From: Xin Long <lucien.xin@gmail.com>
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
Git-commit: 8aec4959d832bae0889a8e2f348973b5e4abffef
Patch-mainline: v4.14-rc7
References: networking-stable-17_11_14

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.

Fixes: c12b395a4664 ("gre: Support GRE over IPv6")
Reported-by: Jianlin Shi <jishi@redhat.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
---
 net/ipv6/ip6_gre.c |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

--- a/net/ipv6/ip6_gre.c
+++ b/net/ipv6/ip6_gre.c
@@ -503,8 +503,8 @@ 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;
@@ -518,9 +518,14 @@ 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);
 }