act_nat: not all of the ICMP packets need an IP header payload

not all of the ICMP packets need an IP header payload, so we check the length
of the skbs only when the packets should have an IP header payload.

Based upon analysis and initial patch by Rodrigo Partearroyo González.
Signed-off-by: Changli Gao <xiaosuo@gmail.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
----
 net/sched/act_nat.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/act_nat.c

@@ -205,7 +205,7 @@ static int tcf_nat(struct sk_buff *skb, struct tc_action *a,
 	{
 		struct icmphdr *icmph;
 
-		if (!pskb_may_pull(skb, ihl + sizeof(*icmph) + sizeof(*iph)))
+		if (!pskb_may_pull(skb, ihl + sizeof(*icmph)))
 			goto drop;
 
 		icmph = (void *)(skb_network_header(skb) + ihl);
@@ -215,6 +215,9 @@ static int tcf_nat(struct sk_buff *skb, struct tc_action *a,
 		    (icmph->type != ICMP_PARAMETERPROB))
 			break;
 
+		if (!pskb_may_pull(skb, ihl + sizeof(*icmph) + sizeof(*iph)))
+			goto drop;
+
 		iph = (void *)(icmph + 1);
 		if (egress)
 			addr = iph->daddr;