[media] vb2: core: Skip planes array verification if pb is NULL

An earlier patch fixing an input validation issue introduced another
issue: vb2_core_dqbuf() is called with pb argument value NULL in some
cases, causing a NULL pointer dereference. Fix this by skipping the
verification as there's nothing to verify.

Fixes: e7e0c3e2 ("[media] videobuf2-core: Check user space planes array in dqbuf")
Signed-off-by: David R <david@unsolicited.net>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Hans Verkuil <hans.verkuil@cisco.com>
Cc: stable@vger.kernel.org # for v4.4 and later
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/media/v4l2-core/videobuf2-core.c

@@ -1648,7 +1648,7 @@ static int __vb2_get_done_vb(struct vb2_queue *q, struct vb2_buffer **vb,
 			     void *pb, int nonblocking)
 {
 	unsigned long flags;
-	int ret;
+	int ret = 0;
 
 	/*
 	 * Wait for at least one buffer to become available on the done_list.
@@ -1664,10 +1664,12 @@ static int __vb2_get_done_vb(struct vb2_queue *q, struct vb2_buffer **vb,
 	spin_lock_irqsave(&q->done_lock, flags);
 	*vb = list_first_entry(&q->done_list, struct vb2_buffer, done_entry);
 	/*
-	 * Only remove the buffer from done_list if v4l2_buffer can handle all
-	 * the planes.
+	 * Only remove the buffer from done_list if all planes can be
+	 * handled. Some cases such as V4L2 file I/O and DVB have pb
+	 * == NULL; skip the check then as there's nothing to verify.
 	 */
-	ret = call_bufop(q, verify_planes_array, *vb, pb);
+	if (pb)
+		ret = call_bufop(q, verify_planes_array, *vb, pb);
 	if (!ret)
 		list_del(&(*vb)->done_entry);
 	spin_unlock_irqrestore(&q->done_lock, flags);