[PATCH] slub: Don't call lockdep_unregister_key() for immature kmem_cache.

[PATCH] slub: Don't call lockdep_unregister_key() for immature kmem_cache.

[Kuniyuki Iwashima]

syzbot reported the lockdep splat below in __kmem_cache_release(). [0]

The problem is that __kmem_cache_release() could be called from
do_kmem_cache_create() before init_kmem_cache_cpus() registers
the lockdep key.

Let's move lockdep_unregister_key() from __kmem_cache_release()
to slab_kmem_cache_release() and do_kmem_cache_create().

[0]:
WARNING: CPU: 1 PID: 6128 at kernel/locking/lockdep.c:6606 lockdep_unregister_key+0x2ca/0x310 kernel/locking/lockdep.c:6606
Modules linked in:
CPU: 1 UID: 0 PID: 6128 Comm: syz.4.21 Not tainted syzkaller #0 PREEMPT_{RT,(full)}
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/18/2025
RIP: 0010:lockdep_unregister_key+0x2ca/0x310 kernel/locking/lockdep.c:6606
Code: 50 e4 0f 48 3b 44 24 10 0f 84 26 fe ff ff e8 bd cd 17 09 e8 e8 ce 17 09 41 f7 c7 00 02 00 00 74 bd fb 40 84 ed 75 bc eb cd 90 <0f> 0b 90 e9 19 ff ff ff 90 0f 0b 90 e9 2a ff ff ff 48 c7 c7 d0 ac
RSP: 0018:ffffc90003e870d0 EFLAGS: 00010002
RAX: eb1525397f5bdf00 RBX: ffff88803c121148 RCX: 1ffff920007d0dfc
RDX: 0000000000000000 RSI: ffffffff8acb1500 RDI: ffffffff8b1dd0e0
RBP: 00000000ffffffea R08: ffffffff8eb5aa37 R09: 1ffffffff1d6b546
R10: dffffc0000000000 R11: fffffbfff1d6b547 R12: 0000000000000000
R13: ffff88814d1b8900 R14: 0000000000000000 R15: 0000000000000203
FS:  00007f773f75e6c0(0000) GS:ffff88812712f000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007ffdaea3af52 CR3: 000000003a5ca000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 __kmem_cache_release+0xe3/0x1e0 mm/slub.c:7696
 do_kmem_cache_create+0x74e/0x790 mm/slub.c:8575
 create_cache mm/slab_common.c:242 [inline]
 __kmem_cache_create_args+0x1ce/0x330 mm/slab_common.c:340
 nfsd_file_cache_init+0x1d6/0x530 fs/nfsd/filecache.c:816
 nfsd_startup_generic fs/nfsd/nfssvc.c:282 [inline]
 nfsd_startup_net fs/nfsd/nfssvc.c:377 [inline]
 nfsd_svc+0x393/0x900 fs/nfsd/nfssvc.c:786
 nfsd_nl_threads_set_doit+0x84a/0x960 fs/nfsd/nfsctl.c:1639
 genl_family_rcv_msg_doit+0x212/0x300 net/netlink/genetlink.c:1115
 genl_family_rcv_msg net/netlink/genetlink.c:1195 [inline]
 genl_rcv_msg+0x60e/0x790 net/netlink/genetlink.c:1210
 netlink_rcv_skb+0x208/0x470 net/netlink/af_netlink.c:2552
 genl_rcv+0x28/0x40 net/netlink/genetlink.c:1219
 netlink_unicast_kernel net/netlink/af_netlink.c:1320 [inline]
 netlink_unicast+0x846/0xa10 net/netlink/af_netlink.c:1346
 netlink_sendmsg+0x805/0xb30 net/netlink/af_netlink.c:1896
 sock_sendmsg_nosec net/socket.c:727 [inline]
 __sock_sendmsg+0x219/0x270 net/socket.c:742
 ____sys_sendmsg+0x508/0x820 net/socket.c:2630
 ___sys_sendmsg+0x21f/0x2a0 net/socket.c:2684
 __sys_sendmsg net/socket.c:2716 [inline]
 __do_sys_sendmsg net/socket.c:2721 [inline]
 __se_sys_sendmsg net/socket.c:2719 [inline]
 __x64_sys_sendmsg+0x1a1/0x260 net/socket.c:2719
 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: 0033:0x7f77400eeec9
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:00007f773f75e038 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 00007f7740345fa0 RCX: 00007f77400eeec9
RDX: 0000000000008004 RSI: 0000200000000180 RDI: 0000000000000006
RBP: 00007f7740171f91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f7740346038 R14: 00007f7740345fa0 R15: 00007ffce616f8d8
 </TASK>

