From: Xin Long <lucien.xin@gmail.com>
Date: Sun, 5 Aug 2018 22:46:07 +0800
Subject: ip6_tunnel: use the right value for ipv4 min mtu check in ip6_tnl_xmit
Patch-mainline: v4.18
Git-commit: 82a40777de12728dedf4075453b694f0d1baee80
References: bsc#1123456

According to RFC791, 68 bytes is the minimum size of IPv4 datagram every
device must be able to forward without further fragmentation while 576
bytes is the minimum size of IPv4 datagram every device has to be able
to receive, so in ip6_tnl_xmit(), 68(IPV4_MIN_MTU) should be the right
value for the ipv4 min mtu check in ip6_tnl_xmit.

While at it, change to use max() instead of if statement.

Fixes: c9fefa08190f ("ip6_tunnel: get the min mtu properly in ip6_tnl_xmit")
Reported-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Michal Kubecek <mkubecek@suse.cz>

---
 net/ipv6/ip6_tunnel.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

--- a/net/ipv6/ip6_tunnel.c
+++ b/net/ipv6/ip6_tunnel.c
@@ -1135,12 +1135,8 @@ int ip6_tnl_xmit(struct sk_buff *skb, struct net_device *dev, __u8 dsfield,
 		max_headroom += 8;
 		mtu -= 8;
 	}
-	if (skb->protocol == htons(ETH_P_IPV6)) {
-		if (mtu < IPV6_MIN_MTU)
-			mtu = IPV6_MIN_MTU;
-	} else if (mtu < 576) {
-		mtu = 576;
-	}
+	mtu = max(mtu, skb->protocol == htons(ETH_P_IPV6) ?
+		       IPV6_MIN_MTU : IPV4_MIN_MTU);
 
 	skb_dst_update_pmtu(skb, mtu);
 	if (skb->len - t->tun_hlen - eth_hlen > mtu && !skb_is_gso(skb)) {