virtio/vsock: fix the transport to work with VMADDR_CID_ANY

The VMADDR_CID_ANY flag used by a socket means that the socket isn't bound
to any specific CID. For example, a host vsock server may want to be bound
with VMADDR_CID_ANY, so that a guest vsock client can connect to the host
server with CID=VMADDR_CID_HOST (i.e. 2), and meanwhile, a host vsock
client can connect to the same local server with CID=VMADDR_CID_LOCAL
(i.e. 1).

The current implementation sets the destination socket's svm_cid to a
fixed CID value after the first client's connection, which isn't an
expected operation. For example, if the guest client first connects to the
host server, the server's svm_cid gets set to VMADDR_CID_HOST, then other
host clients won't be able to connect to the server anymore.

Reproduce steps:
1. Run the host server:
   socat VSOCK-LISTEN:1234,fork -
2. Run a guest client to connect to the host server:
   socat - VSOCK-CONNECT:2:1234
3. Run a host client to connect to the host server:
   socat - VSOCK-CONNECT:1:1234

Without this patch, step 3. above fails to connect, and socat complains
"socat[1720] E connect(5, AF=40 cid:1 port:1234, 16): Connection
reset by peer".
With this patch, the above works well.

Fixes: c0cfa2d8 ("vsock: add multi-transports support")
Signed-off-by: Wei Wang <wei.w.wang@intel.com>
Link: https://lore.kernel.org/r/20211126011823.1760-1-wei.w.wang@intel.comSigned-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>

net/vmw_vsock/virtio_transport_common.c

@@ -1299,7 +1299,8 @@ void virtio_transport_recv_pkt(struct virtio_transport *t,
 	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;
+	if (vsk->local_addr.svm_cid != VMADDR_CID_ANY)
+		vsk->local_addr.svm_cid = dst.svm_cid;
 
 	if (space_available)
 		sk->sk_write_space(sk);