Fixes: 83382af9ddc3 ("slab: Make slub local_(try)lock more precise for LOCKDEP")
Reported-by: syzbot+a6f4d69b9b23404bbabf@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/68e4a3d1.a00a0220.298cc0.0471.GAE@google.com/
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
 mm/slab_common.c |  3 +++
 mm/slub.c        | 11 +++++++----
 2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/mm/slab_common.c b/mm/slab_common.c
index 932d13ada36c..baa934cad7ac 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -479,6 +479,9 @@ static void kmem_cache_release(struct kmem_cache *s)
 
 void slab_kmem_cache_release(struct kmem_cache *s)
 {
+#if !IS_ENABLED(CONFIG_SLUB_TINY) && IS_ENABLED(CONFIG_PREEMPT_RT)
+	lockdep_unregister_key(&s->lock_key);
+#endif
 	__kmem_cache_release(s);
 	kfree_const(s->name);
 	kmem_cache_free(kmem_cache, s);
diff --git a/mm/slub.c b/mm/slub.c
index 584a5ff1828b..8da20552995a 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -7692,9 +7692,6 @@ void __kmem_cache_release(struct kmem_cache *s)
 	if (s->cpu_sheaves)
 		pcs_destroy(s);
 #ifndef CONFIG_SLUB_TINY
-#ifdef CONFIG_PREEMPT_RT
-	lockdep_unregister_key(&s->lock_key);
-#endif
 	free_percpu(s->cpu_slab);
 #endif
 	free_kmem_cache_nodes(s);
@@ -8551,7 +8548,7 @@ int do_kmem_cache_create(struct kmem_cache *s, const char *name,
 	if (s->cpu_sheaves) {
 		err = init_percpu_sheaves(s);
 		if (err)
-			goto out;
+			goto out_unreg_lockdep;
 	}
 
 	err = 0;
@@ -8574,6 +8571,12 @@ int do_kmem_cache_create(struct kmem_cache *s, const char *name,
 	if (err)
 		__kmem_cache_release(s);
 	return err;
+
+out_unreg_lockdep:
+#if !IS_ENABLED(CONFIG_SLUB_TINY) && IS_ENABLED(CONFIG_PREEMPT_RT)
+	lockdep_unregister_key(&s->lock_key);
+#endif
+	goto out;
 }
 
 #ifdef SLAB_SUPPORTS_SYSFS
-- 
2.51.0.710.ga91ca5db03-goog

Re: [PATCH] slub: Don't call lockdep_unregister_key() for immature kmem_cache.

[Vlastimil Babka]

