Merge branch 'net-fix-netfilter-defrag-ip-tunnel-pmtu-blackhole'

Florian Westphal says:

====================
net: fix netfilter defrag/ip tunnel pmtu blackhole

Christian Perle reported a PMTU blackhole due to unexpected interaction
between the ip defragmentation that comes with connection tracking and
ip tunnels.

Unfortunately setting 'nopmtudisc' on the tunnel breaks the test
scenario even without netfilter.

Christinas setup looks like this:
     +--------+       +---------+       +--------+
     |Router A|-------|Wanrouter|-------|Router B|
     |        |.IPIP..|         |..IPIP.|        |
     +--------+       +---------+       +--------+
          /             mtu 1400           \
         /                                  \
 +--------+                                  +--------+
 |Client A|                                  |Client B|
 +--------+                                  +--------+

MTU is 1500 everywhere, except on Router A to Wanrouter and
Wanrouter to Router B.

Router A and Router B use IPIP tunnel interfaces to tunnel traffic
between Client A and Client B over WAN.

Client A sends a 1400 byte UDP datagram to Client B.
This packet gets encapsulated in the IPIP tunnel.

This works, packet is received on client B.

When conntrack (or anything else that forces ip defragmentation) is
enabled on Router A, the packet gets dropped on Router A after
encapsulation because they exceed the link MTU.

Setting the 'nopmtudisc' flag on the IPIP tunnel makes things worse,
no packets pass even in the no-netfilter scenario.

Patch one is a reproducer script for selftest infra.

Patch two is a fix for 'nopmtudisc' behaviour so ip_tunnel will send
an icmp error to Client A.  This allows 'nopmtudisc' tunnel to forward
the UDP datagrams.

Patch three enables ip refragmentation for all reassembled packets, just
like ipv6.
====================

Link: https://lore.kernel.org/r/20210105231523.622-1-fw@strlen.deSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/ip_output.c

@@ -302,7 +302,7 @@ static int __ip_finish_output(struct net *net, struct sock *sk, struct sk_buff *
 	if (skb_is_gso(skb))
 		return ip_finish_output_gso(net, sk, skb, mtu);
 
-	if (skb->len > mtu || (IPCB(skb)->flags & IPSKB_FRAG_PMTU))
+	if (skb->len > mtu || IPCB(skb)->frag_max_size)
 		return ip_fragment(net, sk, skb, mtu, ip_finish_output2);
 
 	return ip_finish_output2(net, sk, skb);

net/ipv4/ip_tunnel.c

@@ -759,8 +759,11 @@ void ip_tunnel_xmit(struct sk_buff *skb, struct net_device *dev,
 		goto tx_error;
 	}
 
-	if (tnl_update_pmtu(dev, skb, rt, tnl_params->frag_off, inner_iph,
-			    0, 0, false)) {
+	df = tnl_params->frag_off;
+	if (skb->protocol == htons(ETH_P_IP) && !tunnel->ignore_df)
+		df |= (inner_iph->frag_off & htons(IP_DF));
+
+	if (tnl_update_pmtu(dev, skb, rt, df, inner_iph, 0, 0, false)) {
 		ip_rt_put(rt);
 		goto tx_error;
 	}
@@ -788,10 +791,6 @@ void ip_tunnel_xmit(struct sk_buff *skb, struct net_device *dev,
 			ttl = ip4_dst_hoplimit(&rt->dst);
 	}
 
-	df = tnl_params->frag_off;
-	if (skb->protocol == htons(ETH_P_IP) && !tunnel->ignore_df)
-		df |= (inner_iph->frag_off&htons(IP_DF));
-
 	max_headroom = LL_RESERVED_SPACE(rt->dst.dev) + sizeof(struct iphdr)
 			+ rt->dst.header_len + ip_encap_hlen(&tunnel->encap);
 	if (max_headroom > dev->needed_headroom)

tools/testing/selftests/netfilter/Makefile

@@ -4,7 +4,8 @@
 TEST_PROGS := nft_trans_stress.sh nft_nat.sh bridge_brouter.sh \
 	conntrack_icmp_related.sh nft_flowtable.sh ipvs.sh \
 	nft_concat_range.sh nft_conntrack_helper.sh \
-	nft_queue.sh nft_meta.sh
+	nft_queue.sh nft_meta.sh \
+	ipip-conntrack-mtu.sh
 
