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@xxxxxxxxxxxx>
---
Changes in v3:
- Moved locking back into callers (Takashi)
- Combined update_widgets exit flow (Takashi)
Changes in v2:
- Introduced widget_mutex relocation
sound/hda/hdac_device.c | 18 ++++++++++++------
1 file changed, 12 insertions(+), 6 deletions(-)
diff --git a/sound/hda/hdac_device.c b/sound/hda/hdac_device.c
index 6907dbefd08c..3842f9d34b7c 100644
--- a/sound/hda/hdac_device.c
+++ b/sound/hda/hdac_device.c
@@ -400,27 +400,33 @@ 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);
--
2.20.1
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