[PATCH v2 1/2] ALSA: hda: Fix widget_mutex incomplete protection

[Evan Green]

The widget_mutex was introduced to serialize callers to
hda_widget_sysfs_{re}init. However, its protection of the sysfs widget array
is incomplete. For example, it is acquired around the call to
hda_widget_sysfs_reinit(), which actually creates the new array, but isn't
still acquired when codec->num_nodes and codec->start_nid is updated. So
the lock ensures one thread sets up the new array at a time, but doesn't
ensure which thread's value will end up in codec->num_nodes. If a larger
num_nodes wins but a smaller array was set up, the next call to
refresh_widgets() will touch free memory as it iterates over codec->num_nodes
that aren't there.

The widget_lock really protects both the tree as well as codec->num_nodes,
start_nid, and end_nid, so make sure it's held across that update. It should
also be held during snd_hdac_get_sub_nodes(), so that a very old read from that
function doesn't end up clobbering a later update.

While in there, move the exit mutex call inside the function. This moves the
mutex closer to the data structure it protects and removes a requirement of
acquiring the somewhat internal widget_lock before calling sysfs_exit.

Fixes: ed180abba7f1 ("ALSA: hda: Fix race between creating and refreshing sysfs entries")

Signed-off-by: Evan Green <evgreen@xxxxxxxxxxxx>

---

Changes in v2:
- Introduced widget_mutex relocation

 sound/hda/hdac_device.c | 19 +++++++++++++------
 sound/hda/hdac_sysfs.c  |  4 ++--
 2 files changed, 15 insertions(+), 8 deletions(-)

diff --git a/sound/hda/hdac_device.c b/sound/hda/hdac_device.c
index 6907dbefd08c..ff3420c5cdc8 100644
--- a/sound/hda/hdac_device.c
+++ b/sound/hda/hdac_device.c
@@ -162,9 +162,7 @@ EXPORT_SYMBOL_GPL(snd_hdac_device_register);
 void snd_hdac_device_unregister(struct hdac_device *codec)
 {
 	if (device_is_registered(&codec->dev)) {
-		mutex_lock(&codec->widget_lock);
 		hda_widget_sysfs_exit(codec);
-		mutex_unlock(&codec->widget_lock);
 		device_del(&codec->dev);
 		snd_hdac_bus_remove_device(codec->bus, codec);
 	}
@@ -402,25 +400,34 @@ int snd_hdac_refresh_widgets(struct hdac_device *codec, bool sysfs)
 	hda_nid_t start_nid;
 	int nums, err;
 
+	/*
+	 * Serialize against multiple threads trying to update the sysfs
+	 * widgets array.
+	 */
+	mutex_lock(&codec->widget_lock);
 	nums = snd_hdac_get_sub_nodes(codec, codec->afg, &start_nid);
 	if (!start_nid || nums <= 0 || nums >= 0xff) {
 		dev_err(&codec->dev, "cannot read sub nodes for FG 0x%02x\n",
 			codec->afg);
-		return -EINVAL;
+		err = -EINVAL;
+		goto unlock;
 	}
 
 	if (sysfs) {
-		mutex_lock(&codec->widget_lock);
 		err = hda_widget_sysfs_reinit(codec, start_nid, nums);
-		mutex_unlock(&codec->widget_lock);
 		if (err < 0)
-			return err;
+			goto unlock;
 	}
 
 	codec->num_nodes = nums;
 	codec->start_nid = start_nid;
 	codec->end_nid = start_nid + nums;
+	mutex_unlock(&codec->widget_lock);
 	return 0;
+
+unlock:
+	mutex_unlock(&codec->widget_lock);
+	return err;
 }
 EXPORT_SYMBOL_GPL(snd_hdac_refresh_widgets);
 
diff --git a/sound/hda/hdac_sysfs.c b/sound/hda/hdac_sysfs.c
index 909d5ef1179c..400ca262e2f8 100644
--- a/sound/hda/hdac_sysfs.c
+++ b/sound/hda/hdac_sysfs.c
@@ -412,13 +412,13 @@ int hda_widget_sysfs_init(struct hdac_device *codec)
 	return 0;
 }
 
-/* call with codec->widget_lock held */
 void hda_widget_sysfs_exit(struct hdac_device *codec)
 {
+	mutex_lock(&codec->widget_lock);
 	widget_tree_free(codec);
+	mutex_unlock(&codec->widget_lock);
 }
 
-/* call with codec->widget_lock held */
 int hda_widget_sysfs_reinit(struct hdac_device *codec,
 			    hda_nid_t start_nid, int num_nodes)
 {
-- 
2.20.1