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locking/lockdep: Invalidate stale class_cache entries for zapped classes

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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: a0b0fd53e1 ("locking/lockdep: Free lock classes that are no longer in use")
Closes: https://lore.kernel.org/netdev/6a8c66dc.4d75e56a.c9a88.0050.GAE@google.com/T/#u
Reported-by: syzbot+2d770620059281e225a4@syzkaller.appspotmail.com
Assisted-by: Gemini:gemini-3.1-pro
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260824155129.676096-1-edumazet@google.com
This commit is contained in:
Eric Dumazet 2026-08-24 15:51:29 +00:00 committed by Peter Zijlstra
parent 2af470916a
commit 02c6be7d67
1 changed files with 42 additions and 8 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

50
kernel/locking/lockdep.c
View File

@ -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 @@ register_lock_class(struct lockdep_map *lock, unsigned int subclass, int force)
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);
/*