On Mon, 10 Aug 2020 18:37:17 +0200, Pavel Machek wrote:
Hi!
commit 80982c7e834e5d4e325b6ce33757012ecafdf0bb upstream.
Some ioctls via OSS sequencer API may race and lead to UAF when the port create and delete are performed concurrently, as spotted by a couple of syzkaller cases. This patch is an attempt to address it by serializing the ioctls with the existing register_mutex.
Basically OSS sequencer API is an obsoleted interface and was designed without much consideration of the concurrency. There are very few applications with it, and the concurrent performance isn't asked, hence this "big hammer" approach should be good enough.
That really is a "big hammer". And I believe it is too big.
In particular, do we need to drop the lock while sleeping in SNDCTL_SEQ_SYNC: => snd_seq_oss_writeq_sync ?
Well, do you see any issue with this for really used applications? If yes, I'd happily take a look and give finer locking.
thanks,
Takashi