KEYS: Fix race between key destruction and finding a keyring by name

commit 94c4554b upstream.

There appears to be a race between:

 (1) key_gc_unused_keys() which frees key->security and then calls
     keyring_destroy() to unlink the name from the name list

 (2) find_keyring_by_name() which calls key_permission(), thus accessing
     key->security, on a key before checking to see whether the key usage is 0
     (ie. the key is dead and might be cleaned up).

Fix this by calling ->destroy() before cleaning up the core key data -
including key->security.
Reported-by: Petr Matousek <pmatouse@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>
[carnil: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

security/keys/gc.c

@@ -172,6 +172,10 @@ static noinline void key_gc_unused_key(struct key *key)
 {
 	key_check(key);
 
+	/* Throw away the key data */
+	if (key->type->destroy)
+		key->type->destroy(key);
+
 	security_key_free(key);
 
 	/* deal with the user's key tracking and quota */
@@ -186,10 +190,6 @@ static noinline void key_gc_unused_key(struct key *key)
 	if (test_bit(KEY_FLAG_INSTANTIATED, &key->flags))
 		atomic_dec(&key->user->nikeys);
 
-	/* now throw away the key memory */
-	if (key->type->destroy)
-		key->type->destroy(key);
-
 	key_user_put(key->user);
 
 	kfree(key->description);