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Subject[PATCH 3.2 139/199] ALSA: seq: Fix race at creating a queue
3.2.87-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Takashi Iwai <tiwai@suse.de>

commit 4842e98f26dd80be3623c4714a244ba52ea096a8 upstream.

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>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
sound/core/seq/seq_queue.c | 33 ++++++++++++++++++++-------------
1 file changed, 20 insertions(+), 13 deletions(-)

--- a/sound/core/seq/seq_queue.c
+++ b/sound/core/seq/seq_queue.c
@@ -183,6 +183,8 @@ 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
*/
@@ -194,11 +196,11 @@ int snd_seq_queue_alloc(int client, int
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;
}

@@ -504,19 +506,9 @@ int snd_seq_queue_timer_set_tempo(int qu
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++;
@@ -531,6 +523,21 @@ int snd_seq_queue_use(int queueid, int c
} 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;
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