sch_dsmark: fix invalid skb_cow() usage

commit aea92fb2 upstream.

skb_cow(skb, sizeof(ip header)) is not very helpful in this context.

First we need to use pskb_may_pull() to make sure the ip header
is in skb linear part, then use skb_try_make_writable() to
address clones issues.

Fixes: 4c30719f ("[PKT_SCHED] dsmark: handle cloned and non-linear skb's")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

net/sched/sch_dsmark.c

@@ -197,9 +197,13 @@ static int dsmark_enqueue(struct sk_buff *skb, struct Qdisc *sch)
 	pr_debug("%s(skb %p,sch %p,[qdisc %p])\n", __func__, skb, sch, p);
 
 	if (p->set_tc_index) {
+		int wlen = skb_network_offset(skb);
+
 		switch (skb->protocol) {
 		case htons(ETH_P_IP):
-			if (skb_cow_head(skb, sizeof(struct iphdr)))
+			wlen += sizeof(struct iphdr);
+			if (!pskb_may_pull(skb, wlen) ||
+			    skb_try_make_writable(skb, wlen))
 				goto drop;
 
 			skb->tc_index = ipv4_get_dsfield(ip_hdr(skb))
@@ -207,7 +211,9 @@ static int dsmark_enqueue(struct sk_buff *skb, struct Qdisc *sch)
 			break;
 
 		case htons(ETH_P_IPV6):
-			if (skb_cow_head(skb, sizeof(struct ipv6hdr)))
+			wlen += sizeof(struct ipv6hdr);
+			if (!pskb_may_pull(skb, wlen) ||
+			    skb_try_make_writable(skb, wlen))
 				goto drop;
 
 			skb->tc_index = ipv6_get_dsfield(ipv6_hdr(skb))