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

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Message-ID: <1526503907.9159.150.camel@codethink.co.uk>
Date:   Wed, 16 May 2018 21:51:47 +0100
From:   Ben Hutchings <ben.hutchings@...ethink.co.uk>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org
Cc:     stable@...r.kernel.org,
        syzbot+8e62ff4e07aa2ce87826@...kaller.appspotmail.com,
        Takashi Iwai <tiwai@...e.de>
Subject: Re: [PATCH 4.4 53/97] ALSA: pcm: Fix UAF at PCM release via PCM
 timer access

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@...e.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@...e.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.)

Ben.

-- 
Ben Hutchings
Software Developer, Codethink Ltd.