Merge branch 'tunnels-fix-ipv4-pmtu-icmp-checksum'

Florian Westphal says:

====================
tunnels: fix ipv4 pmtu icmp checksum

The checksum of the generated ipv4 icmp pmtud message is
only correct if the skb that causes the icmp error generation
is linear.

Fix this and add a selftest for this.
====================

Link: https://lore.kernel.org/r/20230803152653.29535-1-fw@strlen.deSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/ip_tunnel_core.c

@@ -224,7 +224,7 @@ static int iptunnel_pmtud_build_icmp(struct sk_buff *skb, int mtu)
 		.un.frag.__unused	= 0,
 		.un.frag.mtu		= htons(mtu),
 	};
-	icmph->checksum = ip_compute_csum(icmph, len);
+	icmph->checksum = csum_fold(skb_checksum(skb, 0, len, 0));
 	skb_reset_transport_header(skb);
 
 	niph = skb_push(skb, sizeof(*niph));

tools/testing/selftests/net/pmtu.sh

@@ -361,6 +361,7 @@ err_buf=
 tcpdump_pids=
 nettest_pids=
 socat_pids=
+tmpoutfile=
 
 err() {
 	err_buf="${err_buf}${1}
@@ -951,6 +952,7 @@ cleanup() {
 	ip link del veth_A-R1			2>/dev/null
 	ovs-vsctl --if-exists del-port vxlan_a	2>/dev/null
 	ovs-vsctl --if-exists del-br ovs_br0	2>/dev/null
+	rm -f "$tmpoutfile"
 }
 
 mtu() {
@@ -1328,6 +1330,39 @@ test_pmtu_ipvX_over_bridged_vxlanY_or_geneveY_exception() {
 	check_pmtu_value ${exp_mtu} "${pmtu}" "exceeding link layer MTU on bridged ${type} interface"
 	pmtu="$(route_get_dst_pmtu_from_exception "${ns_a}" ${dst})"
 	check_pmtu_value ${exp_mtu} "${pmtu}" "exceeding link layer MTU on locally bridged ${type} interface"
+
+	tmpoutfile=$(mktemp)
+
+	# Flush Exceptions, retry with TCP
+	run_cmd ${ns_a} ip route flush cached ${dst}
+	run_cmd ${ns_b} ip route flush cached ${dst}
+	run_cmd ${ns_c} ip route flush cached ${dst}
+
+	for target in "${ns_a}" "${ns_c}" ; do
+		if [ ${family} -eq 4 ]; then
+			TCPDST=TCP:${dst}:50000
+		else
+			TCPDST="TCP:[${dst}]:50000"
+		fi
+		${ns_b} socat -T 3 -u -6 TCP-LISTEN:50000 STDOUT > $tmpoutfile &
+
+		sleep 1
+
+		dd if=/dev/zero of=/dev/stdout status=none bs=1M count=1 | ${target} socat -T 3 -u STDIN $TCPDST,connect-timeout=3
+
+		size=$(du -sb $tmpoutfile)
+		size=${size%%/tmp/*}
+
+		[ $size -ne 1048576 ] && err "File size $size mismatches exepcted value in locally bridged vxlan test" && return 1
+	done
+
+	rm -f "$tmpoutfile"
+
+	# Check that exceptions were created
+	pmtu="$(route_get_dst_pmtu_from_exception "${ns_c}" ${dst})"
+	check_pmtu_value ${exp_mtu} "${pmtu}" "tcp: exceeding link layer MTU on bridged ${type} interface"
+	pmtu="$(route_get_dst_pmtu_from_exception "${ns_a}" ${dst})"
+	check_pmtu_value ${exp_mtu} "${pmtu}" "tcp exceeding link layer MTU on locally bridged ${type} interface"
 }
 
 test_pmtu_ipv4_br_vxlan4_exception() {