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

[Greg Kroah-Hartman]

On Thu, May 17, 2018 at 12:09:55AM +0200, Takashi Iwai wrote:
> 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@xxxxxxx>
> > > 
> > > 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@xxxxxxx>
> > 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.)

Good catch, I'll queue this up now, thanks.

greg k-h