On 10/7/25 07:25, Kuniyuki Iwashima wrote:
> syzbot reported the lockdep splat below in __kmem_cache_release(). [0]
> 
> The problem is that __kmem_cache_release() could be called from
> do_kmem_cache_create() before init_kmem_cache_cpus() registers
> the lockdep key.
> 
> Let's move lockdep_unregister_key() from __kmem_cache_release()
> to slab_kmem_cache_release() and do_kmem_cache_create().
> 
> [0]:
> WARNING: CPU: 1 PID: 6128 at kernel/locking/lockdep.c:6606 lockdep_unregister_key+0x2ca/0x310 kernel/locking/lockdep.c:6606
> Modules linked in:
> CPU: 1 UID: 0 PID: 6128 Comm: syz.4.21 Not tainted syzkaller #0 PREEMPT_{RT,(full)}
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/18/2025
> RIP: 0010:lockdep_unregister_key+0x2ca/0x310 kernel/locking/lockdep.c:6606
> Code: 50 e4 0f 48 3b 44 24 10 0f 84 26 fe ff ff e8 bd cd 17 09 e8 e8 ce 17 09 41 f7 c7 00 02 00 00 74 bd fb 40 84 ed 75 bc eb cd 90 <0f> 0b 90 e9 19 ff ff ff 90 0f 0b 90 e9 2a ff ff ff 48 c7 c7 d0 ac
> RSP: 0018:ffffc90003e870d0 EFLAGS: 00010002
> RAX: eb1525397f5bdf00 RBX: ffff88803c121148 RCX: 1ffff920007d0dfc
> RDX: 0000000000000000 RSI: ffffffff8acb1500 RDI: ffffffff8b1dd0e0
> RBP: 00000000ffffffea R08: ffffffff8eb5aa37 R09: 1ffffffff1d6b546
> R10: dffffc0000000000 R11: fffffbfff1d6b547 R12: 0000000000000000
> R13: ffff88814d1b8900 R14: 0000000000000000 R15: 0000000000000203
> FS:  00007f773f75e6c0(0000) GS:ffff88812712f000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007ffdaea3af52 CR3: 000000003a5ca000 CR4: 00000000003526f0
> Call Trace:
>  <TASK>
>  __kmem_cache_release+0xe3/0x1e0 mm/slub.c:7696
>  do_kmem_cache_create+0x74e/0x790 mm/slub.c:8575
>  create_cache mm/slab_common.c:242 [inline]
>  __kmem_cache_create_args+0x1ce/0x330 mm/slab_common.c:340
>  nfsd_file_cache_init+0x1d6/0x530 fs/nfsd/filecache.c:816
>  nfsd_startup_generic fs/nfsd/nfssvc.c:282 [inline]
>  nfsd_startup_net fs/nfsd/nfssvc.c:377 [inline]
>  nfsd_svc+0x393/0x900 fs/nfsd/nfssvc.c:786
>  nfsd_nl_threads_set_doit+0x84a/0x960 fs/nfsd/nfsctl.c:1639
>  genl_family_rcv_msg_doit+0x212/0x300 net/netlink/genetlink.c:1115
>  genl_family_rcv_msg net/netlink/genetlink.c:1195 [inline]
>  genl_rcv_msg+0x60e/0x790 net/netlink/genetlink.c:1210
>  netlink_rcv_skb+0x208/0x470 net/netlink/af_netlink.c:2552
>  genl_rcv+0x28/0x40 net/netlink/genetlink.c:1219
>  netlink_unicast_kernel net/netlink/af_netlink.c:1320 [inline]
>  netlink_unicast+0x846/0xa10 net/netlink/af_netlink.c:1346
>  netlink_sendmsg+0x805/0xb30 net/netlink/af_netlink.c:1896
>  sock_sendmsg_nosec net/socket.c:727 [inline]
>  __sock_sendmsg+0x219/0x270 net/socket.c:742
>  ____sys_sendmsg+0x508/0x820 net/socket.c:2630
>  ___sys_sendmsg+0x21f/0x2a0 net/socket.c:2684
>  __sys_sendmsg net/socket.c:2716 [inline]
>  __do_sys_sendmsg net/socket.c:2721 [inline]
>  __se_sys_sendmsg net/socket.c:2719 [inline]
>  __x64_sys_sendmsg+0x1a1/0x260 net/socket.c:2719
>  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: 0033:0x7f77400eeec9
> 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:00007f773f75e038 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
> RAX: ffffffffffffffda RBX: 00007f7740345fa0 RCX: 00007f77400eeec9
> RDX: 0000000000008004 RSI: 0000200000000180 RDI: 0000000000000006
> RBP: 00007f7740171f91 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
> R13: 00007f7740346038 R14: 00007f7740345fa0 R15: 00007ffce616f8d8
>  </TASK>
> 
> Fixes: 83382af9ddc3 ("slab: Make slub local_(try)lock more precise for LOCKDEP")
> Reported-by: syzbot+a6f4d69b9b23404bbabf@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/all/68e4a3d1.a00a0220.298cc0.0471.GAE@google.com/
> Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>

Thanks, added to slab/for-next-fixes

Re: [PATCH] slub: Don't call lockdep_unregister_key() for immature kmem_cache.

[Alexei Starovoitov]

On Tue, Oct 7, 2025 at 12:46 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 10/7/25 07:25, Kuniyuki Iwashima wrote:
> > syzbot reported the lockdep splat below in __kmem_cache_release(). [0]
> >
> > The problem is that __kmem_cache_release() could be called from
> > do_kmem_cache_create() before init_kmem_cache_cpus() registers
> > the lockdep key.

Hmm. If that's the case then alloc_kmem_cache_cpus() wasn't called yet
and s->cpu_slab == NULL, so any kmalloc/kmem_cache_alloc from
that slub will crash. The above sounds like it's a race condition,
but it's not. It's a weird error path in do_kmem_cache_create()
due to syzbot pressure.
So the fix isn't quite right.
There is no need to sprinkle #ifdef all over the code.

> >
> > Let's move lockdep_unregister_key() from __kmem_cache_release()
> > to slab_kmem_cache_release() and do_kmem_cache_create().

I wouldn't.

> Thanks, added to slab/for-next-fixes

The following is imo much cleaner fix:

diff --git a/mm/slub.c b/mm/slub.c
index 584a5ff1828b..0a1fbddb77f8 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -7693,7 +7693,8 @@ void __kmem_cache_release(struct kmem_cache *s)
                pcs_destroy(s);
 #ifndef CONFIG_SLUB_TINY
 #ifdef CONFIG_PREEMPT_RT
-       lockdep_unregister_key(&s->lock_key);
+       if (s->cpu_slab)
+               lockdep_unregister_key(&s->lock_key);
 #endif
        free_percpu(s->cpu_slab);
 #endif

compile tested only...

