[PATCH 3.19 04/27] ipv6: Dont reduce hop limit for an interface

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

3.19-stable review patch.  If anyone has any objections, please let me know.

------------------

From: "D.S. Ljungmark" <ljungmark@modio.se>

[ Upstream commit 6fd99094de2b83d1d4c8457f2c83483b2828e75a ]

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 net/ipv6/ndisc.c |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

--- a/net/ipv6/ndisc.c
+++ b/net/ipv6/ndisc.c
@@ -1216,7 +1216,14 @@ static void ndisc_router_discovery(struc
 	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);