netdev - [PATCH v2 net-next 06/14] tcp: accecn: handle unexpected AccECN negotiation feedback

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <20250918162133.111922-7-chia-yu.chang@nokia-bell-labs.com>
Date: Thu, 18 Sep 2025 18:21:25 +0200
From: chia-yu.chang@...ia-bell-labs.com
To: pabeni@...hat.com,
	edumazet@...gle.com,
	linux-doc@...r.kernel.org,
	corbet@....net,
	horms@...nel.org,
	dsahern@...nel.org,
	kuniyu@...zon.com,
	bpf@...r.kernel.org,
	netdev@...r.kernel.org,
	dave.taht@...il.com,
	jhs@...atatu.com,
	kuba@...nel.org,
	stephen@...workplumber.org,
	xiyou.wangcong@...il.com,
	jiri@...nulli.us,
	davem@...emloft.net,
	andrew+netdev@...n.ch,
	donald.hunter@...il.com,
	ast@...erby.net,
	liuhangbin@...il.com,
	shuah@...nel.org,
	linux-kselftest@...r.kernel.org,
	ij@...nel.org,
	ncardwell@...gle.com,
	koen.de_schepper@...ia-bell-labs.com,
	g.white@...lelabs.com,
	ingemar.s.johansson@...csson.com,
	mirja.kuehlewind@...csson.com,
	cheshire@...le.com,
	rs.ietf@....at,
	Jason_Livingood@...cast.com,
	vidhi_goel@...le.com
Cc: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>
Subject: [PATCH v2 net-next 06/14] tcp: accecn: handle unexpected AccECN negotiation feedback

From: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>

According to Section 3.1.2 of AccECN spec (RFC9768), if a TCP Client
has sent a SYN requesting AccECN feedback with (AE,CWR,ECE) = (1,1,1)
then receives a SYN/ACK with the currently reserved combination
(AE,CWR,ECE) = (1,0,1) but it does not have logic specific to such a
combination, the Client MUST enable AccECN mode as if the SYN/ACK
confirmed that the Server supported AccECN and as if it fed back that
the IP-ECN field on the SYN had arrived unchanged.

Signed-off-by: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>
---
 include/net/tcp_ecn.h | 44 ++++++++++++++++++++++++++++++-------------
 1 file changed, 31 insertions(+), 13 deletions(-)

diff --git a/include/net/tcp_ecn.h b/include/net/tcp_ecn.h
index fb4fcb641d24..7ceb191893c6 100644
--- a/include/net/tcp_ecn.h
+++ b/include/net/tcp_ecn.h
@@ -467,6 +467,26 @@ static inline u8 tcp_accecn_option_init(const struct sk_buff *skb,
 	return TCP_ACCECN_OPT_COUNTER_SEEN;
 }
 
+static inline void tcp_ecn_rcv_synack_accecn(struct sock *sk,
+					     const struct sk_buff *skb, u8 dsf)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	tcp_ecn_mode_set(tp, TCP_ECN_MODE_ACCECN);
+	tp->syn_ect_rcv = dsf & INET_ECN_MASK;
+	/* Demand Accurate ECN option in response to the SYN on the SYN/ACK
+	 * and the TCP server will try to send one more packet with an AccECN
+	 * Option at a later point during the connection.
+	 */
+	if (tp->rx_opt.accecn &&
+	    tp->saw_accecn_opt < TCP_ACCECN_OPT_COUNTER_SEEN) {
+		u8 saw_opt = tcp_accecn_option_init(skb, tp->rx_opt.accecn);
+
+		tcp_accecn_saw_opt_fail_recv(tp, saw_opt);
+		tp->accecn_opt_demand = 2;
+	}
+}
+
 /* See Table 2 of the AccECN draft */
 static inline void tcp_ecn_rcv_synack(struct sock *sk, const struct sk_buff *skb,
 				      const struct tcphdr *th, u8 ip_dsfield)
@@ -489,13 +509,11 @@ static inline void tcp_ecn_rcv_synack(struct sock *sk, const struct sk_buff *skb
 		tcp_ecn_mode_set(tp, TCP_ECN_DISABLED);
 		break;
 	case 0x1:
-	case 0x5:
 		/* +========+========+============+=============+
 		 * | A      | B      |  SYN/ACK   |  Feedback   |
 		 * |        |        |    B->A    |  Mode of A  |
 		 * |        |        | AE CWR ECE |             |
 		 * +========+========+============+=============+
-		 * | AccECN | Nonce  | 1   0   1  | (Reserved)  |
 		 * | AccECN | ECN    | 0   0   1  | Classic ECN |
 		 * | Nonce  | AccECN | 0   0   1  | Classic ECN |
 		 * | ECN    | AccECN | 0   0   1  | Classic ECN |
@@ -503,20 +521,20 @@ static inline void tcp_ecn_rcv_synack(struct sock *sk, const struct sk_buff *skb
 		 */
 		if (tcp_ca_no_fallback_rfc3168(sk))
 			tcp_ecn_mode_set(tp, TCP_ECN_DISABLED);
-		else if (tcp_ecn_mode_pending(tp))
-			/* Downgrade from AccECN, or requested initially */
+		else
 			tcp_ecn_mode_set(tp, TCP_ECN_MODE_RFC3168);
 		break;
-	default:
-		tcp_ecn_mode_set(tp, TCP_ECN_MODE_ACCECN);
-		tp->syn_ect_rcv = ip_dsfield & INET_ECN_MASK;
-		if (tp->rx_opt.accecn &&
-		    tp->saw_accecn_opt < TCP_ACCECN_OPT_COUNTER_SEEN) {
-			u8 saw_opt = tcp_accecn_option_init(skb, tp->rx_opt.accecn);
-
-			tcp_accecn_saw_opt_fail_recv(tp, saw_opt);
-			tp->accecn_opt_demand = 2;
+	case 0x5:
+		if (tcp_ecn_mode_pending(tp)) {
+			tcp_ecn_rcv_synack_accecn(sk, skb, ip_dsfield);
+			if (INET_ECN_is_ce(ip_dsfield)) {
+				tp->received_ce++;
+				tp->received_ce_pending++;
+			}
 		}
+		break;
+	default:
+		tcp_ecn_rcv_synack_accecn(sk, skb, ip_dsfield);
 		if (INET_ECN_is_ce(ip_dsfield) &&
 		    tcp_accecn_validate_syn_feedback(sk, ace,
 						     tp->syn_ect_snt)) {
-- 
2.34.1