A good catch! The fix patch is below.
thanks,
Takashi
-- 8< --
From: Takashi Iwai <
ti...@suse.de>
Subject: [PATCH] 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 <
dvy...@google.com>
Cc: <
sta...@vger.kernel.org>
Signed-off-by: Takashi Iwai <
ti...@suse.de>
---
sound/core/seq/seq_queue.c | 33 ++++++++++++++++++++-------------
1 file changed, 20 insertions(+), 13 deletions(-)
diff --git a/sound/core/seq/seq_queue.c b/sound/core/seq/seq_queue.c
index 0bec02e89d51..450c5187eecb 100644
--- a/sound/core/seq/seq_queue.c
+++ b/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;
--
2.11.0