netdev - [PATCH] tcp: don't ignore ECN CWR on pure ACK

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Message-Id: <20200623145343.7546-1-denis.kirjanov@suse.com>
Date:   Tue, 23 Jun 2020 17:53:43 +0300
From:   Denis Kirjanov <kda@...ux-powerpc.org>
To:     netdev@...r.kernel.org
Subject: [PATCH] tcp: don't ignore ECN CWR on pure ACK

there is a problem with the CWR flag set in an incoming ACK segment
and it leads to the situation when the ECE flag is latched forever

the following packetdrill script shows what happens:

// Stack receives incoming segments with CE set
+0.1 <[ect0]  . 11001:12001(1000) ack 1001 win 65535
+0.0 <[ce]    . 12001:13001(1000) ack 1001 win 65535
+0.0 <[ect0] P. 13001:14001(1000) ack 1001 win 65535

// Stack repsonds with ECN ECHO
+0.0 >[noecn]  . 1001:1001(0) ack 12001
+0.0 >[noecn] E. 1001:1001(0) ack 13001
+0.0 >[noecn] E. 1001:1001(0) ack 14001

// Write a packet
+0.1 write(3, ..., 1000) = 1000
+0.0 >[ect0] PE. 1001:2001(1000) ack 14001

// Pure ACK received
+0.01 <[noecn] W. 14001:14001(0) ack 2001 win 65535

// Since CWR was sent, this packet should NOT have ECE set

+0.1 write(3, ..., 1000) = 1000
+0.0 >[ect0]  P. 2001:3001(1000) ack 14001
// but Linux will still keep ECE latched here, with packetdrill
// flagging a missing ECE flag, expecting
// >[ect0] PE. 2001:3001(1000) ack 14001
// in the script

In the situation above we will continue to send ECN ECHO packets
and trigger the peer to reduce the congestion window.

Signed-off-by: Denis Kirjanov <denis.kirjanov@...e.com>
---
 net/ipv4/tcp_input.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 12fda8f27b08..e00b88c8598d 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -3696,8 +3696,13 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
 				      &rexmit);
 	}
 
-	if ((flag & FLAG_FORWARD_PROGRESS) || !(flag & FLAG_NOT_DUP))
+	if ((flag & FLAG_FORWARD_PROGRESS) || !(flag & FLAG_NOT_DUP)) {
+		/* If we miss the CWR flag then ECE will be
+		 * latched forever.
+		 */
+		tcp_ecn_accept_cwr(sk, skb);
 		sk_dst_confirm(sk);
+	}
 
 	delivered = tcp_newly_delivered(sk, delivered, flag);
 	lost = tp->lost - lost;			/* freshly marked lost */
@@ -4800,8 +4805,6 @@ static void tcp_data_queue(struct sock *sk, struct sk_buff *skb)
 	skb_dst_drop(skb);
 	__skb_pull(skb, tcp_hdr(skb)->doff * 4);
 
-	tcp_ecn_accept_cwr(sk, skb);
-
 	tp->rx_opt.dsack = 0;
 
 	/*  Queue data for delivery to the user.
-- 
2.27.0