KEYS: request_key() should return -ENOKEY if the constructed key is negative

request_key() should return -ENOKEY if the key it constructs has been
negatively instantiated.

Without this, request_key() can return an unusable key to its caller,
and if the caller then does key_validate() that won't catch the problem.

Signed-off-by: David Howells <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

security/keys/request_key.c

···
 			  intr ? TASK_INTERRUPTIBLE : TASK_UNINTERRUPTIBLE);
 	if (ret < 0)
 		return ret;
+	if (test_bit(KEY_FLAG_NEGATIVE, &key->flags))
+		return -ENOKEY;
 	return key_validate(key);
 }
 EXPORT_SYMBOL(wait_for_key_construction);