ALSA: seq: Don't allow resizing pool in use

tiwai · tiwai · commit d85739367c6d · 2018-03-08T08:59:26.000+01:00
This is a fix for a (sort of) fallout in the recent commit d15d662 ("ALSA: seq: Fix racy pool initializations") for CVE-2018-1000004. As the pool resize deletes the existing cells, it may lead to a race when another thread is writing concurrently, eventually resulting a UAF. A simple workaround is not to allow the pool resizing when the pool is in use. It's an invalid behavior in anyway. Fixes: d15d662 ("ALSA: seq: Fix racy pool initializations") Reported-by: 范龙飞 <long7573@126.com> Reported-by: Nicolai Stange <nstange@suse.de> Cc: <stable@vger.kernel.org> Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/sound/core/seq/seq_clientmgr.c b/sound/core/seq/seq_clientmgr.c
@@ -1838,6 +1838,9 @@ static int snd_seq_ioctl_set_client_pool(struct snd_seq_client *client,
 	    (! snd_seq_write_pool_allocated(client) ||
 	     info->output_pool != client->pool->size)) {
 		if (snd_seq_write_pool_allocated(client)) {
+			/* is the pool in use? */
+			if (atomic_read(&client->pool->counter))
+				return -EBUSY;
 			/* remove all existing cells */
 			snd_seq_pool_mark_closing(client->pool);
 			snd_seq_queue_client_leave_cells(client->number);
