vhost/net: fix heads usage of ubuf_info

ubuf info allocator uses guest controlled head as an index,
so a malicious guest could put the same head entry in the ring twice,
and we will get two callbacks on the same value.
To fix use upend_idx which is guaranteed to be unique.
Reported-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Cc: stable@kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/vhost/net.c

@@ -339,7 +339,8 @@ static void handle_tx(struct vhost_net *net)
 				msg.msg_controllen = 0;
 				ubufs = NULL;
 			} else {
-				struct ubuf_info *ubuf = &vq->ubuf_info[head];
+				struct ubuf_info *ubuf;
+				ubuf = vq->ubuf_info + vq->upend_idx;
 
 				vq->heads[vq->upend_idx].len =
 					VHOST_DMA_IN_PROGRESS;