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Date:   Thu, 17 May 2018 00:09:55 +0200
Message-ID: <s5h36yr9rrg.wl-tiwai@suse.de>
From:   Takashi Iwai <tiwai@suse.de>
To:     Ben Hutchings <ben.hutchings@codethink.co.uk>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-kernel@vger.kernel.org, stable@vger.kernel.org,
        syzbot+8e62ff4e07aa2ce87826@syzkaller.appspotmail.com
Subject: Re: [PATCH 4.4 53/97] ALSA: pcm: Fix UAF at PCM release via PCM timer access
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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