linux-kernel - [PATCH 3.16.y-ckt 105/144] ipv6: Don't reduce hop limit for an interface

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Message-Id: <1429630329-21748-106-git-send-email-luis.henriques@canonical.com>
Date:	Tue, 21 Apr 2015 16:31:30 +0100
From:	Luis Henriques <luis.henriques@...onical.com>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	kernel-team@...ts.ubuntu.com
Cc:	"D.S. Ljungmark" <ljungmark@...io.se>,
	"David S. Miller" <davem@...emloft.net>,
	Luis Henriques <luis.henriques@...onical.com>
Subject: [PATCH 3.16.y-ckt 105/144] ipv6: Don't reduce hop limit for an interface

3.16.7-ckt10 -stable review patch.  If anyone has any objections, please let me know.

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

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

commit 6fd99094de2b83d1d4c8457f2c83483b2828e75a 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@...io.se>
Acked-by: Hannes Frederic Sowa <hannes@...essinduktion.org>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Luis Henriques <luis.henriques@...onical.com>
---
 net/ipv6/ndisc.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/net/ipv6/ndisc.c b/net/ipv6/ndisc.c
index ca8d4ea48a5d..e2eb53c719dc 100644
--- a/net/ipv6/ndisc.c
+++ b/net/ipv6/ndisc.c
@@ -1190,7 +1190,14 @@ 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);
--
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