af_iucv: fix recvmsg by replacing skb_pull() function

When receiving data messages, the "BUG_ON(skb->len < skb->data_len)" in
the skb_pull() function triggers a kernel panic.

Replace the skb_pull logic by a per skb offset as advised by
Eric Dumazet.

Signed-off-by: Ursula Braun <[e-mail]>
Signed-off-by: Frank Blaschka <[e-mail]>
Reviewed-by: Hendrik Brueckner <[e-mail]>
Acked-by: Eric Dumazet <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/iucv/af_iucv.h

···
 					       enum iucv_tx_notify n);
 };
 
+struct iucv_skb_cb {
+	u32	class;		/* target class of message */
+	u32	tag;		/* tag associated with message */
+	u32	offset;		/* offset for skb receival */
+};
+
+#define IUCV_SKB_CB(__skb)	((struct iucv_skb_cb *)&((__skb)->cb[0]))
+
 /* iucv socket options (SOL_IUCV) */
 #define SO_IPRMDATA_MSG	0x0080		/* send/recv IPRM_DATA msgs */
 #define SO_MSGLIMIT	0x1000		/* get/set IUCV MSGLIMIT */

net/iucv/af_iucv.c

···
 
 #define TRGCLS_SIZE	(sizeof(((struct iucv_message *)0)->class))
 
-/* macros to set/get socket control buffer at correct offset */
-#define CB_TAG(skb)	((skb)->cb)		/* iucv message tag */
-#define CB_TAG_LEN	(sizeof(((struct iucv_message *) 0)->tag))
-#define CB_TRGCLS(skb)	((skb)->cb + CB_TAG_LEN) /* iucv msg target class */
-#define CB_TRGCLS_LEN	(TRGCLS_SIZE)
-
 #define __iucv_sock_wait(sk, condition, timeo, ret)			\
 do {									\
 	DEFINE_WAIT(__wait);						\
···
 
 	/* increment and save iucv message tag for msg_completion cbk */
 	txmsg.tag = iucv->send_tag++;
-	memcpy(CB_TAG(skb), &txmsg.tag, CB_TAG_LEN);
+	IUCV_SKB_CB(skb)->tag = txmsg.tag;
 
 	if (iucv->transport == AF_IUCV_TRANS_HIPER) {
 		atomic_inc(&iucv->msg_sent);
···
 			return -ENOMEM;
 
 		/* copy target class to control buffer of new skb */
-		memcpy(CB_TRGCLS(nskb), CB_TRGCLS(skb), CB_TRGCLS_LEN);
+		IUCV_SKB_CB(nskb)->class = IUCV_SKB_CB(skb)->class;
 
 		/* copy data fragment */
 		memcpy(nskb->data, skb->data + copied, size);
···
 
 	/* store msg target class in the second 4 bytes of skb ctrl buffer */
 	/* Note: the first 4 bytes are reserved for msg tag */
-	memcpy(CB_TRGCLS(skb), &msg->class, CB_TRGCLS_LEN);
+	IUCV_SKB_CB(skb)->class = msg->class;
 
 	/* check for special IPRM messages (e.g. iucv_sock_shutdown) */
 	if ((msg->flags & IUCV_IPRMDATA) && len > 7) {
···
 		}
 	}
 
+	IUCV_SKB_CB(skb)->offset = 0;
 	if (sock_queue_rcv_skb(sk, skb))
 		skb_queue_head(&iucv_sk(sk)->backlog_skb_q, skb);
 }
···
 	unsigned int copied, rlen;
 	struct sk_buff *skb, *rskb, *cskb;
 	int err = 0;
+	u32 offset;
 
 	msg->msg_namelen = 0;
 
···
 		return err;
 	}
 
-	rlen   = skb->len;		/* real length of skb */
+	offset = IUCV_SKB_CB(skb)->offset;
+	rlen   = skb->len - offset;		/* real length of skb */
 	copied = min_t(unsigned int, rlen, len);
 	if (!rlen)
 		sk->sk_shutdown = sk->sk_shutdown | RCV_SHUTDOWN;
 
 	cskb = skb;
-	if (skb_copy_datagram_iovec(cskb, 0, msg->msg_iov, copied)) {
+	if (skb_copy_datagram_iovec(cskb, offset, msg->msg_iov, copied)) {
 		if (!(flags & MSG_PEEK))
 			skb_queue_head(&sk->sk_receive_queue, skb);
 		return -EFAULT;
···
 	 * get the trgcls from the control buffer of the skb due to
 	 * fragmentation of original iucv message. */
 	err = put_cmsg(msg, SOL_IUCV, SCM_IUCV_TRGCLS,
-			CB_TRGCLS_LEN, CB_TRGCLS(skb));
+		       sizeof(IUCV_SKB_CB(skb)->class),
+		       (void *)&IUCV_SKB_CB(skb)->class);
 	if (err) {
 		if (!(flags & MSG_PEEK))
 			skb_queue_head(&sk->sk_receive_queue, skb);
···
 
 		/* SOCK_STREAM: re-queue skb if it contains unreceived data */
 		if (sk->sk_type == SOCK_STREAM) {
-			skb_pull(skb, copied);
-			if (skb->len) {
-				skb_queue_head(&sk->sk_receive_queue, skb);
+			if (copied < rlen) {
+				IUCV_SKB_CB(skb)->offset = offset + copied;
 				goto done;
 			}
 		}
···
 		spin_lock_bh(&iucv->message_q.lock);
 		rskb = skb_dequeue(&iucv->backlog_skb_q);
 		while (rskb) {
+			IUCV_SKB_CB(rskb)->offset = 0;
 			if (sock_queue_rcv_skb(sk, rskb)) {
 				skb_queue_head(&iucv->backlog_skb_q,
 						rskb);
···
 		spin_lock_irqsave(&list->lock, flags);
 
 		while (list_skb != (struct sk_buff *)list) {
-			if (!memcmp(&msg->tag, CB_TAG(list_skb), CB_TAG_LEN)) {
+			if (msg->tag != IUCV_SKB_CB(list_skb)->tag) {
 				this = list_skb;
 				break;
 			}
···
 	skb_pull(skb, sizeof(struct af_iucv_trans_hdr));
 	skb_reset_transport_header(skb);
 	skb_reset_network_header(skb);
+	IUCV_SKB_CB(skb)->offset = 0;
 	spin_lock(&iucv->message_q.lock);
 	if (skb_queue_empty(&iucv->backlog_skb_q)) {
 		if (sock_queue_rcv_skb(sk, skb)) {
···
 		/* fall through and receive zero length data */
 	case 0:
 		/* plain data frame */
-		memcpy(CB_TRGCLS(skb), &trans_hdr->iucv_hdr.class,
-		       CB_TRGCLS_LEN);
+		IUCV_SKB_CB(skb)->class = trans_hdr->iucv_hdr.class;
 		err = afiucv_hs_callback_rx(sk, skb);
 		break;
 	default: