On Wed, 16 May 2018 22:51:47 +0200, Ben Hutchings wrote:
On Sun, 2018-04-22 at 15:53 +0200, Greg Kroah-Hartman wrote:
4.4-stable review patch. If anyone has any objections, please let me know.
From: Takashi Iwai tiwai@suse.de
commit a820ccbe21e8ce8e86c39cd1d3bc8c7d1cbb949b upstream.
The PCM runtime object is created and freed dynamically at PCM stream open / close time. This is tracked via substream->runtime, and it's cleared at snd_pcm_detach_substream().
The runtime object assignment is protected by PCM open_mutex, so for all PCM operations, it's safely handled. However, each PCM substream provides also an ALSA timer interface, and user-space can access to this while closing a PCM substream. This may eventually lead to a UAF, as snd_pcm_timer_resolution() tries to access the runtime while clearing it in other side.
Fortunately, it's the only concurrent access from the PCM timer, and it merely reads runtime->timer_resolution field. So, we can avoid the race by reordering kfree() and wrapping the substream->runtime clearance with the corresponding timer lock.
[...]
This seems to depend on:
commit f65e0d299807d8a11812845c972493c3f9a18e10 Author: Takashi Iwai tiwai@suse.de Date: Wed Feb 10 12:47:03 2016 +0100
ALSA: timer: Call notifier in the same spinlock
(But I'm not totally convinced that snd_pcm_timer_resolution() is always called with the timer lock held, even after that.)
Indeed, there seem more codes calling the callback without the lock. I'll send the fix patches.
thanks,
Takashi