Re: [PATCH 4.4 53/97] ALSA: pcm: Fix UAF at PCM release via PCM timer access

[Ben Hutchings]

On Sun, 2018-04-22 at 15:53 +0200, Greg Kroah-Hartman wrote:
> 4.4-stable review patch.ÂÂIf anyone has any objections, please let me know.
> 
> ------------------
> 
> From: Takashi Iwai <tiwai@xxxxxxx>
> 
> commit a820ccbe21e8ce8e86c39cd1d3bc8c7d1cbb949b upstream.
>
> The PCM runtime object is created and freed dynamically at PCM stream
> open / close time.ÂÂThis 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.ÂÂHowever, each PCM substream
> provides also an ALSA timer interface, and user-space can access to
> this while closing a PCM substream.ÂÂThis 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.ÂÂSo, 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@xxxxxxx>
Date:ÂÂÂWed Feb 10 12:47:03 2016 +0100

ÂÂÂÂALSA: 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.)

Ben.

-- 
Ben Hutchings
Software Developer, Codethink Ltd.