af_iucv: exploit target message class support of IUCV

The first 4 bytes of data to be sent are stored additionally into
the message class field of the send request. A receiving target
program (not an af_iucv socket program) can make use of this
information to pre-screen incoming messages.
Signed-off-by: NUrsula Braun <braunu@de.ibm.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/iucv/af_iucv.c

@@ -644,6 +644,7 @@ static int iucv_sock_sendmsg(struct kiocb *iocb, struct socket *sock,
 		}
 
 		txmsg.class = 0;
+		memcpy(&txmsg.class, skb->data, skb->len >= 4 ? 4 : skb->len);
 		txmsg.tag = iucv->send_tag++;
 		memcpy(skb->cb, &txmsg.tag, 4);
 		skb_queue_tail(&iucv->send_skb_q, skb);