 LDLIBS = -lmnl
 TEST_GEN_FILES =  nf-queue

tools/testing/selftests/netfilter/ipip-conntrack-mtu.sh

+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+
+# Kselftest framework requirement - SKIP code is 4.
+ksft_skip=4
+
+# Conntrack needs to reassemble fragments in order to have complete
+# packets for rule matching.  Reassembly can lead to packet loss.
+
+# Consider the following setup:
+#            +--------+       +---------+       +--------+
+#            |Router A|-------|Wanrouter|-------|Router B|
+#            |        |.IPIP..|         |..IPIP.|        |
+#            +--------+       +---------+       +--------+
+#           /                  mtu 1400                   \
+#          /                                               \
+#+--------+                                                 +--------+
+#|Client A|                                                 |Client B|
+#|        |                                                 |        |
+#+--------+                                                 +--------+
+
+# Router A and Router B use IPIP tunnel interfaces to tunnel traffic
+# between Client A and Client B over WAN. Wanrouter has MTU 1400 set
+# on its interfaces.
+
+rnd=$(mktemp -u XXXXXXXX)
+rx=$(mktemp)
+
+r_a="ns-ra-$rnd"
+r_b="ns-rb-$rnd"
+r_w="ns-rw-$rnd"
+c_a="ns-ca-$rnd"
+c_b="ns-cb-$rnd"
+
+checktool (){
+	if ! $1 > /dev/null 2>&1; then
+		echo "SKIP: Could not $2"
+		exit $ksft_skip
+	fi
+}
+
+checktool "iptables --version" "run test without iptables"
+checktool "ip -Version" "run test without ip tool"
+checktool "which nc" "run test without nc (netcat)"
+checktool "ip netns add ${r_a}" "create net namespace"
+
+for n in ${r_b} ${r_w} ${c_a} ${c_b};do
+	ip netns add ${n}
+done
+
+cleanup() {
+	for n in ${r_a} ${r_b} ${r_w} ${c_a} ${c_b};do
+		ip netns del ${n}
+	done
+	rm -f ${rx}
+}
+
+trap cleanup EXIT
+
+test_path() {
+	msg="$1"
+
+	ip netns exec ${c_b} nc -n -w 3 -q 3 -u -l -p 5000 > ${rx} < /dev/null &
+
+	sleep 1
+	for i in 1 2 3; do
+		head -c1400 /dev/zero | tr "\000" "a" | ip netns exec ${c_a} nc -n -w 1 -u 192.168.20.2 5000
+	done
+
+	wait
+
+	bytes=$(wc -c < ${rx})
+
+	if [ $bytes -eq 1400 ];then
+		echo "OK: PMTU $msg connection tracking"
+	else
+		echo "FAIL: PMTU $msg connection tracking: got $bytes, expected 1400"
+		exit 1
+	fi
+}
+
+# Detailed setup for Router A
+# ---------------------------
+# Interfaces:
+# eth0: 10.2.2.1/24
+# eth1: 192.168.10.1/24
+# ipip0: No IP address, local 10.2.2.1 remote 10.4.4.1
+# Routes:
+# 192.168.20.0/24 dev ipip0    (192.168.20.0/24 is subnet of Client B)
+# 10.4.4.1 via 10.2.2.254      (Router B via Wanrouter)
+# No iptables rules at all.
+
+ip link add veth0 netns ${r_a} type veth peer name veth0 netns ${r_w}
+ip link add veth1 netns ${r_a} type veth peer name veth0 netns ${c_a}
+
+l_addr="10.2.2.1"
+r_addr="10.4.4.1"
+ip netns exec ${r_a} ip link add ipip0 type ipip local ${l_addr} remote ${r_addr} mode ipip || exit $ksft_skip
+
+for dev in lo veth0 veth1 ipip0; do
+    ip -net ${r_a} link set $dev up
+done
+
+ip -net ${r_a} addr add 10.2.2.1/24 dev veth0
+ip -net ${r_a} addr add 192.168.10.1/24 dev veth1
+
+ip -net ${r_a} route add 192.168.20.0/24 dev ipip0
+ip -net ${r_a} route add 10.4.4.0/24 via 10.2.2.254
+
+ip netns exec ${r_a} sysctl -q net.ipv4.conf.all.forwarding=1 > /dev/null
+
+# Detailed setup for Router B
+# ---------------------------
