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: Jiri Slaby <jslaby@suse.cz>

diff --git a/include/net/ip6_route.h b/include/net/ip6_route.h
index 2b786b7e35850fe4b4b147a8a0cdcf0b010d2cb9..a210b33fc8e2f8749d124c798e2018a34ee941db 100644 (file)
--- a/include/net/ip6_route.h
+++ b/include/net/ip6_route.h
@@ -32,6 +32,11 @@ struct route_info {
 #define RT6_LOOKUP_F_SRCPREF_PUBLIC    0x00000010
 #define RT6_LOOKUP_F_SRCPREF_COA       0x00000020
 
+/* We do not (yet ?) support IPv6 jumbograms (RFC 2675)
+ * Unlike IPv4, hdr->seg_len doesn't include the IPv6 header
+ */
+#define IP6_MAX_MTU (0xFFFF + sizeof(struct ipv6hdr))
+
 /*
  * rt6_srcprefs2flags() and rt6_flags2srcprefs() translate
  * between IPV6_ADDR_PREFERENCES socket option values

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 77f81beabbd34840b66d0806419ee59e63eb6aba..b4bb6a29aa16669f6e54cefea4220ffa420de95e 100644 (file)
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -1329,7 +1329,7 @@ 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;
 
@@ -1339,7 +1339,8 @@ 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;