In the Linux kernel, the following vulnerability has been resolved:

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

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.