vhost/vsock: fix vhost virtio_vsock_pkt use-after-free

Stash the packet length in a local variable before handing over
ownership of the packet to virtio_transport_recv_pkt() or
virtio_transport_free_pkt().

This patch solves the use-after-free since pkt is no longer guaranteed
to be alive.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/vhost/vsock.c

@@ -307,6 +307,8 @@ static void vhost_vsock_handle_tx_kick(struct vhost_work *work)
 
 	vhost_disable_notify(&vsock->dev, vq);
 	for (;;) {
+		u32 len;
+
 		if (!vhost_vsock_more_replies(vsock)) {
 			/* Stop tx until the device processes already
 			 * pending replies.  Leave tx virtqueue
@@ -334,13 +336,15 @@ static void vhost_vsock_handle_tx_kick(struct vhost_work *work)
 			continue;
 		}
 
+		len = pkt->len;
+
 		/* Only accept correctly addressed packets */
 		if (le64_to_cpu(pkt->hdr.src_cid) == vsock->guest_cid)
 			virtio_transport_recv_pkt(pkt);
 		else
 			virtio_transport_free_pkt(pkt);
 
-		vhost_add_used(vq, head, sizeof(pkt->hdr) + pkt->len);
+		vhost_add_used(vq, head, sizeof(pkt->hdr) + len);
 		added = true;
 	}