netdev - [PATCH v1 net] tcp: Refine SYN handling for PAWS.

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Date:   Mon, 27 Mar 2023 16:06:28 -0700
From:   Kuniyuki Iwashima <kuniyu@...zon.com>
To:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>
CC:     Kuniyuki Iwashima <kuniyu@...zon.com>,
        Kuniyuki Iwashima <kuni1840@...il.com>,
        <netdev@...r.kernel.org>
Subject: [PATCH v1 net] tcp: Refine SYN handling for PAWS.

Our Network Load Balancer (NLB) [0] has multiple nodes with different
IP addresses, and each node forwards TCP flows from clients to backend
targets.  NLB has an option to preserve the client's source IP address
and port when routing packets to backend targets.

When a client connects to two different NLB nodes, they may select the
same backend target.  Then, if the client has used the same source IP
and port, the two flows at the backend side will have the same 4-tuple.

While testing around such cases, I saw these sequences on the backend
target.

IP 10.0.0.215.60000 > 10.0.3.249.10000: Flags [S], seq 2819965599, win 62727, options [mss 8365,sackOK,TS val 1029816180 ecr 0,nop,wscale 7], length 0
IP 10.0.3.249.10000 > 10.0.0.215.60000: Flags [S.], seq 3040695044, ack 2819965600, win 62643, options [mss 8961,sackOK,TS val 1224784076 ecr 1029816180,nop,wscale 7], length 0
IP 10.0.0.215.60000 > 10.0.3.249.10000: Flags [.], ack 1, win 491, options [nop,nop,TS val 1029816181 ecr 1224784076], length 0
IP 10.0.0.215.60000 > 10.0.3.249.10000: Flags [S], seq 2681819307, win 62727, options [mss 8365,sackOK,TS val 572088282 ecr 0,nop,wscale 7], length 0
IP 10.0.3.249.10000 > 10.0.0.215.60000: Flags [.], ack 1, win 490, options [nop,nop,TS val 1224794914 ecr 1029816181,nop,nop,sack 1 {4156821004:4156821005}], length 0

It seems to be working correctly, but the last ACK was generated by
tcp_send_dupack() and PAWSEstab was increased.  This is because the
second connection has a smaller timestamp than the first one.

In this case, we should send a challenge ACK instead of a dup ACK and
increase the correct counter to rate-limit it properly.

Let's check the SYN bit after the PAWS tests to avoid adding unnecessary
overhead for most packets.

Link: https://docs.aws.amazon.com/elasticloadbalancing/latest/network/introduction.html [0]
Link: https://docs.aws.amazon.com/elasticloadbalancing/latest/network/load-balancer-target-groups.html#client-ip-preservation [1]
Fixes: 0c24604b68fc ("tcp: implement RFC 5961 4.2")
Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.com>
---
 net/ipv4/tcp_input.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index cc072d2cfcd8..89fca4c18530 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -5714,6 +5714,8 @@ static bool tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
 	    tp->rx_opt.saw_tstamp &&
 	    tcp_paws_discard(sk, skb)) {
 		if (!th->rst) {
+			if (unlikely(th->syn))
+				goto syn_challenge;
 			NET_INC_STATS(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
 			if (!tcp_oow_rate_limited(sock_net(sk), skb,
 						  LINUX_MIB_TCPACKSKIPPEDPAWS,
-- 
2.30.2