locking/lockdep: Avoid potential access of invalid memory in lock_class

It was found that reading /proc/lockdep after a lockdep splat may
potentially cause an access to freed memory if lockdep_unregister_key()
is called after the splat but before access to /proc/lockdep [1]. This
is due to the fact that graph_lock() call in lockdep_unregister_key()
fails after the clearing of debug_locks by the splat process.

After lockdep_unregister_key() is called, the lock_name may be freed
but the corresponding lock_class structure still have a reference to
it. That invalid memory pointer will then be accessed when /proc/lockdep
is read by a user and a use-after-free (UAF) error will be reported if
KASAN is enabled.

To fix this problem, lockdep_unregister_key() is now modified to always
search for a matching key irrespective of the debug_locks state and
zap the corresponding lock class if a matching one is found.

[1] https://lore.kernel.org/lkml/77f05c15-81b6-bddd-9650-80d5f23fe330@i-love.sakura.ne.jp/

Fixes: 8b39adbee805 ("locking/lockdep: Make lockdep_unregister_key() honor 'debug_locks' again")
Reported-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lkml.kernel.org/r/20220103023558.1377055-1-longman@redhat.com

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 89b3df51fd988a3d6ac09a7d3a70ad88bb87e21e..2e6892ec3756b25b5042a7357762996d1bfce295 100644 (file)
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -6287,7 +6287,13 @@ void lockdep_reset_lock(struct lockdep_map *lock)
                lockdep_reset_lock_reg(lock);
 }
 
-/* Unregister a dynamically allocated key. */
+/*
+ * Unregister a dynamically allocated key.
+ *
+ * Unlike lockdep_register_key(), a search is always done to find a matching
+ * key irrespective of debug_locks to avoid potential invalid access to freed
+ * memory in lock_class entry.
+ */
 void lockdep_unregister_key(struct lock_class_key *key)
 {
        struct hlist_head *hash_head = keyhashentry(key);
@@ -6302,10 +6308,8 @@ void lockdep_unregister_key(struct lock_class_key *key)
                return;
 
        raw_local_irq_save(flags);
-       if (!graph_lock())
-               goto out_irq;
+       lockdep_lock();
 
-       pf = get_pending_free();
        hlist_for_each_entry_rcu(k, hash_head, hash_entry) {
                if (k == key) {
                        hlist_del_rcu(&k->hash_entry);
@@ -6313,11 +6317,13 @@ void lockdep_unregister_key(struct lock_class_key *key)
                        break;
                }
        }
-       WARN_ON_ONCE(!found);
-       __lockdep_free_key_range(pf, key, 1);
-       call_rcu_zapped(pf);
-       graph_unlock();
-out_irq:
+       WARN_ON_ONCE(!found && debug_locks);
+       if (found) {
+               pf = get_pending_free();
+               __lockdep_free_key_range(pf, key, 1);
+               call_rcu_zapped(pf);
+       }
+       lockdep_unlock();
        raw_local_irq_restore(flags);
 
        /* Wait until is_dynamic_key() has finished accessing k->hash_entry. */