Re: [PATCH] ALSA: sh: aica: reorder cleanup operations to avoid UAF bug

[Takashi Iwai]

On Mon, 25 Mar 2024 04:39:46 +0100,
Duoming Zhou 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@xxxxxxxxxx>
> ---
>  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.


thanks,

Takashi


>  	flush_work(&(dreamcastcard->spu_dma_work));
> -	del_timer(&dreamcastcard->timer);
>  	dreamcastcard->substream = NULL;
>  	kfree(dreamcastcard->channel);
>  	spu_disable();
> -- 
> 2.17.1
>