From: Georg Kohmann <geokohma@cisco.com>
Subject: net: ipv6: Discard next-hop MTU less than minimum link MTU
Git-commit: 4a65dff81a04f874fa6915c7f069b4dc2c4010e4
Patch-mainline: v5.10
References: bsc#1191241
Acked-by: Jiri Bohac <jbohac@suse.cz>

When a ICMPV6_PKT_TOOBIG report a next-hop MTU that is less than the IPv6
minimum link MTU, the estimated path MTU is reduced to the minimum link
MTU. This behaviour breaks TAHI IPv6 Core Conformance Test v6LC4.1.6:
Packet Too Big Less than IPv6 MTU.

Referring to RFC 8201 section 4: "If a node receives a Packet Too Big
message reporting a next-hop MTU that is less than the IPv6 minimum link
MTU, it must discard it. A node must not reduce its estimate of the Path
MTU below the IPv6 minimum link MTU on receipt of a Packet Too Big
message."

Drop the path MTU update if reported MTU is less than the minimum link MTU.

Signed-off-by: Georg Kohmann <geokohma@cisco.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index fb075d9545b9..27430d6e1a99 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -1408,7 +1408,8 @@ static void __ip6_rt_update_pmtu(struct
 		saddr = NULL;
 	}
 	dst_confirm_neigh(dst, daddr);
-	mtu = max_t(u32, mtu, IPV6_MIN_MTU);
+	if (mtu < IPV6_MIN_MTU)
+		return;
 	if (mtu >= dst_mtu(dst))
 		return;
 
-- 
2.33.0