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

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@126.com>
Reported-by: Nicolai Stange <nstange@suse.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

1 files changed, 3 insertions, 0 deletions

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);