Commit ad608fbc authored by Sakari Ailus's avatar Sakari Ailus Committed by Mauro Carvalho Chehab
Browse files

media: v4l: event: Prevent freeing event subscriptions while accessed 



The event subscriptions are added to the subscribed event list while
holding a spinlock, but that lock is subsequently released while still
accessing the subscription object. This makes it possible to unsubscribe
the event --- and freeing the subscription object's memory --- while
the subscription object is simultaneously accessed.

Prevent this by adding a mutex to serialise the event subscription and
unsubscription. This also gives a guarantee to the callback ops that the
add op has returned before the del op is called.

This change also results in making the elems field less special:
subscriptions are only added to the event list once they are fully
initialised.

Signed-off-by: default avatarSakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: default avatarHans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: default avatarLaurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: stable@vger.kernel.org # for 4.14 and up
Fixes: c3b5b024 ("V4L/DVB: V4L: Events: Add backend")
Signed-off-by: default avatarMauro Carvalho Chehab <mchehab+samsung@kernel.org>
parent 324493fb

drivers/media/v4l2-core/v4l2-event.c

+20 −18
Original line number Diff line number Diff line
@@ -115,14 +115,6 @@ static void __v4l2_event_queue_fh(struct v4l2_fh *fh, const struct v4l2_event *e 
	if (sev == NULL)

		return;



	/*

	 * If the event has been added to the fh->subscribed list, but its

	 * add op has not completed yet elems will be 0, treat this as

	 * not being subscribed.

	 */

	if (!sev->elems)

		return;



	/* Increase event sequence number on fh. */

	fh->sequence++;
@@ -208,6 +200,7 @@ int v4l2_event_subscribe(struct v4l2_fh *fh, 
	struct v4l2_subscribed_event *sev, *found_ev;

	unsigned long flags;

	unsigned i;

	int ret = 0;



	if (sub->type == V4L2_EVENT_ALL)

		return -EINVAL;
@@ -225,31 +218,36 @@ int v4l2_event_subscribe(struct v4l2_fh *fh, 
	sev->flags = sub->flags;

	sev->fh = fh;

	sev->ops = ops;

	sev->elems = elems;



	mutex_lock(&fh->subscribe_lock);



	spin_lock_irqsave(&fh->vdev->fh_lock, flags);

	found_ev = v4l2_event_subscribed(fh, sub->type, sub->id);

	if (!found_ev)

		list_add(&sev->list, &fh->subscribed);

	spin_unlock_irqrestore(&fh->vdev->fh_lock, flags);



	if (found_ev) {

		/* Already listening */

		kvfree(sev);

		return 0; /* Already listening */

		goto out_unlock;

	}



	if (sev->ops && sev->ops->add) {

		int ret = sev->ops->add(sev, elems);

		ret = sev->ops->add(sev, elems);

		if (ret) {

			sev->ops = NULL;

			v4l2_event_unsubscribe(fh, sub);

			return ret;

			kvfree(sev);

			goto out_unlock;

		}

	}



	/* Mark as ready for use */

	sev->elems = elems;

	spin_lock_irqsave(&fh->vdev->fh_lock, flags);

	list_add(&sev->list, &fh->subscribed);

	spin_unlock_irqrestore(&fh->vdev->fh_lock, flags);



	return 0;

out_unlock:

	mutex_unlock(&fh->subscribe_lock);



	return ret;

}

EXPORT_SYMBOL_GPL(v4l2_event_subscribe);
@@ -288,6 +286,8 @@ int v4l2_event_unsubscribe(struct v4l2_fh *fh, 
		return 0;

	}



	mutex_lock(&fh->subscribe_lock);



	spin_lock_irqsave(&fh->vdev->fh_lock, flags);



	sev = v4l2_event_subscribed(fh, sub->type, sub->id);
@@ -305,6 +305,8 @@ int v4l2_event_unsubscribe(struct v4l2_fh *fh, 
	if (sev && sev->ops && sev->ops->del)

		sev->ops->del(sev);



	mutex_unlock(&fh->subscribe_lock);



	kvfree(sev);



	return 0;

drivers/media/v4l2-core/v4l2-fh.c

+2 −0
Original line number Diff line number Diff line
@@ -45,6 +45,7 @@ void v4l2_fh_init(struct v4l2_fh *fh, struct video_device *vdev) 
	INIT_LIST_HEAD(&fh->available);

	INIT_LIST_HEAD(&fh->subscribed);

	fh->sequence = -1;

	mutex_init(&fh->subscribe_lock);

}

EXPORT_SYMBOL_GPL(v4l2_fh_init);
@@ -90,6 +91,7 @@ void v4l2_fh_exit(struct v4l2_fh *fh) 
		return;

	v4l_disable_media_source(fh->vdev);

	v4l2_event_unsubscribe_all(fh);

	mutex_destroy(&fh->subscribe_lock);

	fh->vdev = NULL;

}

EXPORT_SYMBOL_GPL(v4l2_fh_exit);

include/media/v4l2-fh.h

+4 −0
Original line number Diff line number Diff line
@@ -38,10 +38,13 @@ struct v4l2_ctrl_handler; 
 * @prio: priority of the file handler, as defined by &enum v4l2_priority

 *

 * @wait: event' s wait queue

 * @subscribe_lock: serialise changes to the subscribed list; guarantee that

 *		    the add and del event callbacks are orderly called

 * @subscribed: list of subscribed events

 * @available: list of events waiting to be dequeued

 * @navailable: number of available events at @available list

 * @sequence: event sequence number

 *

 * @m2m_ctx: pointer to &struct v4l2_m2m_ctx

 */

struct v4l2_fh {
@@ -52,6 +55,7 @@ struct v4l2_fh { 


	/* Events */

	wait_queue_head_t	wait;

	struct mutex		subscribe_lock;

	struct list_head	subscribed;

	struct list_head	available;

	unsigned int		navailable;

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