net: flow_dissector: fail on evil iph->ihl

commit 6f092343 upstream.

We don't validate iph->ihl which may lead a dead loop if we meet a IPIP
skb whose iph->ihl is zero. Fix this by failing immediately when iph->ihl
is evil (less than 5).

This issue were introduced by commit ec5efe79
(rps: support IPIP encapsulation).

Cc: Eric Dumazet <edumazet@google.com>
Cc: Petr Matousek <pmatouse@redhat.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.2: the affected code is in __skb_get_rxhash()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

net/core/dev.c

@@ -2609,6 +2609,8 @@ void __skb_get_rxhash(struct sk_buff *skb)
 			goto done;
 
 		ip = (const struct iphdr *) (skb->data + nhoff);
+		if (ip->ihl < 5)
+			goto done;
 		if (ip_is_fragment(ip))
 			ip_proto = 0;
 		else