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

On Fri, Jan 13, 2023 at 03:26:39PM +0100, Takashi Iwai wrote:
> 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 ]

Thanks for the backport, now queued up.

greg k-h

On 13. 01. 23 15:26, Takashi Iwai wrote:
> 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>

Reviewed-by: Jaroslav Kysela <perex@perex.cz>

-- 
Jaroslav Kysela <perex@perex.cz>
Linux Sound Maintainer; ALSA Project; Red Hat, Inc.