ALSA: control: Fix race between adding and removing a user element

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>

sound/core/control.c

@@ -348,6 +348,40 @@ static int snd_ctl_find_hole(struct snd_card *card, unsigned int count)
 	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
@@ -364,45 +398,18 @@ static int snd_ctl_find_hole(struct snd_card *card, unsigned int count)
  */
 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:
@@ -1361,9 +1368,12 @@ static int snd_ctl_elem_add(struct snd_ctl_file *file,
 		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);
 	/*
@@ -1374,10 +1384,10 @@ static int snd_ctl_elem_add(struct snd_ctl_file *file,
 	 * 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;
 }