[PATCH 5.10.y] ALSA: pcm: Properly take rwsem lock in ctl_elem_read_user/ctl_elem_write_user to prevent UAF

[Takashi Iwai <tiwai@suse.de>]

From: Clement Lecigne <clecigne@google.com>

[ Note: this is a fix that works around the bug equivalently as the
  two upstream commits:
   1fa4445f9adf ("ALSA: control - introduce snd_ctl_notify_one() helper")
   56b88b50565c ("ALSA: pcm: Move rwsem lock inside snd_ctl_elem_read to prevent UAF")
  but in a simpler way to fit with older stable trees -- tiwai ]

Add missing locking in ctl_elem_read_user/ctl_elem_write_user which can be
easily triggered and turned into an use-after-free.

Example code paths with SNDRV_CTL_IOCTL_ELEM_READ:

64-bits:
snd_ctl_ioctl
  snd_ctl_elem_read_user
    [takes controls_rwsem]
    snd_ctl_elem_read [lock properly held, all good]
    [drops controls_rwsem]

32-bits (compat):
snd_ctl_ioctl_compat
  snd_ctl_elem_write_read_compat
    ctl_elem_write_read
      snd_ctl_elem_read [missing lock, not good]

CVE-2023-0266 was assigned for this issue.

Signed-off-by: Clement Lecigne <clecigne@google.com>
Cc: stable@kernel.org # 5.12 and older
Signed-off-by: Takashi Iwai <tiwai@suse.de>
---

Greg, this is a patch for the last ALSA PCM UCM fix for the older
stable trees.  Please take this to 5.10.y and older stable trees.
Thanks!

 sound/core/control_compat.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/sound/core/control_compat.c b/sound/core/control_compat.c
index 97467f6a32a1..980ab3580f1b 100644
--- a/sound/core/control_compat.c
+++ b/sound/core/control_compat.c
@@ -304,7 +304,9 @@ static int ctl_elem_read_user(struct snd_card *card,
 	err = snd_power_wait(card, SNDRV_CTL_POWER_D0);
 	if (err < 0)
 		goto error;
+	down_read(&card->controls_rwsem);
 	err = snd_ctl_elem_read(card, data);
+	up_read(&card->controls_rwsem);
 	if (err < 0)
 		goto error;
 	err = copy_ctl_value_to_user(userdata, valuep, data, type, count);
@@ -332,7 +334,9 @@ static int ctl_elem_write_user(struct snd_ctl_file *file,
 	err = snd_power_wait(card, SNDRV_CTL_POWER_D0);
 	if (err < 0)
 		goto error;
+	down_write(&card->controls_rwsem);
 	err = snd_ctl_elem_write(card, file, data);
+	up_write(&card->controls_rwsem);
 	if (err < 0)
 		goto error;
 	err = copy_ctl_value_to_user(userdata, valuep, data, type, count);
-- 
2.35.3