[PATCH] ipv6: Don't reduce hop limit for an interface

[PATCH] ipv6: Don't reduce hop limit for an interface

[D. S. Ljungmark]

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From 3ae93eb68a06ab2d9c984c6708dbc9e5f3bc8251 Mon Sep 17 00:00:00 2001
From: "D.S. Ljungmark" <ljungmark@modio.se>
Date: Wed, 25 Mar 2015 09:28:15 +0100
Subject: [PATCH] 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>
---
 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 471ed24..14ecdaf 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);
-- 
2.1.0



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Re: [PATCH] ipv6: Don't reduce hop limit for an interface

[Hannes Frederic Sowa]

On Wed, Mar 25, 2015, at 09:29, D. S. Ljungmark wrote:
> From 3ae93eb68a06ab2d9c984c6708dbc9e5f3bc8251 Mon Sep 17 00:00:00 2001
> From: "D.S. Ljungmark" <ljungmark@modio.se>
> Date: Wed, 25 Mar 2015 09:28:15 +0100
> Subject: [PATCH] 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>
> ---
>  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 471ed24..14ecdaf 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");
> +               }

I think this is the simplest solution to this problem and solves the
problem by default, thanks!

Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>

Re: [PATCH] ipv6: Don't reduce hop limit for an interface

[David Miller]

From: "D. S. Ljungmark" <spider@takeit.se>
Date: Wed, 25 Mar 2015 09:29:01 +0100

> From 3ae93eb68a06ab2d9c984c6708dbc9e5f3bc8251 Mon Sep 17 00:00:00 2001
> From: "D.S. Ljungmark" <ljungmark@modio.se>
> Date: Wed, 25 Mar 2015 09:28:15 +0100
> Subject: [PATCH] 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>

Applied and queued up for -stable.