[tip: locking/urgent] locking/lockdep: Invalidate stale class_cache entries for zapped classes
From: tip-bot2 for Eric Dumazet
Date: Wed Sep 02 2026 - 03:41:14 EST
The following commit has been merged into the locking/urgent branch of tip:
Commit-ID: 02c6be7d675b21d81f0ba3a524346850a8c0e3bf
Gitweb: https://git.kernel.org/tip/02c6be7d675b21d81f0ba3a524346850a8c0e3bf
Author: Eric Dumazet <edumazet@xxxxxxxxxx>
AuthorDate: Mon, 24 Aug 2026 15:51:29
Committer: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Wed, 02 Sep 2026 09:18:03 +02:00
locking/lockdep: Invalidate stale class_cache entries for zapped classes
syzbot reported a lockdep splat hitting DEBUG_LOCKS_WARN_ON(1) in
hlock_class() due to an invalid class_idx:
WARNING: kernel/locking/lockdep.c:238 at __lock_acquire+0x382/0x2cf0 kernel/locking/lockdep.c:5203
Workqueue: wg-crypt-wg0 wg_packet_tx_worker
RIP: 0010:hlock_class kernel/locking/lockdep.c:238 [inline]
RIP: 0010:check_wait_context kernel/locking/lockdep.c:4870 [inline]
RIP: 0010:__lock_acquire+0x389/0x2cf0 kernel/locking/lockdep.c:5203
Call Trace:
<IRQ>
lock_acquire+0x106/0x350 kernel/locking/lockdep.c:5886
_raw_spin_lock+0x2e/0x40 kernel/locking/spinlock.c:173
tcp_tsq_handler+0x29/0x200 net/ipv4/tcp_output.c:1291
tcp_tsq_workfn+0x384/0x410 net/ipv4/tcp_output.c:1325
...
When a lock class is zapped (e.g. during module unload or key
unregistration), zap_class() clears the class's bit in
lock_classes_in_use and removes it from the class hash table.
However, existing lockdep_map instances embedded in data structures
may still retain a pointer to the zapped class in their class_cache[]
array.
When __lock_acquire() subsequently runs on such a lock, it finds
lock->class_cache[subclass] != NULL, skipping register_lock_class()
and assigning hlock->class_idx to the index of the zapped class. When
check_wait_context() or hlock_class() inspects the held_lock, it finds
!test_bit(class_idx, lock_classes_in_use) and warns. Furthermore, if
the zapped slot is subsequently re-allocated to an unrelated lock key,
the stale class_cache entry would erroneously match the unrelated
class (ABA issue).
Add lock_class_cache_is_valid() to validate that the cached class is
within lock_classes bounds, still allocated in lock_classes_in_use
(using uninstrumented arch_test_bit() in __always_inline context so it
is safe in noinstr contexts like match_held_lock()), and that
class->key matches the expected subkey (taking lockdep_set_subclass()
overrides into account). Also use READ_ONCE()/WRITE_ONCE() when
accessing class_cache[]. If the entry is invalid or stale, fall back
to register_lock_class() / look_up_lock_class().
Fixes: a0b0fd53e1e6 ("locking/lockdep: Free lock classes that are no longer in use")
Closes: https://lore.kernel.org/netdev/6a8c66dc.4d75e56a.c9a88.0050.GAE@xxxxxxxxxx/T/#u
Reported-by: syzbot+2d770620059281e225a4@xxxxxxxxxxxxxxxxxxxxxxxxx
Assisted-by: Gemini:gemini-3.1-pro
Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://patch.msgid.link/20260824155129.676096-1-edumazet@xxxxxxxxxx
---
kernel/locking/lockdep.c | 50 ++++++++++++++++++++++++++++++++-------
1 file changed, 42 insertions(+), 8 deletions(-)
diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 25d77d4..763f798 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -963,6 +963,34 @@ look_up_lock_class(const struct lockdep_map *lock, unsigned int subclass)
return NULL;
}
+static __always_inline bool lock_class_cache_is_valid(const struct lockdep_map *lock,
+ const struct lock_class *class,
+ unsigned int subclass)
+{
+ unsigned int class_subclass;
+
+ if (!class)
+ return false;
+
+ if (unlikely(class < lock_classes || class >= lock_classes + MAX_LOCKDEP_KEYS))
+ return false;
+
+ if (unlikely(!arch_test_bit(class - lock_classes, lock_classes_in_use)))
+ return false;
+
+ if (unlikely(!lock->key))
+ return false;
+
+ class_subclass = subclass ? subclass : class->subclass;
+ if (unlikely(class_subclass >= MAX_LOCKDEP_SUBCLASSES))
+ return false;
+
+ if (unlikely(READ_ONCE(class->key) != lock->key->subkeys + class_subclass))
+ return false;
+
+ return true;
+}
+
/*
* Static locks do not have their class-keys yet - for them the key is
* the lock object itself. If the lock is in the per cpu area, the
@@ -1395,9 +1423,9 @@ out_unlock_set:
out_set_class_cache:
if (!subclass || force)
- lock->class_cache[0] = class;
+ WRITE_ONCE(lock->class_cache[0], class);
else if (subclass < NR_LOCKDEP_CACHING_CLASSES)
- lock->class_cache[subclass] = class;
+ WRITE_ONCE(lock->class_cache[subclass], class);
/*
* Hash collision, did we smoke some? We found a class with a matching
@@ -4957,7 +4985,7 @@ void lockdep_init_map_type(struct lockdep_map *lock, const char *name,
int i;
for (i = 0; i < NR_LOCKDEP_CACHING_CLASSES; i++)
- lock->class_cache[i] = NULL;
+ WRITE_ONCE(lock->class_cache[i], NULL);
#ifdef CONFIG_LOCK_STAT
lock->cpu = raw_smp_processor_id();
@@ -5022,12 +5050,15 @@ EXPORT_SYMBOL_GPL(__lockdep_no_track__);
void lockdep_set_lock_cmp_fn(struct lockdep_map *lock, lock_cmp_fn cmp_fn,
lock_print_fn print_fn)
{
- struct lock_class *class = lock->class_cache[0];
+ struct lock_class *class = READ_ONCE(lock->class_cache[0]);
unsigned long flags;
raw_local_irq_save(flags);
lockdep_recursion_inc();
+ if (!lock_class_cache_is_valid(lock, class, 0))
+ class = NULL;
+
if (!class)
class = register_lock_class(lock, 0, 0);
@@ -5119,8 +5150,11 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
if (DEBUG_LOCKS_WARN_ON(subclass >= MAX_LOCKDEP_SUBCLASSES))
return 0;
- if (subclass < NR_LOCKDEP_CACHING_CLASSES)
- class = lock->class_cache[subclass];
+ if (subclass < NR_LOCKDEP_CACHING_CLASSES) {
+ class = READ_ONCE(lock->class_cache[subclass]);
+ if (!lock_class_cache_is_valid(lock, class, subclass))
+ class = NULL;
+ }
/*
* Not cached?
*/
@@ -5323,9 +5357,9 @@ static noinstr int match_held_lock(const struct held_lock *hlock,
return 1;
if (hlock->references) {
- const struct lock_class *class = lock->class_cache[0];
+ const struct lock_class *class = READ_ONCE(lock->class_cache[0]);
- if (!class)
+ if (!lock_class_cache_is_valid(lock, class, 0))
class = look_up_lock_class(lock, 0);
/*