ipv6: Don't reduce hop limit for an interface

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>

1 files changed, 8 insertions, 1 deletions

diff --git a/net/ipv6/ndisc.c b/net/ipv6/ndisc.c
index 471ed24aabae..14ecdaf06bf7 100644
--- a/net/ipv6/ndisc.c
+++ b/net/ipv6/ndisc.c
@@ -1218,7 +1218,14 @@ static void ndisc_router_discovery(struct sk_buff *skb)
         if (rt)
                 rt6_set_expires(rt, 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_PRINTK(2, warn, "RA: Got route advertisement with lower hop_limit than current\n");
+                }
                 if (rt)
                         dst_metric_set(&rt->dst, RTAX_HOPLIMIT,
                                        ra_msg->icmph.icmp6_hop_limit);