Subject: [PATCH v2] IPv4: Tunnel: Fix effective path mtu calculation

From: Oliver Herms <[email-masked]>
Date: Wed, 24 Jun 2020 20:44:12 +0200

The calculation of the effective tunnel mtu, that is used to create
mtu exceptions if necessary, is currently not done correctly. This
leads to unnecessary entires in the IPv6 route cache for any
packet send through the tunnel.

The root cause is, that "dev->hard_header_len" is subtracted from the
tunnel destionations path mtu. Thus subtracting too much, if
dev->hard_header_len is filled in. This is that case for SIT tunnels
where hard_header_len is the underlyings dev hard_header_len (e.g. 14
for ethernet) + 20 bytes IP header (see net/ipv6/sit.c:1091).

However, the MTU of the path is exclusive of the ethernet header
and the 20 bytes for the IP header are being subtracted separately
already. Thus hard_header_len is removed from this calculation.

For IPIP and GRE tunnels this doesn't change anything as hard_header_len
is zero in those cases anyways.

Fixes: c54419321455 ("GRE: Refactor GRE tunneling code.")
Signed-off-by: Oliver Herms <[email-masked]>
---
 net/ipv4/ip_tunnel.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/net/ipv4/ip_tunnel.c b/net/ipv4/ip_tunnel.c
index f4f1d11eab50..871d28bd29fa 100644
--- a/net/ipv4/ip_tunnel.c
+++ b/net/ipv4/ip_tunnel.c
@@ -495,8 +495,7 @@ static int tnl_update_pmtu(struct net_device *dev, struct sk_buff *skb,
 	pkt_size = skb->len - tunnel_hlen - dev->hard_header_len;
 
 	if (df)
-		mtu = dst_mtu(&rt->dst) - dev->hard_header_len
-					- sizeof(struct iphdr) - tunnel_hlen;
+		mtu = dst_mtu(&rt->dst) - sizeof(struct iphdr) - tunnel_hlen;
 	else
 		mtu = skb_valid_dst(skb) ? dst_mtu(skb_dst(skb)) : dev->mtu;
 
-- 
2.25.1