ipv6: Limit mtu to 65575 bytes

[ Upstream commit 30f78d8ebf7f514801e71b88a10c948275168518 ]

Francois reported that setting big mtu on loopback device could prevent
tcp sessions making progress.

We do not support (yet ?) IPv6 Jumbograms and cook corrupted packets.

We must limit the IPv6 MTU to (65535 + 40) bytes in theory.

Tested:

ifconfig lo mtu 70000
netperf -H ::1

Before patch : Throughput :   0.05 Mbits

After patch : Throughput : 35484 Mbits

Reported-by: Francois WELLENREITER <f.wellenreiter@gmail.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
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>

1 files changed, 3 insertions, 2 deletions

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 3fde3e977862..b2614b22622b 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -1236,7 +1236,7 @@ static unsigned int ip6_mtu(const struct dst_entry *dst)
         unsigned int mtu = dst_metric_raw(dst, RTAX_MTU);
 
         if (mtu)
-                return mtu;
+                goto out;
 
         mtu = IPV6_MIN_MTU;
 
@@ -1246,7 +1246,8 @@ static unsigned int ip6_mtu(const struct dst_entry *dst)
                 mtu = idev->cnf.mtu6;
         rcu_read_unlock();
 
-        return mtu;
+out:
+        return min_t(unsigned int, mtu, IP6_MAX_MTU);
 }
 
 static struct dst_entry *icmp6_dst_gc_list;