[PATCH] ALSA: pcm: prevent UAF in snd_pcm_info

[Nick Desaulniers]

From: Robb Glasser <rglas...@google.com>

When the device descriptor is closed, the `substream->runtime` pointer
is freed. But another thread may be in the ioctl handler, case
SNDRV_CTL_IOCTL_PCM_INFO. This case calls snd_pcm_info_user() which
calls snd_pcm_info() which accesses the now freed `substream->runtime`.

Signed-off-by: Robb Glasser <rglas...@google.com>
Signed-off-by: Nick Desaulniers <ndesaulni...@google.com>
---
 sound/core/pcm.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/sound/core/pcm.c b/sound/core/pcm.c
index 9070f277f8db..09ee8c6b9f75 100644
--- a/sound/core/pcm.c
+++ b/sound/core/pcm.c
@@ -153,7 +153,9 @@ static int snd_pcm_control_ioctl(struct snd_card *card,
                                err = -ENXIO;
                                goto _error;
                        }
+                       mutex_lock(&pcm->open_mutex);
                        err = snd_pcm_info_user(substream, info);
+                       mutex_unlock(&pcm->open_mutex);
                _error:
                        mutex_unlock(&register_mutex);
                        return err;
-- 
2.15.0.531.g2ccb3012c9-goog