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Message-ID: <864a35d.75f2.18e7a2b487c.Coremail.duoming@zju.edu.cn>
Date: Tue, 26 Mar 2024 17:50:40 +0800 (GMT+08:00)
From: duoming@....edu.cn
To: "Takashi Iwai" <tiwai@...e.de>
Cc: 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 09:25:26 +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:
> >
> > I think if we leave the cleanup operations unchanged, the UAF could
> > still happen. The scenario is shown below:
> >
> > (Thread 1) | (Thread 2)
> > | run_spu_dma() //worker
> > | if (snd_pcm_running(dreamcastcard->substream))
> > snd_aicapcm_pcm_close() |
> > ... |
> > | mod_timer()
> > flush_work() |
> > del_timer() | aica_period_elapsed() //timer
> > kfree(dreamcastcard->channel) | schedule_work()
> > | run_spu_dma() //worker
> > ... | dreamcastcard->channel-> //USE
> >
> > So we should implement PCM sync_stop ops like below.
> >
> > > - 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()
> >
> > Thank you for your suggestions, the improved plan is shown below:
> >
> > diff --git a/sound/sh/aica.c b/sound/sh/aica.c
> > index 320ac792c7f..2989407606f 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);
> > }
> > }
> >
> > @@ -290,6 +291,8 @@ static void aica_period_elapsed(struct timer_list *t)
> > /*timer function - so cannot sleep */
> > int play_period;
> > struct snd_pcm_runtime *runtime;
> > + if (!snd_pcm_running(substream))
> > + return;
> > runtime = substream->runtime;
> > dreamcastcard = substream->pcm->private_data;
> > /* Have we played out an additional period? */
> > @@ -350,12 +353,20 @@ static int snd_aicapcm_pcm_open(struct snd_pcm_substream
> > return 0;
> > }
> >
> > +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;
> > +}
> > +
> > static int snd_aicapcm_pcm_close(struct snd_pcm_substream
> > *substream)
> > {
> > struct snd_card_aica *dreamcastcard = substream->pcm->private_data;
> > - flush_work(&(dreamcastcard->spu_dma_work));
> > - del_timer(&dreamcastcard->timer);
> > + snd_aicapcm_pcm_sync_stop(substream);
> > dreamcastcard->substream = NULL;
> > kfree(dreamcastcard->channel);
> > spu_disable();
>
> Don't call it here explicitly. sync_stop is a PCM op, i.e. add
> snd_aicapcm_pcm_sync_stop to snd_aicapcm_playback_ops definition,
> instead.
> Then it'll be called from PCM core appropriately when needed
> (e.g. before calling the close callback via hw_free call).
Thank you for your suggestions! I have sent the v2 that adds
snd_aicapcm_pcm_sync_stop to snd_aicapcm_playback_ops definition.
https://patchwork.kernel.org/project/alsa-devel/patch/20240326094238.95442-1-duoming@zju.edu.cn/
Best regards,
Duoming Zhou
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