On Wed, 26 Jun 2019 23:22:19 +0200, Evan Green wrote:
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.
OK, right, this fix is needed no matter whether to take my other change to skip hda_widget_sysfs_init() call in hda_widget_sysfs_reinit().
However...
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.
... this doesn't look better from consistency POV. The whole code in hdac_sysfs.c doesn't take any lock in itself. The protection is supposed to be done in the caller side. So, let's keep as is now.
Also...
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;
There is no need of two mutex_unlock() here. They can be unified,
codec->num_nodes = nums; codec->start_nid = start_nid; codec->end_nid = start_nid + nums; unlock: mutex_unlock(&codec->widget_lock); return err;
Could you refresh this and resubmit?
thanks,
Takashi