+# Interfaces:
+# eth0: 10.4.4.1/24
+# eth1: 192.168.20.1/24
+# ipip0: No IP address, local 10.4.4.1 remote 10.2.2.1
+# Routes:
+# 192.168.10.0/24 dev ipip0    (192.168.10.0/24 is subnet of Client A)
+# 10.2.2.1 via 10.4.4.254      (Router A via Wanrouter)
+# No iptables rules at all.
+
+ip link add veth0 netns ${r_b} type veth peer name veth1 netns ${r_w}
+ip link add veth1 netns ${r_b} type veth peer name veth0 netns ${c_b}
+
+l_addr="10.4.4.1"
+r_addr="10.2.2.1"
+
+ip netns exec ${r_b} ip link add ipip0 type ipip local ${l_addr} remote ${r_addr} mode ipip || exit $ksft_skip
+
+for dev in lo veth0 veth1 ipip0; do
+	ip -net ${r_b} link set $dev up
+done
+
+ip -net ${r_b} addr add 10.4.4.1/24 dev veth0
+ip -net ${r_b} addr add 192.168.20.1/24 dev veth1
+
+ip -net ${r_b} route add 192.168.10.0/24 dev ipip0
+ip -net ${r_b} route add 10.2.2.0/24 via 10.4.4.254
+ip netns exec ${r_b} sysctl -q net.ipv4.conf.all.forwarding=1 > /dev/null
+
+# Client A
+ip -net ${c_a} addr add 192.168.10.2/24 dev veth0
+ip -net ${c_a} link set dev lo up
+ip -net ${c_a} link set dev veth0 up
+ip -net ${c_a} route add default via 192.168.10.1
+
+# Client A
+ip -net ${c_b} addr add 192.168.20.2/24 dev veth0
+ip -net ${c_b} link set dev veth0 up
+ip -net ${c_b} link set dev lo up
+ip -net ${c_b} route add default via 192.168.20.1
+
+# Wan
+ip -net ${r_w} addr add 10.2.2.254/24 dev veth0
+ip -net ${r_w} addr add 10.4.4.254/24 dev veth1
+
+ip -net ${r_w} link set dev lo up
+ip -net ${r_w} link set dev veth0 up mtu 1400
+ip -net ${r_w} link set dev veth1 up mtu 1400
+
+ip -net ${r_a} link set dev veth0 mtu 1400
+ip -net ${r_b} link set dev veth0 mtu 1400
+
+ip netns exec ${r_w} sysctl -q net.ipv4.conf.all.forwarding=1 > /dev/null
+
+# Path MTU discovery
+# ------------------
+# Running tracepath from Client A to Client B shows PMTU discovery is working
+# as expected:
+#
+# clienta:~# tracepath 192.168.20.2
+# 1?: [LOCALHOST]                      pmtu 1500
+# 1:  192.168.10.1                                          0.867ms
+# 1:  192.168.10.1                                          0.302ms
+# 2:  192.168.10.1                                          0.312ms pmtu 1480
+# 2:  no reply
+# 3:  192.168.10.1                                          0.510ms pmtu 1380
+# 3:  192.168.20.2                                          2.320ms reached
+# Resume: pmtu 1380 hops 3 back 3
+
+# ip netns exec ${c_a} traceroute --mtu 192.168.20.2
+
+# Router A has learned PMTU (1400) to Router B from Wanrouter.
+# Client A has learned PMTU (1400 - IPIP overhead = 1380) to Client B
+# from Router A.
+
+#Send large UDP packet
+#---------------------
+#Now we send a 1400 bytes UDP packet from Client A to Client B:
+
+# clienta:~# head -c1400 /dev/zero | tr "\000" "a" | nc -u 192.168.20.2 5000
+test_path "without"
+
+# The IPv4 stack on Client A already knows the PMTU to Client B, so the
+# UDP packet is sent as two fragments (1380 + 20). Router A forwards the
+# fragments between eth1 and ipip0. The fragments fit into the tunnel and
+# reach their destination.
+
+#When sending the large UDP packet again, Router A now reassembles the
+#fragments before routing the packet over ipip0. The resulting IPIP
+#packet is too big (1400) for the tunnel PMTU (1380) to Router B, it is
+#dropped on Router A before sending.
+
+ip netns exec ${r_a} iptables -A FORWARD -m conntrack --ctstate NEW
+test_path "with"