ALSA: mixer: oss: Fix racy access to slots

commit 411cef6adfb38a5bb6bd9af3941b28198e7fb680 upstream.

The OSS mixer can reassign the mapping slots dynamically via proc
file.  Although the addition and deletion of those slots are protected
by mixer->reg_mutex, the access to slots aren't, hence this may cause
UAF when the slots in use are deleted concurrently.

This patch applies the mixer->reg_mutex in all appropriate code paths
(i.e. the ioctl functions) that may access slots.

Reported-by: syzbot+9988f17cf72a1045a189@syzkaller.appspotmail.com
Reviewed-by: Jaroslav Kysela <perex@perex.cz>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/00000000000036adc005ceca9175@google.com
Link: https://lore.kernel.org/r/20211020164846.922-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 33 insertions, 10 deletions

diff --git a/sound/core/oss/mixer_oss.c b/sound/core/oss/mixer_oss.c
index 379bf486ccc7..a22b6acc816d 100644
--- a/sound/core/oss/mixer_oss.c
+++ b/sound/core/oss/mixer_oss.c
@@ -145,11 +145,13 @@ static int snd_mixer_oss_devmask(struct snd_mixer_oss_file *fmixer)
 
 	if (mixer == NULL)
 		return -EIO;
+	mutex_lock(&mixer->reg_mutex);
 	for (chn = 0; chn < 31; chn++) {
 		pslot = &mixer->slots[chn];
 		if (pslot->put_volume || pslot->put_recsrc)
 			result |= 1 << chn;
 	}
+	mutex_unlock(&mixer->reg_mutex);
 	return result;
 }
 
@@ -161,11 +163,13 @@ static int snd_mixer_oss_stereodevs(struct snd_mixer_oss_file *fmixer)
 
 	if (mixer == NULL)
 		return -EIO;
+	mutex_lock(&mixer->reg_mutex);
 	for (chn = 0; chn < 31; chn++) {
 		pslot = &mixer->slots[chn];
 		if (pslot->put_volume && pslot->stereo)
 			result |= 1 << chn;
 	}
+	mutex_unlock(&mixer->reg_mutex);
 	return result;
 }
 
@@ -176,6 +180,7 @@ static int snd_mixer_oss_recmask(struct snd_mixer_oss_file *fmixer)
 
 	if (mixer == NULL)
 		return -EIO;
+	mutex_lock(&mixer->reg_mutex);
 	if (mixer->put_recsrc && mixer->get_recsrc) {	/* exclusive */
 		result = mixer->mask_recsrc;
 	} else {
@@ -187,6 +192,7 @@ static int snd_mixer_oss_recmask(struct snd_mixer_oss_file *fmixer)
 				result |= 1 << chn;
 		}
 	}
+	mutex_unlock(&mixer->reg_mutex);
 	return result;
 }
 
@@ -197,11 +203,12 @@ static int snd_mixer_oss_get_recsrc(struct snd_mixer_oss_file *fmixer)
 
 	if (mixer == NULL)
 		return -EIO;
+	mutex_lock(&mixer->reg_mutex);
 	if (mixer->put_recsrc && mixer->get_recsrc) {	/* exclusive */
-		int err;
 		unsigned int index;
-		if ((err = mixer->get_recsrc(fmixer, &index)) < 0)
-			return err;
+		result = mixer->get_recsrc(fmixer, &index);
+		if (result < 0)
+			goto unlock;
 		result = 1 << index;
 	} else {
 		struct snd_mixer_oss_slot *pslot;
@@ -216,7 +223,10 @@ static int snd_mixer_oss_get_recsrc(struct snd_mixer_oss_file *fmixer)
 			}
 		}
 	}
-	return mixer->oss_recsrc = result;
+	mixer->oss_recsrc = result;
+ unlock:
+	mutex_unlock(&mixer->reg_mutex);
+	return result;
 }
 
 static int snd_mixer_oss_set_recsrc(struct snd_mixer_oss_file *fmixer, int recsrc)
@@ -229,6 +239,7 @@ static int snd_mixer_oss_set_recsrc(struct snd_mixer_oss_file *fmixer, int recsr
 
 	if (mixer == NULL)
 		return -EIO;
+	mutex_lock(&mixer->reg_mutex);
 	if (mixer->get_recsrc && mixer->put_recsrc) {	/* exclusive input */
 		if (recsrc & ~mixer->oss_recsrc)
 			recsrc &= ~mixer->oss_recsrc;
@@ -254,6 +265,7 @@ static int snd_mixer_oss_set_recsrc(struct snd_mixer_oss_file *fmixer, int recsr
 			}
 		}
 	}
+	mutex_unlock(&mixer->reg_mutex);
 	return result;
 }
 
@@ -265,6 +277,7 @@ static int snd_mixer_oss_get_volume(struct snd_mixer_oss_file *fmixer, int slot)
 
 	if (mixer == NULL || slot > 30)
 		return -EIO;
+	mutex_lock(&mixer->reg_mutex);
 	pslot = &mixer->slots[slot];
 	left = pslot->volume[0];
 	right = pslot->volume[1];
@@ -272,15 +285,21 @@ static int snd_mixer_oss_get_volume(struct snd_mixer_oss_file *fmixer, int slot)
 		result = pslot->get_volume(fmixer, pslot, &left, &right);
 	if (!pslot->stereo)
 		right = left;
-	if (snd_BUG_ON(left < 0 || left > 100))
-		return -EIO;
-	if (snd_BUG_ON(right < 0 || right > 100))
-		return -EIO;
+	if (snd_BUG_ON(left < 0 || left > 100)) {
+		result = -EIO;
+		goto unlock;
+	}
+	if (snd_BUG_ON(right < 0 || right > 100)) {
+		result = -EIO;
+		goto unlock;
+	}
 	if (result >= 0) {
 		pslot->volume[0] = left;
 		pslot->volume[1] = right;
 	 	result = (left & 0xff) | ((right & 0xff) << 8);
 	}
+ unlock:
+	mutex_unlock(&mixer->reg_mutex);
 	return result;
 }
 
@@ -293,6 +312,7 @@ static int snd_mixer_oss_set_volume(struct snd_mixer_oss_file *fmixer,
 
 	if (mixer == NULL || slot > 30)
 		return -EIO;
+	mutex_lock(&mixer->reg_mutex);
 	pslot = &mixer->slots[slot];
 	if (left > 100)
 		left = 100;
@@ -303,10 +323,13 @@ static int snd_mixer_oss_set_volume(struct snd_mixer_oss_file *fmixer,
 	if (pslot->put_volume)
 		result = pslot->put_volume(fmixer, pslot, left, right);
 	if (result < 0)
-		return result;
+		goto unlock;
 	pslot->volume[0] = left;
 	pslot->volume[1] = right;
- 	return (left & 0xff) | ((right & 0xff) << 8);
+	result = (left & 0xff) | ((right & 0xff) << 8);
+ unlock:
+	mutex_lock(&mixer->reg_mutex);
+	return result;
 }
 
 static int snd_mixer_oss_ioctl1(struct snd_mixer_oss_file *fmixer, unsigned int cmd, unsigned long arg)