[added to the 4.1 stable tree] ALSA: pcm: prevent UAF in snd_pcm_info

[Sasha Levin <Alexander.Levin@xxxxxxxxxxxxx>]

From: Robb Glasser <rglasser@xxxxxxxxxx>

This patch has been added to the  stable tree. If you have any
objections, please let us know.

===============

[ Upstream commit 362bca57f5d78220f8b5907b875961af9436e229 ]

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`.

Note: this fixes CVE-2017-0861

Signed-off-by: Robb Glasser <rglasser@xxxxxxxxxx>
Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
Signed-off-by: Sasha Levin <alexander.levin@xxxxxxxxxxxxx>
---
 sound/core/pcm.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/sound/core/pcm.c b/sound/core/pcm.c
index f6e7fdd354de..7c60cb289719 100644
--- a/sound/core/pcm.c
+++ b/sound/core/pcm.c
@@ -149,7 +149,9 @@ 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;
-- 
2.11.0