vhost: allow batching hint without size

commit 95932ab upstream.

Commit e2ae38c ("vhost: fix hung thread due to erroneous iotlb
entries") tries to reject the IOTLB message whose size is zero. But
the size is not necessarily meaningful, one example is the batching
hint, so the commit breaks that.

Fixing this be reject zero size message only if the message is used to
update/invalidate the IOTLB.

Fixes: e2ae38c ("vhost: fix hung thread due to erroneous iotlb entries")
Reported-by: Eli Cohen <elic@nvidia.com>
Cc: Anirudh Rayabharam <mail@anirudhrb.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20220310075211.4801-1-jasowang@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Eli Cohen <elic@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/vhost/vhost.c

@@ -1170,7 +1170,9 @@ ssize_t vhost_chr_write_iter(struct vhost_dev *dev,
 		goto done;
 	}
 
-	if (msg.size == 0) {
+	if ((msg.type == VHOST_IOTLB_UPDATE ||
+	     msg.type == VHOST_IOTLB_INVALIDATE) &&
+	     msg.size == 0) {
 		ret = -EINVAL;
 		goto done;
 	}