPU#0: syz.3.25/6203
WARNING: kernel/locking/rwsem.c:1381 at up_write+0x3a2/0x420 kernel/locking/rwsem.c:1643, CPU#0: syz.3.25/6203
Modules linked in:
CPU: 0 UID: 0 PID: 6203 Comm: syz.3.25 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/18/2025
RIP: 0010:__up_write kernel/locking/rwsem.c:1380 [inline]
RIP: 0010:up_write+0x3a2/0x420 kernel/locking/rwsem.c:1643
Code: d0 48 c7 c7 80 ff aa 8b 48 c7 c6 a0 01 ab 8b 48 8b 14 24 4c 89 f1 4d 89 e0 4c 8b 4c 24 08 41 52 e8 83 37 e6 ff 48 83 c4 08 90 <0f> 0b 90 90 e9 6d fd ff ff 48 c7 c1 74 37 c3 8f 80 e1 07 80 c1 03
RSP: 0018:ffffc90003d2faf0 EFLAGS: 00010296
RAX: 4aab14382228fb00 RBX: ffffffff8e48df00 RCX: ffff88803408dac0
RDX: 0000000000000000 RSI: 0000000000000001 RDI: 0000000000000002
RBP: dffffc0000000000 R08: 0000000000000003 R09: 0000000000000004
R10: dffffc0000000000 R11: fffffbfff1c3a654 R12: ffff88803389dac0
R13: ffffffff8e48df58 R14: ffffffff8e48df00 R15: 1ffffffff1c91be1
FS:  00007f7cd6feb6c0(0000) GS:ffff888125a0a000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000001b33163fff CR3: 0000000027b4e000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 namespace_unlock+0x486/0x760 fs/namespace.c:1705
 class_namespace_excl_destructor fs/namespace.c:96 [inline]
 copy_mnt_ns+0x6e5/0x880 fs/namespace.c:4176
 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:3198
 __do_sys_unshare kernel/fork.c:3269 [inline]
 __se_sys_unshare kernel/fork.c:3267 [inline]
 __x64_sys_unshare+0x38/0x50 kernel/fork.c:3267
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xfa/0xfa0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f7cd618eec9
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:00007f7cd6feb038 EFLAGS: 00000246 ORIG_RAX: 0000000000000110
RAX: ffffffffffffffda RBX: 00007f7cd63e6180 RCX: 00007f7cd618eec9
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000040020000
RBP: 00007f7cd6211f91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f7cd63e6218 R14: 00007f7cd63e6180 R15: 00007ffc3db53b78
 </TASK>


---
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[PATCH Next] copy_mnt_ns(): Remove unnecessary unlock

[Edward Adam Davis]

This code segment is already protected by guards, namespace_unlock()
should not appear here.

Reported-by: syzbot+0d671007a95cd2835e05@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=0d671007a95cd2835e05
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
---
 fs/namespace.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index ac1aedafe05e..c22febeda1ac 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -4134,7 +4134,6 @@ struct mnt_namespace *copy_mnt_ns(u64 flags, struct mnt_namespace *ns,
 	new = copy_tree(old, old->mnt.mnt_root, copy_flags);
 	if (IS_ERR(new)) {
 		emptied_ns = new_ns;
-		namespace_unlock();
 		return ERR_CAST(new);
 	}
 	if (user_ns != ns->user_ns) {
-- 
2.43.0

Re: [PATCH Next] copy_mnt_ns(): Remove unnecessary unlock

[Jan Kara]

On Wed 24-09-25 18:29:04, Edward Adam Davis wrote:
> This code segment is already protected by guards, namespace_unlock()
> should not appear here.
> 
> Reported-by: syzbot+0d671007a95cd2835e05@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=0d671007a95cd2835e05
> Signed-off-by: Edward Adam Davis <eadavis@qq.com>

