vsock/virtio: update credit only if socket is not closed

If the socket is closed or is being released, some resources used by
virtio_transport_space_update() such as 'vsk->trans' may be released.

To avoid a use after free bug we should only update the available credit
when we are sure the socket is still open and we have the lock held.

Fixes: 06a8fc78 ("VSOCK: Introduce virtio_vsock_common.ko")
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Link: https://lore.kernel.org/r/20210208144454.84438-1-sgarzare@redhat.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/vmw_vsock/virtio_transport_common.c

@@ -1130,8 +1130,6 @@ void virtio_transport_recv_pkt(struct virtio_transport *t,
 
 	vsk = vsock_sk(sk);
 
-	space_available = virtio_transport_space_update(sk, pkt);
-
 	lock_sock(sk);
 
 	/* Check if sk has been closed before lock_sock */
@@ -1142,6 +1140,8 @@ void virtio_transport_recv_pkt(struct virtio_transport *t,
 		goto free_pkt;
 	}
 
+	space_available = virtio_transport_space_update(sk, pkt);
+
 	/* Update CID in case it has changed after a transport reset event */
 	vsk->local_addr.svm_cid = dst.svm_cid;