[alsa-devel] [PATCH 1/2] ALSA: seq: Don't allow resizing pool in use

[Takashi Iwai]

This is a fix for a (sort of) fallout in the recent commit
d15d662e89fc ("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: d15d662e89fc ("ALSA: seq: Fix racy pool initializations")
Reported-by: 范龙飞 <long7573 at 126.com>
Reported-by: Nicolai Stange <nstange at suse.de>
Cc: <stable at vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai at suse.de>
---
 sound/core/seq/seq_clientmgr.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/sound/core/seq/seq_clientmgr.c b/sound/core/seq/seq_clientmgr.c
index 04d4db44fae5..d41ce3ed62ca 100644
--- 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);
-- 
2.16.2