[PATCH] locking/lockdep: Fix NULL pointer dereference in __lock_set_class()

[Naveen Kumar Chaudhary]

register_lock_class() can return NULL when the lock class pool is
exhausted, graph_lock() fails, or key validation fails. However,
__lock_set_class() uses the return value directly in pointer arithmetic
without a NULL check:

  class = register_lock_class(lock, subclass, 0);
  hlock->class_idx = class - lock_classes;

If class is NULL, this computes a wild offset that corrupts
hlock->class_idx. The subsequent reacquire_held_locks() call will
invoke hlock_class() with this corrupted index, leading to a NULL or
out-of-bounds pointer dereference.

Add the missing NULL check, consistent with how __lock_acquire() already
handles this case at the same call site.

Fixes: 64aa348edc61 ("lockdep: lock_set_subclass - reset a held lock's subclass")
Signed-off-by: Naveen Kumar Chaudhary <naveen.osdev@xxxxxxxxx>
---
 kernel/locking/lockdep.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 2d4c5bab5af8..e0de81114824 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -5437,6 +5437,8 @@ __lock_set_class(struct lockdep_map *lock, const char *name,
 			      lock->wait_type_outer,
 			      lock->lock_type);
 	class = register_lock_class(lock, subclass, 0);
+	if (!class)
+		return 0;
 	hlock->class_idx = class - lock_classes;
 
 	curr->lockdep_depth = i;
-- 
2.43.0