Re: [PATCH] slub: Don't call lockdep_unregister_key() for immature kmem_cache.

[Vlastimil Babka]

On 10/7/25 18:11, Alexei Starovoitov wrote:
> On Tue, Oct 7, 2025 at 12:46 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>>
>> On 10/7/25 07:25, Kuniyuki Iwashima wrote:
>> > syzbot reported the lockdep splat below in __kmem_cache_release(). [0]
>> >
>> > The problem is that __kmem_cache_release() could be called from
>> > do_kmem_cache_create() before init_kmem_cache_cpus() registers
>> > the lockdep key.
> 
> Hmm. If that's the case then alloc_kmem_cache_cpus() wasn't called yet
> and s->cpu_slab == NULL, so any kmalloc/kmem_cache_alloc from
> that slub will crash. The above sounds like it's a race condition,
> but it's not. It's a weird error path in do_kmem_cache_create()

Indeed.

> due to syzbot pressure.
> So the fix isn't quite right.
> There is no need to sprinkle #ifdef all over the code.
> 
>> >
>> > Let's move lockdep_unregister_key() from __kmem_cache_release()
>> > to slab_kmem_cache_release() and do_kmem_cache_create().
> 
> I wouldn't.
> 
>> Thanks, added to slab/for-next-fixes
> 
> The following is imo much cleaner fix:

Yes much better, I'm changing it. Thanks!
> diff --git a/mm/slub.c b/mm/slub.c
> index 584a5ff1828b..0a1fbddb77f8 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -7693,7 +7693,8 @@ void __kmem_cache_release(struct kmem_cache *s)
>                 pcs_destroy(s);
>  #ifndef CONFIG_SLUB_TINY
>  #ifdef CONFIG_PREEMPT_RT
> -       lockdep_unregister_key(&s->lock_key);
> +       if (s->cpu_slab)
> +               lockdep_unregister_key(&s->lock_key);
>  #endif
>         free_percpu(s->cpu_slab);
>  #endif
> 
> compile tested only...

Re: [PATCH] slub: Don't call lockdep_unregister_key() for immature kmem_cache.

[Harry Yoo]

On Tue, Oct 07, 2025 at 08:07:54PM +0200, Vlastimil Babka wrote:
> On 10/7/25 18:11, Alexei Starovoitov wrote:
> > On Tue, Oct 7, 2025 at 12:46 AM Vlastimil Babka <vbabka@suse.cz> wrote:
> >>
> >> On 10/7/25 07:25, Kuniyuki Iwashima wrote:
> >> > syzbot reported the lockdep splat below in __kmem_cache_release(). [0]
> >> >
> >> > The problem is that __kmem_cache_release() could be called from
> >> > do_kmem_cache_create() before init_kmem_cache_cpus() registers
> >> > the lockdep key.
> > 
> > Hmm. If that's the case then alloc_kmem_cache_cpus() wasn't called yet
> > and s->cpu_slab == NULL, so any kmalloc/kmem_cache_alloc from
> > that slub will crash. The above sounds like it's a race condition,
> > but it's not. It's a weird error path in do_kmem_cache_create()
> 
> Indeed.
> 
> > due to syzbot pressure.
> > So the fix isn't quite right.
> > There is no need to sprinkle #ifdef all over the code.
> > 
> >> >
> >> > Let's move lockdep_unregister_key() from __kmem_cache_release()
> >> > to slab_kmem_cache_release() and do_kmem_cache_create().
> > 
> > I wouldn't.
> > 
> >> Thanks, added to slab/for-next-fixes
> > 
> > The following is imo much cleaner fix:
> 
> Yes much better, I'm changing it. Thanks!
> > diff --git a/mm/slub.c b/mm/slub.c
> > index 584a5ff1828b..0a1fbddb77f8 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -7693,7 +7693,8 @@ void __kmem_cache_release(struct kmem_cache *s)
> >                 pcs_destroy(s);
> >  #ifndef CONFIG_SLUB_TINY
> >  #ifdef CONFIG_PREEMPT_RT
> > -       lockdep_unregister_key(&s->lock_key);
> > +       if (s->cpu_slab)
> > +               lockdep_unregister_key(&s->lock_key);
> >  #endif
> >         free_percpu(s->cpu_slab);
> >  #endif
> > 
> > compile tested only...

Looks good to me,
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>

-- 
Cheers,
Harry / Hyeonggon