ipv6: Don't reduce hop limit for an interface

commit 6fd99094 upstream.

A local route may have a lower hop_limit set than global routes do.

RFC 3756, Section 4.2.7, "Parameter Spoofing"

>   1.  The attacker includes a Current Hop Limit of one or another small
>       number which the attacker knows will cause legitimate packets to
>       be dropped before they reach their destination.

>   As an example, one possible approach to mitigate this threat is to
>   ignore very small hop limits.  The nodes could implement a
>   configurable minimum hop limit, and ignore attempts to set it below
>   said limit.
Signed-off-by: D.S. Ljungmark <ljungmark@modio.se>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.2: adjust ND_PRINTK() usage]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
(cherry picked from commit f10f7d2a)
Signed-off-by: Willy Tarreau <w@1wt.eu>

net/ipv6/ndisc.c

@@ -1244,7 +1244,14 @@ static void ndisc_router_discovery(struct sk_buff *skb)
 		rt->rt6i_expires = jiffies + (HZ * lifetime);
 
 	if (ra_msg->icmph.icmp6_hop_limit) {
-		in6_dev->cnf.hop_limit = ra_msg->icmph.icmp6_hop_limit;
+		/* Only set hop_limit on the interface if it is higher than
+		 * the current hop_limit.
+		 */
+		if (in6_dev->cnf.hop_limit < ra_msg->icmph.icmp6_hop_limit) {
+			in6_dev->cnf.hop_limit = ra_msg->icmph.icmp6_hop_limit;
+		} else {
+			ND_PRINTK2(KERN_WARNING "RA: Got route advertisement with lower hop_limit than current\n");
+		}
 		if (rt)
 			rt->u.dst.metrics[RTAX_HOPLIMIT-1] = ra_msg->icmph.icmp6_hop_limit;
 	}