[PATCH 6.6 356/396] ALSA: sh: aica: reorder cleanup operations to avoid UAF bugs

Greg Kroah-Hartman
2024-04-01T16:54:09Z

From: Duoming Zhou <duoming@zju.edu.cn>

commit 051e0840ffa8ab25554d6b14b62c9ab9e4901457 upstream.

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@suse.de>
Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
Message-ID: <20240326094238.95442-1-duoming@zju.edu.cn>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
sound/sh/aica.c | 17 ++++++++++++++---
1 file changed, 14 insertions(+), 3 deletions(-)

--- a/sound/sh/aica.c
+++ b/sound/sh/aica.c
@@ -278,7 +278,8 @@ static void run_spu_dma(struct work_stru
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 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 +353,19 @@ static int snd_aicapcm_pcm_open(struct s
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);
dreamcastcard->substream = NULL;
kfree(dreamcastcard->channel);
spu_disable();
@@ -401,6 +411,7 @@ static const struct snd_pcm_ops snd_aica
.prepare = snd_aicapcm_pcm_prepare,
.trigger = snd_aicapcm_pcm_trigger,
.pointer = snd_aicapcm_pcm_pointer,
+ .sync_stop = snd_aicapcm_pcm_sync_stop,
};

/* TO DO: set up to handle more than one pcm instance */