[CVE-2015-2922][Precise] ipv6: Don't reduce hop limit for an interface

Luis Henriques luis.henriques at canonical.com
Wed Apr 8 14:16:59 UTC 2015


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

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 at modio.se>
Acked-by: Hannes Frederic Sowa <hannes at stressinduktion.org>
Signed-off-by: David S. Miller <davem at davemloft.net>
(backported from commit 6fd99094de2b83d1d4c8457f2c83483b2828e75a)
[ luis: replaced ND_PRINTK macro by ND_PRINTK2 ]
BugLink: http://bugs.launchpad.net/bugs/1441103
CVE-2015-2922
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 net/ipv6/ndisc.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/net/ipv6/ndisc.c b/net/ipv6/ndisc.c
index 62096d81f7eb..f723096722db 100644
--- a/net/ipv6/ndisc.c
+++ b/net/ipv6/ndisc.c
@@ -1277,7 +1277,15 @@ 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)
 			dst_metric_set(&rt->dst, RTAX_HOPLIMIT,
 				       ra_msg->icmph.icmp6_hop_limit);




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