skbuff: don't corrupt mac_header on skb expansion

The skb mac_header field is sometimes NULL (or ~0u) as a sentinel
value. The places where skb is expanded add an offset which would
change this flag into an invalid pointer (or offset).
Signed-off-by: NStephen Hemminger <shemminger@vyatta.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/skbuff.c

@@ -661,7 +661,8 @@ static void copy_skb_header(struct sk_buff *new, const struct sk_buff *old)
 	/* {transport,network,mac}_header are relative to skb->head */
 	new->transport_header += offset;
 	new->network_header   += offset;
-	new->mac_header	      += offset;
+	if (skb_mac_header_was_set(new))
+		new->mac_header	      += offset;
 #endif
 	skb_shinfo(new)->gso_size = skb_shinfo(old)->gso_size;
 	skb_shinfo(new)->gso_segs = skb_shinfo(old)->gso_segs;
@@ -843,7 +844,8 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
 	skb->tail	      += off;
 	skb->transport_header += off;
 	skb->network_header   += off;
-	skb->mac_header	      += off;
+	if (skb_mac_header_was_set(skb))
+		skb->mac_header += off;
 	skb->csum_start       += nhead;
 	skb->cloned   = 0;
 	skb->hdr_len  = 0;
@@ -935,7 +937,8 @@ struct sk_buff *skb_copy_expand(const struct sk_buff *skb,
 #ifdef NET_SKBUFF_DATA_USES_OFFSET
 	n->transport_header += off;
 	n->network_header   += off;
-	n->mac_header	    += off;
+	if (skb_mac_header_was_set(skb))
+		n->mac_header += off;
 #endif
 
 	return n;