[PATCH 3.16 025/410] ALSA: seq: Don't allow resizing pool in use

[Ben Hutchings]

3.16.57-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Takashi Iwai <tiwai@xxxxxxx>

commit d85739367c6d56e475c281945c68fdb05ca74b4c upstream.

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: èéé <long7573@xxxxxxx>
Reported-by: Nicolai Stange <nstange@xxxxxxx>
Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 sound/core/seq/seq_clientmgr.c | 3 +++
 1 file changed, 3 insertions(+)

--- a/sound/core/seq/seq_clientmgr.c
+++ b/sound/core/seq/seq_clientmgr.c
@@ -1929,6 +1929,9 @@ static int snd_seq_ioctl_set_client_pool
 	    (! 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);