ipv6: stop sending PTB packets for MTU < 1280

Reduce the attack vector and stop generating IPv6 Fragment Header for
paths with an MTU smaller than the minimum required IPv6 MTU
size (1280 byte) - called atomic fragments.

See IETF I-D "Deprecating the Generation of IPv6 Atomic Fragments" [1]
for more information and how this "feature" can be misused.

[1] https://tools.ietf.org/html/draft-ietf-6man-deprecate-atomfrag-generation-00

Signed-off-by: Fernando Gont <fgont@si6networks.com>
Signed-off-by: Hagen Paul Pfeifer <hagen@jauu.net>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 2 insertions, 5 deletions

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index c91083156edb..166e33bed222 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -1160,12 +1160,9 @@ static void ip6_rt_update_pmtu(struct dst_entry *dst, struct sock *sk,
                 struct net *net = dev_net(dst->dev);
 
                 rt6->rt6i_flags |= RTF_MODIFIED;
-                if (mtu < IPV6_MIN_MTU) {
-                        u32 features = dst_metric(dst, RTAX_FEATURES);
+                if (mtu < IPV6_MIN_MTU)
                         mtu = IPV6_MIN_MTU;
-                        features |= RTAX_FEATURE_ALLFRAG;
-                        dst_metric_set(dst, RTAX_FEATURES, features);
-                }
+
                 dst_metric_set(dst, RTAX_MTU, mtu);
                 rt6_update_expires(rt6, net->ipv6.sysctl.ip6_rt_mtu_expires);
         }