Patch "ip6_tunnel: get the min mtu properly in ip6_tnl_xmit" has been added to the 4.14-stable tree

This is a note to let you know that I've just added the patch titled

ip6_tunnel: get the min mtu properly in ip6_tnl_xmit

to the 4.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
ip6_tunnel-get-the-min-mtu-properly-in-ip6_tnl_xmit.patch
and it can be found in the queue-4.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Xin Long <[email protected]>
Date: Mon, 18 Dec 2017 14:26:21 +0800
Subject: ip6_tunnel: get the min mtu properly in ip6_tnl_xmit

From: Xin Long <[email protected]>

[ Upstream commit c9fefa08190fc879fb2e681035d7774e0a8c5170 ]

Now it's using IPV6_MIN_MTU as the min mtu in ip6_tnl_xmit, but IPV6_MIN_MTU actually only works when the inner packet is ipv6.

With IPV6_MIN_MTU for ipv4 packets, the new pmtu for inner dst couldn't be set less than 1280. It would cause tx_err and the packet to be dropped when the outer dst pmtu is close to 1280.

Jianlin found it by running ipv4 traffic with the topo:

(client) gre6 <---> eth1 (route) eth2 <---> gre6 (server)

After changing eth2 mtu to 1300, the performance became very low, or the connection was even broken. The issue also affects ip4ip6 and ip6ip6 tunnels.

So if the inner packet is ipv4, 576 should be considered as the min mtu.

Note that for ip4ip6 and ip6ip6 tunnels, the inner packet can only be ipv4 or ipv6, but for gre6 tunnel, it may also be ARP. This patch using 576 as the min mtu for non-ipv6 packet works for all those cases.

---
 net/ipv6/ip6_tunnel.c |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

@@ -1131,8 +1131,13 @@ route_lookup:
 		max_headroom += 8;
 		mtu -= 8;
 	}
-	if (mtu < IPV6_MIN_MTU)
-		mtu = IPV6_MIN_MTU;
+	if (skb->protocol == htons(ETH_P_IPV6)) {
+		if (mtu < IPV6_MIN_MTU)
+			mtu = IPV6_MIN_MTU;
+	} else if (mtu < 576) {
+		mtu = 576;
+	}
+
 	if (skb_dst(skb) && !t->parms.collect_md)
 		skb_dst(skb)->ops->update_pmtu(skb_dst(skb), NULL, skb, mtu);
 	if (skb->len - t->tun_hlen - eth_hlen > mtu && !skb_is_gso(skb)) {

Patches currently in stable-queue which might be from [email protected] are

queue-4.14/ip_gre-remove-the-incorrect-mtu-limit-for-ipgre-tap.patch queue-4.14/ip6_gre-remove-the-incorrect-mtu-limit-for-ipgre-tap.patch queue-4.14/ip6_tunnel-allow-ip6gre-dev-mtu-to-be-set-below-1280.patch queue-4.14/vxlan-update-skb-dst-pmtu-on-tx-path.patch queue-4.14/ip6_tunnel-get-the-min-mtu-properly-in-ip6_tnl_xmit.patch queue-4.14/sctp-fix-the-issue-that-a-__u16-variable-may-overflow-in-sctp_ulpq_renege.patch