Indeed. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza

> ---
>  fs/namespace.c | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/fs/namespace.c b/fs/namespace.c
> index ac1aedafe05e..c22febeda1ac 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -4134,7 +4134,6 @@ struct mnt_namespace *copy_mnt_ns(u64 flags, struct mnt_namespace *ns,
>  	new = copy_tree(old, old->mnt.mnt_root, copy_flags);
>  	if (IS_ERR(new)) {
>  		emptied_ns = new_ns;
> -		namespace_unlock();
>  		return ERR_CAST(new);
>  	}
>  	if (user_ns != ns->user_ns) {
> -- 
> 2.43.0
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Re: [PATCH Next] copy_mnt_ns(): Remove unnecessary unlock

[Al Viro]

On Wed, Sep 24, 2025 at 02:03:56PM +0200, Jan Kara wrote:
> On Wed 24-09-25 18:29:04, Edward Adam Davis wrote:
> > This code segment is already protected by guards, namespace_unlock()
> > should not appear here.
> > 
> > Reported-by: syzbot+0d671007a95cd2835e05@syzkaller.appspotmail.com
> > Closes: https://syzkaller.appspot.com/bug?extid=0d671007a95cd2835e05
> > Signed-off-by: Edward Adam Davis <eadavis@qq.com>
> 
> Indeed. Feel free to add:
> 
> Reviewed-by: Jan Kara <jack@suse.cz>

I wonder where does that line come from, though.  Mismerge somewhere?
d7b7253a0adc "copy_mnt_ns(): use guards" includes this:

@@ -4185,13 +4186,11 @@ struct mnt_namespace *copy_mnt_ns(unsigned long flags, struct mnt_namespace *ns,
 	new = copy_tree(old, old->mnt.mnt_root, copy_flags);
 	if (IS_ERR(new)) {
 		emptied_ns = new_ns;
-		namespace_unlock();
 		return ERR_CAST(new);
 	}
 	if (user_ns != ns->user_ns) {
-		lock_mount_hash();
+		guard(mount_writer)();
 		lock_mnt_tree(new);
-		unlock_mount_hash();
 	}
 	new_ns->root = new;
 

Re: [PATCH Next] copy_mnt_ns(): Remove unnecessary unlock

[Christian Brauner]

On Wed, Sep 24, 2025 at 06:56:41PM +0100, Al Viro wrote:
> On Wed, Sep 24, 2025 at 02:03:56PM +0200, Jan Kara wrote:
> > On Wed 24-09-25 18:29:04, Edward Adam Davis wrote:
> > > This code segment is already protected by guards, namespace_unlock()
> > > should not appear here.
> > > 
> > > Reported-by: syzbot+0d671007a95cd2835e05@syzkaller.appspotmail.com
> > > Closes: https://syzkaller.appspot.com/bug?extid=0d671007a95cd2835e05
> > > Signed-off-by: Edward Adam Davis <eadavis@qq.com>
> > 
> > Indeed. Feel free to add:
> > 
> > Reviewed-by: Jan Kara <jack@suse.cz>
> 
> I wonder where does that line come from, though.  Mismerge somewhere?
> d7b7253a0adc "copy_mnt_ns(): use guards" includes this:
> 
> @@ -4185,13 +4186,11 @@ struct mnt_namespace *copy_mnt_ns(unsigned long flags, struct mnt_namespace *ns,
>  	new = copy_tree(old, old->mnt.mnt_root, copy_flags);
>  	if (IS_ERR(new)) {
>  		emptied_ns = new_ns;
> -		namespace_unlock();
>  		return ERR_CAST(new);
>  	}
>  	if (user_ns != ns->user_ns) {
> -		lock_mount_hash();
> +		guard(mount_writer)();
>  		lock_mnt_tree(new);
> -		unlock_mount_hash();
>  	}
>  	new_ns->root = new;
>  

Fwiw, I just pulled next-20250926 and I don't see this bug. So I'm not
sure it's even a merge conflict or it was in an earlier -next version.