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

From: Takashi Iwai
Date: Mon Jul 01 2019 - 10:09:02 EST

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


> 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.


> 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;
return err;

Could you refresh this and resubmit?