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Message-ID: <875xx9wjv6.wl-tiwai@suse.de>
Date: Tue, 26 Mar 2024 08:08:29 +0100
From: Takashi Iwai <tiwai@...e.de>
To: duoming@....edu.cn
Cc: "Takashi Iwai" <tiwai@...e.de>,
linux-sound@...r.kernel.org,
linux-kernel@...r.kernel.org,
tiwai@...e.com,
perex@...ex.cz
Subject: Re: [PATCH] ALSA: sh: aica: reorder cleanup operations to avoid UAF bug
On Tue, 26 Mar 2024 07:24:14 +0100,
duoming@....edu.cn wrote:
>
> On Mon, 25 Mar 2024 15:44:26 +0100 Takashi Iwai wrote:
> > > > > The dreamcastcard->timer could schedule the spu_dma_work and the
> > > > > spu_dma_work could also arm the dreamcastcard->timer.
> > > > >
> > > > > When the Yamaha AICA card is closing, the dreamcastcard->channel
> > > > > will be deallocated. But it could still be dereferenced in the
> > > > > worker thread. The reason is that del_timer() will return directly
> > > > > regardless of whether the timer handler is running or not and the
> > > > > worker could be rescheduled in the timer handler. As a result, the
> > > > > UAF bug will happen. The racy situation is shown below:
> > > > >
> > > > > (Thread 1) | (Thread 2)
> > > > > snd_aicapcm_pcm_close() |
> > > > > ... | run_spu_dma() //worker
> > > > > | mod_timer()
> > > > > flush_work() |
> > > > > del_timer() | aica_period_elapsed() //timer
> > > > > kfree(dreamcastcard->channel) | schedule_work()
> > > > > | run_spu_dma() //worker
> > > > > ... | dreamcastcard->channel-> //USE
> > > > >
> > > > > In order to mitigate this bug, use timer_shutdown_sync() to shutdown
> > > > > the timer and then use flush_work() to cancel the worker.
> > > > >
> > > > > Fixes: 198de43d758c ("[ALSA] Add ALSA support for the SEGA Dreamcast PCM device")
> > > > > Signed-off-by: Duoming Zhou <duoming@....edu.cn>
> > > > > ---
> > > > > sound/sh/aica.c | 2 +-
> > > > > 1 file changed, 1 insertion(+), 1 deletion(-)
> > > > >
> > > > > diff --git a/sound/sh/aica.c b/sound/sh/aica.c
> > > > > index 320ac792c7f..bc68a3903f2 100644
> > > > > --- a/sound/sh/aica.c
> > > > > +++ b/sound/sh/aica.c
> > > > > @@ -354,8 +354,8 @@ static int snd_aicapcm_pcm_close(struct snd_pcm_substream
> > > > > *substream)
> > > > > {
> > > > > struct snd_card_aica *dreamcastcard = substream->pcm->private_data;
> > > > > + timer_shutdown_sync(&dreamcastcard->timer);
> > > >
> > > > I thought this call invalidates the timer object, hence it can't be
> > > > used again; i.e. it breaks when the stream is re-opened, I suppose?
> > > >
> > > > In general timer_shutdown*() is used for the code path to clean up the
> > > > driver (or the object the timer belongs to). The PCM close is only
> > > > about the PCM stream, and it's not the place.
> > > >
> > > > A proper fix would be rather to implement two things:
> > > > - Call mod_timer() conditionally in run_spu_dma()
> > > > - Implement PCM sync_stop op to cancel/flush the work
> > > >
> > > > The former alone should suffice to fix the UAF in your scenario,
> > > > though. The latter will cover other possible corner cases.
> > >
> > > Thank you for your time and reply! I know using timer_shutdown_sync()
> > > is not proper. In order to solve the problem, I add a shutdown flag
> > > in the struct snd_card_aica and set the flag to true when the PCM
> > > stream is closing. Then call mod_timer() conditionally in run_spu_dma().
> > > What's more, use del_timer_sync() to stop the timer and put it before
> > > flush_work(). As a result, both timer and worker could be stopped safely.
> > > The detail is shown below:
> >
> > You can use the existing API to check the PCM running state, e.g.
> >
> > --- a/sound/sh/aica.c
> > +++ b/sound/sh/aica.c
> > @@ -278,7 +278,8 @@ static void run_spu_dma(struct work_struct *work)
> > dreamcastcard->clicks++;
> > if (unlikely(dreamcastcard->clicks >= AICA_PERIOD_NUMBER))
> > dreamcastcard->clicks %= AICA_PERIOD_NUMBER;
> > - mod_timer(&dreamcastcard->timer, jiffies + 1);
> > + if (snd_pcm_running(dreamcastcard->substream))
> > + mod_timer(&dreamcastcard->timer, jiffies + 1);
> > }
> > }
>
> Thank you for your suggestions, The following is a new plan using the
> existing API to mitigate the bug.
>
> diff --git a/sound/sh/aica.c b/sound/sh/aica.c
> index 320ac792c7fe..bc003dd91a82 100644
> --- a/sound/sh/aica.c
> +++ b/sound/sh/aica.c
> @@ -278,7 +278,8 @@ static void run_spu_dma(struct work_struct *work)
> dreamcastcard->clicks++;
> if (unlikely(dreamcastcard->clicks >= AICA_PERIOD_NUMBER))
> dreamcastcard->clicks %= AICA_PERIOD_NUMBER;
> - mod_timer(&dreamcastcard->timer, jiffies + 1);
> + if (snd_pcm_running(dreamcastcard->substream))
> + mod_timer(&dreamcastcard->timer, jiffies + 1);
> }
> }
>
> @@ -316,6 +317,7 @@ static void spu_begin_dma(struct snd_pcm_substream *substream)
> struct snd_pcm_runtime *runtime;
> runtime = substream->runtime;
> dreamcastcard = substream->pcm->private_data;
> + __snd_pcm_set_state(runtime, SNDRV_PCM_STATE_RUNNING);
Such an explicit state change isn't needed, rather wrong.
The above condition check is performed only when kicked off from the
timer handler, and that's always after the stream started.
> @@ -354,8 +357,9 @@ static int snd_aicapcm_pcm_close(struct snd_pcm_substream
> *substream)
> {
> struct snd_card_aica *dreamcastcard = substream->pcm->private_data;
> + __snd_pcm_set_state(substream->runtime, SNDRV_PCM_STATE_DISCONNECTED);
This breaks things again! You don't disconnect the device at closing
the stream at all. And the state change is handled in PCM core side,
not in the driver side.
> + del_timer_sync(&dreamcastcard->timer);
> flush_work(&(dreamcastcard->spu_dma_work));
> - del_timer(&dreamcastcard->timer);
I'd leave this unchanged. The UAF itself is covered by the stream
state check. And, if any, we can change more properly:
- Add the same PCM state check at the beginning of
aica_period_elapsed(), and bail out immediately if not running
- Implement PCM sync_stop ops:
it should have like
static int snd_aicapcm_pcm_sync_stop(struct snd_pcm_substream *substream)
{
struct snd_card_aica *dreamcastcard = substream->pcm->private_data;
del_timer_sync(&dreamcastcard->timer);
cancel_work_sync(&dreamcastcard->spu_dma_work);
return 0;
}
and get rid of the corresponding calls from snd_aicapcm_pcm_close()
thanks,
Takashi
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