Patch "ip6_gre: update dst pmtu if dev mtu has been updated by toobig in __gre6_xmit" has been added to the 4.9-stable tree

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

ip6_gre: update dst pmtu if dev mtu has been updated by toobig in __gre6_xmit

to the 4.9-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_gre-update-dst-pmtu-if-dev-mtu-has-been-updated-by-toobig-in-__gre6_xmit.patch
and it can be found in the queue-4.9 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: Thu, 26 Oct 2017 19:27:17 +0800
Subject: ip6_gre: update dst pmtu if dev mtu has been updated by toobig in __gre6_xmit

From: Xin Long <[email protected]>

[ Upstream commit 8aec4959d832bae0889a8e2f348973b5e4abffef ]

When receiving a Toobig icmpv6 packet, ip6gre_err would just set tunnel dev's mtu, that's not enough. For skb_dst(skb)'s pmtu may still be using the old value, it has no chance to be updated with tunnel dev's mtu.

Jianlin found this issue by reducing route's mtu while running netperf, the performance went to 0.

ip6ip6 and ip4ip6 tunnel can work well with this, as they lookup the upper dst and update_pmtu it's pmtu or icmpv6_send a Toobig to upper socket after setting tunnel dev's mtu.

We couldn't do that for ip6_gre, as gre's inner packet could be any protocol, it's difficult to handle them (like lookup upper dst) in a good way.

So this patch is to fix it by updating skb_dst(skb)'s pmtu when dev->mtu < skb_dst(skb)'s pmtu in tx path. It's safe to do this update there, as usually dev->mtu <= skb_dst(skb)'s pmtu and no performance regression can be caused by this.

---
 net/ipv6/ip6_gre.c |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

@@ -508,8 +508,8 @@ static netdev_tx_t __gre6_xmit(struct sk
 			       __u32 *pmtu, __be16 proto)
 {
 	struct ip6_tnl *tunnel = netdev_priv(dev);
-	__be16 protocol = (dev->type == ARPHRD_ETHER) ?
-			  htons(ETH_P_TEB) : proto;
+	struct dst_entry *dst = skb_dst(skb);
+	__be16 protocol;
 
 	if (dev->type == ARPHRD_ETHER)
 		IPCB(skb)->flags = 0;

@@ -523,9 +523,14 @@ static netdev_tx_t __gre6_xmit(struct sk
 		tunnel->o_seqno++;
 
 	/* Push GRE header. */
+	protocol = (dev->type == ARPHRD_ETHER) ? htons(ETH_P_TEB) : proto;
 	gre_build_header(skb, tunnel->tun_hlen, tunnel->parms.o_flags,
 			 protocol, tunnel->parms.o_key, htonl(tunnel->o_seqno));
 
+	/* TooBig packet may have updated dst->dev's mtu */
+	if (dst && dst_mtu(dst) > dst->dev->mtu)
+		dst->ops->update_pmtu(dst, NULL, skb, dst->dev->mtu);
+
 	return ip6_tnl_xmit(skb, dev, dsfield, fl6, encap_limit, pmtu,
 			    NEXTHDR_GRE);
 }

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

queue-4.9/sctp-reset-owner-sk-for-data-chunks-on-out-queues-when-migrating-a-sock.patch queue-4.9/ipip-only-increase-err_count-for-some-certain-type-icmp-in-ipip_err.patch queue-4.9/sctp-full-support-for-ipv6-ip_nonlocal_bind-ip_freebind.patch queue-4.9/sctp-add-the-missing-sock_owned_by_user-check-in-sctp_icmp_redirect.patch queue-4.9/ip6_gre-only-increase-err_count-for-some-certain-type-icmpv6-in-ip6gre_err.patch queue-4.9/ip6_gre-update-dst-pmtu-if-dev-mtu-has-been-updated-by-toobig-in-__gre6_xmit.patch