[PATCH 4.9 30/50] ALSA: control: Fix race between adding and removing a user element

Greg Kroah-Hartman
2018-12-04T11:08:39Z

From: Takashi Iwai <tiwai@suse.de>

commit e1a7bfe3807974e66f971f2589d4e0197ec0fced upstream.

The procedure for adding a user control element has some window opened
for race against the concurrent removal of a user element. This was
caught by syzkaller, hitting a KASAN use-after-free error.

This patch addresses the bug by wrapping the whole procedure to add a
user control element with the card->controls_rwsem, instead of only
around the increment of card->user_ctl_count.

This required a slight code refactoring, too. The function
snd_ctl_add() is split to two parts: a core function to add the
control element and a part calling it. The former is called from the
function for adding a user control element inside the controls_rwsem.

One change to be noted is that snd_ctl_notify() for adding a control
element gets called inside the controls_rwsem as well while it was
called outside the rwsem. But this should be OK, as snd_ctl_notify()
takes another (finer) rwlock instead of rwsem, and the call of
snd_ctl_notify() inside rwsem is already done in another code path.

Reported-by: syzbot+dc09047bce3820621ba2@syzkaller.appspotmail.com
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
sound/core/control.c | 80 ++++++++++++++++++++++++++++-----------------------
1 file changed, 45 insertions(+), 35 deletions(-)

--- a/sound/core/control.c
+++ b/sound/core/control.c
@@ -346,6 +346,40 @@ static int snd_ctl_find_hole(struct snd_
return 0;
}

+/* add a new kcontrol object; call with card->controls_rwsem locked */
+static int __snd_ctl_add(struct snd_card *card, struct snd_kcontrol *kcontrol)
+{
+ struct snd_ctl_elem_id id;
+ unsigned int idx;
+ unsigned int count;
+
+ id = kcontrol->id;
+ if (id.index > UINT_MAX - kcontrol->count)
+ return -EINVAL;
+
+ if (snd_ctl_find_id(card, &id)) {
+ dev_err(card->dev,
+ "control %i:%i:%i:%s:%i is already present\n",
+ id.iface, id.device, id.subdevice, id.name, id.index);
+ return -EBUSY;
+ }
+
+ if (snd_ctl_find_hole(card, kcontrol->count) < 0)
+ return -ENOMEM;
+
+ list_add_tail(&kcontrol->list, &card->controls);
+ card->controls_count += kcontrol->count;
+ kcontrol->id.numid = card->last_numid + 1;
+ card->last_numid += kcontrol->count;
+
+ id = kcontrol->id;
+ count = kcontrol->count;
+ for (idx = 0; idx < count; idx++, id.index++, id.numid++)
+ snd_ctl_notify(card, SNDRV_CTL_EVENT_MASK_ADD, &id);
+
+ return 0;
+}
+
/**
* snd_ctl_add - add the control instance to the card
* @card: the card instance
@@ -362,45 +396,18 @@ static int snd_ctl_find_hole(struct snd_
*/
int snd_ctl_add(struct snd_card *card, struct snd_kcontrol *kcontrol)
{
- struct snd_ctl_elem_id id;
- unsigned int idx;
- unsigned int count;
int err = -EINVAL;

if (! kcontrol)
return err;
if (snd_BUG_ON(!card || !kcontrol->info))
goto error;
- id = kcontrol->id;
- if (id.index > UINT_MAX - kcontrol->count)
- goto error;

down_write(&card->controls_rwsem);
- if (snd_ctl_find_id(card, &id)) {
- up_write(&card->controls_rwsem);
- dev_err(card->dev, "control %i:%i:%i:%s:%i is already present\n",
- id.iface,
- id.device,
- id.subdevice,
- id.name,
- id.index);
- err = -EBUSY;
- goto error;
- }
- if (snd_ctl_find_hole(card, kcontrol->count) < 0) {
- up_write(&card->controls_rwsem);
- err = -ENOMEM;
- goto error;
- }
- list_add_tail(&kcontrol->list, &card->controls);
- card->controls_count += kcontrol->count;
- kcontrol->id.numid = card->last_numid + 1;
- card->last_numid += kcontrol->count;
- id = kcontrol->id;
- count = kcontrol->count;
+ err = __snd_ctl_add(card, kcontrol);
up_write(&card->controls_rwsem);
- for (idx = 0; idx < count; idx++, id.index++, id.numid++)
- snd_ctl_notify(card, SNDRV_CTL_EVENT_MASK_ADD, &id);
+ if (err < 0)
+ goto error;
return 0;

error:
@@ -1354,9 +1361,12 @@ static int snd_ctl_elem_add(struct snd_c
kctl->tlv.c = snd_ctl_elem_user_tlv;

/* This function manage to free the instance on failure. */
- err = snd_ctl_add(card, kctl);
- if (err < 0)
- return err;
+ down_write(&card->controls_rwsem);
+ err = __snd_ctl_add(card, kctl);
+ if (err < 0) {
+ snd_ctl_free_one(kctl);
+ goto unlock;
+ }
offset = snd_ctl_get_ioff(kctl, &info->id);
snd_ctl_build_ioff(&info->id, kctl, offset);
/*
@@ -1367,10 +1377,10 @@ static int snd_ctl_elem_add(struct snd_c
* which locks the element.
*/

- down_write(&card->controls_rwsem);
card->user_ctl_count++;
- up_write(&card->controls_rwsem);

+ unlock:
+ up_write(&card->controls_rwsem);
return 0;
}