From: duoming@zju.edu.cn
To: "Takashi Iwai" <tiwai@suse.de>
Cc: linux-sound@vger.kernel.org, linux-kernel@vger.kernel.org,
tiwai@suse.com, perex@perex.cz
Subject: Re: [PATCH] ALSA: sh: aica: reorder cleanup operations to avoid UAF bug
Date: Mon, 25 Mar 2024 22:26:42 +0800 (GMT+08:00) [thread overview]
Message-ID: <43e102f3.61dc.18e7601a2f2.Coremail.duoming@zju.edu.cn> (raw)
In-Reply-To: <871q7yybeb.wl-tiwai@suse.de>
On Mon, 25 Mar 2024 09:16:12 +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@zju.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:
diff --git a/sound/sh/aica.c b/sound/sh/aica.c
index 320ac792c7fe..dab005eda7f0 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 (!dreamcastcard->shutdown)
+ mod_timer(&dreamcastcard->timer, jiffies + 1);
}
}
@@ -347,6 +348,7 @@ static int snd_aicapcm_pcm_open(struct snd_pcm_substream
dreamcastcard->clicks = 0;
dreamcastcard->current_period = 0;
dreamcastcard->dma_check = 0;
+ dreamcastcard->shutdown = false;
return 0;
}
@@ -354,8 +356,9 @@ static int snd_aicapcm_pcm_close(struct snd_pcm_substream
*substream)
{
struct snd_card_aica *dreamcastcard = substream->pcm->private_data;
+ dreamcastcard->shutdown = true;
+ del_timer_sync(&dreamcastcard->timer);
flush_work(&(dreamcastcard->spu_dma_work));
- del_timer(&dreamcastcard->timer);
dreamcastcard->substream = NULL;
kfree(dreamcastcard->channel);
spu_disable();
diff --git a/sound/sh/aica.h b/sound/sh/aica.h
index 021b132e088e..59a9deaf3dd3 100644
--- a/sound/sh/aica.h
+++ b/sound/sh/aica.h
@@ -65,4 +65,5 @@ struct snd_card_aica {
struct timer_list timer;
int master_volume;
int dma_check;
+ bool shutdown;
};
I would be very appreciate, if you could give me some suggestions
about the above solution.
Best regards,
Duoming Zhou
next prev parent reply other threads:[~2024-03-25 14:29 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-03-25 3:39 [PATCH] ALSA: sh: aica: reorder cleanup operations to avoid UAF bug Duoming Zhou
2024-03-25 8:16 ` Takashi Iwai
2024-03-25 14:26 ` duoming [this message]
2024-03-25 14:44 ` Takashi Iwai
2024-03-26 6:24 ` duoming
2024-03-26 7:08 ` Takashi Iwai
2024-03-26 8:06 ` duoming
2024-03-26 8:25 ` Takashi Iwai
2024-03-26 9:50 ` duoming
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