From: Chih-Yang Hsia <paulhsia@chromium.org>
To: Takashi Iwai <tiwai@suse.de>
Cc: alsa-devel@alsa-project.org, Mark Brown <broonie@kernel.org>,
linux-kernel@vger.kernel.org, Takashi Iwai <tiwai@suse.com>
Subject: Re: [alsa-devel] [PATCH 0/2] ALSA: pcm: Fix race condition in runtime access
Date: Wed, 13 Nov 2019 15:24:41 +0800 [thread overview]
Message-ID: <CAJaf1TaZzsPdydcMZMemVSkjRvhYvx7ZxY2JEvExQ56B+MjQLQ@mail.gmail.com> (raw)
In-Reply-To: <s5h1rud7yel.wl-tiwai@suse.de>
On Wed, Nov 13, 2019 at 2:16 AM Takashi Iwai <tiwai@suse.de> wrote:
>
> On Tue, 12 Nov 2019 18:17:13 +0100,
> paulhsia wrote:
> >
> > Since
> > - snd_pcm_detach_substream sets runtime to null without stream lock and
> > - snd_pcm_period_elapsed checks the nullity of the runtime outside of
> > stream lock.
> >
> > This will trigger null memory access in snd_pcm_running() call in
> > snd_pcm_period_elapsed.
>
> Well, if a stream is detached, it means that the stream must have been
> already closed; i.e. it's already a clear bug in the driver that
> snd_pcm_period_elapsed() is called against such a stream.
>
> Or am I missing other possible case?
>
>
> thanks,
>
> Takashi
>
In multithreaded environment, it is possible to have to access both
`interrupt_handler` (from irq) and `substream close` (from
snd_pcm_release) at the same time.
Therefore, in driver implementation, if "substream close function" and
the "code section where snd_pcm_period_elapsed() in" do not hold the
same lock, then the following things can happen:
1. interrupt_handler -> goes into snd_pcm_period_elapsed with a valid
sustream pointer
2. snd_pcm_release_substream: call close without blocking
3. snd_pcm_release_substream: call snd_pcm_detache_substream and set
substream->runtime to NULL
4. interrupt_handler -> call snd_pcm_runtime() and crash while
accessing fields in `substream->runtime`
e.g. In intel8x0.c driver for ac97 device,
In driver intel8x0.c, `snd_pcm_period_elapsed` is called after
checking `ichdev->substream` in `snd_intel8x0_update`.
And if a `snd_pcm_release` call from alsa-lib and pass through close()
and run to snd_pcm_detach_substream() in another thread, it's possible
to trigger a crash.
I can reproduce the issue within a multithread VM easily.
My patches are trying to provide a basic protection for this situation
(and internal pcm lock between detach and elapsed), since
- the usage of `snd_pcm_period_elapsed` does not warn callers about
the possible race if the driver does not force the order for `calling
snd_pcm_period_elapsed` and `close` by lock and
- lots of drivers already have this hidden issue and I can't fix them
one by one (You can check the "snd_pcm_period_elapsed usage" and the
"close implementation" within all the drivers). The most common
mistake is that
- Checking if the substream is null and call into snd_pcm_period_elapsed
- But `close` can happen anytime, pass without block and
snd_pcm_detach_substream will be trigger right after it
Best,
Paul
> >
> > paulhsia (2):
> > ALSA: pcm: Fix stream lock usage in snd_pcm_period_elapsed()
> > ALSA: pcm: Use stream lock in snd_pcm_detach_substream()
> >
> > sound/core/pcm.c | 8 +++++++-
> > sound/core/pcm_lib.c | 8 ++++++--
> > 2 files changed, 13 insertions(+), 3 deletions(-)
> >
> > --
> > 2.24.0.rc1.363.gb1bccd3e3d-goog
> >
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next prev parent reply other threads:[~2019-11-13 7:25 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-11-12 17:17 [alsa-devel] [PATCH 0/2] ALSA: pcm: Fix race condition in runtime access paulhsia
2019-11-12 17:17 ` [alsa-devel] [PATCH 1/2] ALSA: pcm: Fix stream lock usage in snd_pcm_period_elapsed() paulhsia
2019-11-12 17:17 ` [alsa-devel] [PATCH 2/2] ALSA: pcm: Use stream lock in snd_pcm_detach_substream() paulhsia
2019-11-12 18:16 ` [alsa-devel] [PATCH 0/2] ALSA: pcm: Fix race condition in runtime access Takashi Iwai
2019-11-13 7:24 ` Chih-Yang Hsia [this message]
2019-11-13 9:47 ` Takashi Iwai
2019-11-13 11:36 ` Takashi Iwai
2019-11-14 14:16 ` Chih-Yang Hsia
2019-11-14 14:20 ` Takashi Iwai
2019-11-14 16:37 ` Chih-Yang Hsia
2019-11-14 17:00 ` Takashi Iwai
2019-11-15 15:36 ` Chih-Yang Hsia
[not found] ` <CAJaf1Ta1tqYMCTaWxeL82gfY8Fg6hidLjHO3FFiqU7yyn5oVPg@mail.gmail.com>
[not found] ` <s5hy2whi1gw.wl-tiwai@suse.de>
2019-11-15 17:04 ` Chih-Yang Hsia
2019-11-15 17:07 ` Takashi Iwai
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