ALSA: hda: Fix widget_mutex incomplete protection

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.

Fixes: ed180abba7f1 ("ALSA: hda: Fix race between creating and refreshing sysfs entries")
Signed-off-by: Evan Green <evgreen@chromium.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

diff --git a/sound/hda/hdac_device.c b/sound/hda/hdac_device.c
index 4769f4c03e148a567288a61a92d833c8e983bef9..11050bfd8068d26d3bec198f9d63160779cc2696 100644 (file)
--- a/sound/hda/hdac_device.c
+++ b/sound/hda/hdac_device.c
@@ -399,27 +399,33 @@ static void setup_fg_nodes(struct hdac_device *codec)
 int snd_hdac_refresh_widgets(struct hdac_device *codec, bool sysfs)
 {
        hda_nid_t start_nid;
-       int nums, err;
+       int nums, err = 0;
 
+       /*
+        * 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;
-       return 0;
+unlock:
+       mutex_unlock(&codec->widget_lock);
+       return err;
 }
 EXPORT_SYMBOL_GPL(snd_hdac_refresh_widgets);