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

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 perform lockdep_unregister_key() only when init_kmem_cache_cpus()
has been done, which we can determine by checking s->cpu_slab

[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>

[alexei.starovoitov@gmail.com: simplify the fix]
Link: https://lore.kernel.org/all/20251007052534.2776661-1-kuniyu@google.com/
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>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

diff --git a/mm/slub.c b/mm/slub.c
index 44aa0e3f48eed3ce5b2b35425aa03ae87a54a2b6..135c408e051528389fe4b5952bcdd84f455956d4 100644 (file)
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -7701,7 +7701,8 @@ 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