[3.8.y.z extended stable] Patch "ipv6: Limit mtu to 65575 bytes" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    ipv6: Limit mtu to 65575 bytes

to the linux-3.8.y-queue branch of the 3.8.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.8.y-queue

This patch is scheduled to be released in version 3.8.13.24.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.8.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From 8ffa52dadeddd2e23c5e0a9ed230b3948c534f2e Mon Sep 17 00:00:00 2001
From: Eric Dumazet <edumazet at google.com>
Date: Thu, 10 Apr 2014 21:23:36 -0700
Subject: 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 at gmail.com>
Signed-off-by: Eric Dumazet <edumazet at google.com>
Acked-by: YOSHIFUJI Hideaki <yoshfuji at linux-ipv6.org>
Acked-by: Hannes Frederic Sowa <hannes at stressinduktion.org>
Signed-off-by: David S. Miller <davem at davemloft.net>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 include/net/ip6_route.h | 5 +++++
 net/ipv6/route.c        | 5 +++--
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/include/net/ip6_route.h b/include/net/ip6_route.h
index 27d8318..4fca183 100644
--- a/include/net/ip6_route.h
+++ b/include/net/ip6_route.h
@@ -31,6 +31,11 @@ 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 35cc19f..4c2cb88 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -1238,7 +1238,7 @@ 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;

@@ -1248,7 +1248,8 @@ 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;
--
1.9.1