ALSA: seq: Fix race at creating a queue

When a sequencer queue is created in snd_seq_queue_alloc(),it adds the
new queue element to the public list before referencing it.  Thus the
queue might be deleted before the call of snd_seq_queue_use(), and it
results in the use-after-free error, as spotted by syzkaller.

The fix is to reference the queue object at the right time.
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/seq/seq_queue.c

@@ -181,6 +181,8 @@ void __exit snd_seq_queues_delete(void)
 	}
 }
 
+static void queue_use(struct snd_seq_queue *queue, int client, int use);
+
 /* allocate a new queue -
  * return queue index value or negative value for error
  */
@@ -192,11 +194,11 @@ int snd_seq_queue_alloc(int client, int locked, unsigned int info_flags)
 	if (q == NULL)
 		return -ENOMEM;
 	q->info_flags = info_flags;
+	queue_use(q, client, 1);
 	if (queue_list_add(q) < 0) {
 		queue_delete(q);
 		return -ENOMEM;
 	}
-	snd_seq_queue_use(q->queue, client, 1); /* use this queue */
 	return q->queue;
 }
 
@@ -502,19 +504,9 @@ int snd_seq_queue_timer_set_tempo(int queueid, int client,
 	return result;
 }
 
-
-/* use or unuse this queue -
- * if it is the first client, starts the timer.
- * if it is not longer used by any clients, stop the timer.
- */
-int snd_seq_queue_use(int queueid, int client, int use)
+/* use or unuse this queue */
+static void queue_use(struct snd_seq_queue *queue, int client, int use)
 {
-	struct snd_seq_queue *queue;
-
-	queue = queueptr(queueid);
-	if (queue == NULL)
-		return -EINVAL;
-	mutex_lock(&queue->timer_mutex);
 	if (use) {
 		if (!test_and_set_bit(client, queue->clients_bitmap))
 			queue->clients++;
@@ -529,6 +521,21 @@ int snd_seq_queue_use(int queueid, int client, int use)
 	} else {
 		snd_seq_timer_close(queue);
 	}
+}
+
+/* use or unuse this queue -
+ * if it is the first client, starts the timer.
+ * if it is not longer used by any clients, stop the timer.
+ */
+int snd_seq_queue_use(int queueid, int client, int use)
+{
+	struct snd_seq_queue *queue;
+
+	queue = queueptr(queueid);
+	if (queue == NULL)
+		return -EINVAL;
+	mutex_lock(&queue->timer_mutex);
+	queue_use(queue, client, use);
 	mutex_unlock(&queue->timer_mutex);
 	queuefree(queue);
 	return 0;