Re: [PATCH v2] ALSA: sh: aica: reorder cleanup operations to avoid UAF bugs

[Takashi Iwai]

On Tue, 26 Mar 2024 10:42:38 +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 snd_pcm_substream is closing, the aica_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 and other possible corner cases,
> call mod_timer() conditionally in run_spu_dma(), then implement
> PCM sync_stop op to cancel both the timer and worker. The sync_stop
> op will be called from PCM core appropriately when needed.
> 
> Fixes: 198de43d758c ("[ALSA] Add ALSA support for the SEGA Dreamcast PCM device")
> Suggested-by: Takashi Iwai <tiwai@xxxxxxx>
> Signed-off-by: Duoming Zhou <duoming@xxxxxxxxxx>
> ---
> Changes in v2:
>   - call mod_timer() conditionally and implement PCM sync_stop op.

Thanks, applied now.


Takashi