[PATCH] lockdep: fix NULL pointer dereference in __lock_set_class()

[Xiang Gao]

From: Xiang Gao <gaoxiang17@xxxxxxxxxx>

register_lock_class() can return NULL on failure (e.g., exceeding
MAX_LOCKDEP_KEYS or lock_keys_in_use overflow). __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 garbage negative offset that corrupts
hlock->class_idx (a bitfield). Any subsequent hlock_class() call on
this hlock returns a garbage pointer, leading to memory corruption or
a crash.

The other call site in __lock_acquire() (line 5112) already handles
this correctly with an explicit NULL check. Add the same guard here.

Fixes: 64aa348edc61 ("lockdep: lock_set_subclass - reset a held lock's subclass")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Xiang Gao <gaoxiang17@xxxxxxxxxx>
---
 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;
-- 
2.34.1