[PATCH v6 22/26] tcp: authopt: Try to respect rnextkeyid from SYN on SYNACK

[Leonard Crestez <cdleonard@gmail.com>]

According to the RFC we should use the key that the peer suggests via
rnextkeyid.

This is currently done by storing recv_rnextkeyid in tcp_authopt_info
but this does not work for the SYNACK case because the tcp_request_sock
does not hold an info pointer for reasons of memory usage.

Handle this by storing recv_rnextkeyid inside tcp_request_sock. This
doesn't increase the memory usage because there are unused bytes at the
end.

Signed-off-by: Leonard Crestez <cdleonard@gmail.com>
---
 include/linux/tcp.h    |  6 ++++++
 net/ipv4/tcp_authopt.c | 26 ++++++++++++++++++++------
 net/ipv4/tcp_input.c   | 12 ++++++++++++
 3 files changed, 38 insertions(+), 6 deletions(-)

diff --git a/include/linux/tcp.h b/include/linux/tcp.h
index 551942883f06..6a4ff0ed55c6 100644
--- a/include/linux/tcp.h
+++ b/include/linux/tcp.h
@@ -125,10 +125,13 @@ struct tcp_options_received {
 	u8	saw_unknown:1,	/* Received unknown option		*/
 		unused:7;
 	u8	num_sacks;	/* Number of SACK blocks		*/
 	u16	user_mss;	/* mss requested by user in ioctl	*/
 	u16	mss_clamp;	/* Maximal mss, negotiated at connection setup */
+#if IS_ENABLED(CONFIG_TCP_AUTHOPT)
+	u8	rnextkeyid;
+#endif
 };
 
 static inline void tcp_clear_options(struct tcp_options_received *rx_opt)
 {
 	rx_opt->tstamp_ok = rx_opt->sack_ok = 0;
@@ -163,10 +166,13 @@ struct tcp_request_sock {
 	u32				rcv_nxt; /* the ack # by SYNACK. For
 						  * FastOpen it's the seq#
 						  * after data-in-SYN.
 						  */
 	u8				syn_tos;
+#if IS_ENABLED(CONFIG_TCP_AUTHOPT)
+	u8				recv_rnextkeyid;
+#endif
 };
 
 static inline struct tcp_request_sock *tcp_rsk(const struct request_sock *req)
 {
 	return (struct tcp_request_sock *)req;
diff --git a/net/ipv4/tcp_authopt.c b/net/ipv4/tcp_authopt.c
index 00d749aa1025..3596fc1fb770 100644
--- a/net/ipv4/tcp_authopt.c
+++ b/net/ipv4/tcp_authopt.c
@@ -1,7 +1,9 @@
 // SPDX-License-Identifier: GPL-2.0-or-later
 
+#include "linux/tcp.h"
+#include "net/tcp_states.h"
 #include <net/tcp_authopt.h>
 #include <net/ip.h>
 #include <net/ipv6.h>
 #include <net/tcp.h>
 #include <linux/kref.h>
@@ -435,21 +437,33 @@ struct tcp_authopt_key_info *__tcp_authopt_select_key(const struct sock *sk,
 {
 	struct tcp_authopt_key_info *key, *new_key = NULL;
 	struct netns_tcp_authopt *net = sock_net_tcp_authopt(sk);
 
 	/* Listen sockets don't refer to any specific connection so we don't try
-	 * to keep using the same key and ignore any received keyids.
+	 * to keep using the same key.
+	 * The rnextkeyid is stored in tcp_request_sock
 	 */
 	if (sk->sk_state == TCP_LISTEN) {
-		int send_keyid = -1;
-
+		int send_id = -1;
+		struct tcp_request_sock *rsk;
+
+		if (WARN_ONCE(addr_sk->sk_state != TCP_NEW_SYN_RECV, "bad socket state"))
+			return NULL;
+		rsk = tcp_rsk((struct request_sock *)addr_sk);
+		/* Forcing a specific send_keyid on a listen socket forces it for
+		 * all clients so is unlikely to be useful.
+		 */
 		if (info->flags & TCP_AUTHOPT_FLAG_LOCK_KEYID)
-			send_keyid = info->send_keyid;
-		key = tcp_authopt_lookup_send(net, addr_sk, send_keyid);
+			send_id = info->send_keyid;
+		else
+			send_id = rsk->recv_rnextkeyid;
+		key = tcp_authopt_lookup_send(net, addr_sk, send_id);
+		/* If no key found with specific send_id try anything else. */
+		if (!key)
+			key = tcp_authopt_lookup_send(net, addr_sk, -1);
 		if (key)
 			*rnextkeyid = key->recv_id;
-
 		return key;
 	}
 
 	if (locked) {
 		sock_owned_by_me(sk);
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index c09d42614b2b..6f2af45f4271 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -4105,10 +4105,18 @@ void tcp_parse_options(const struct net *net,
 				/*
 				 * The MD5 Hash has already been
 				 * checked (see tcp_v{4,6}_do_rcv()).
 				 */
 				break;
+#endif
+#ifdef CONFIG_TCP_AUTHOPT
+			case TCPOPT_AUTHOPT:
+				/* Hash has already been checked.
+				 * We parse rnextkeyid here so we can match it on synack
+				 */
+				opt_rx->rnextkeyid = ptr[1];
+				break;
 #endif
 			case TCPOPT_FASTOPEN:
 				tcp_parse_fastopen_option(
 					opsize - TCPOLEN_FASTOPEN_BASE,
 					ptr, th->syn, foc, false);
@@ -6958,10 +6966,14 @@ int tcp_conn_request(struct request_sock_ops *rsk_ops,
 		tcp_clear_options(&tmp_opt);
 
 	if (IS_ENABLED(CONFIG_SMC) && want_cookie)
 		tmp_opt.smc_ok = 0;
 
+#if IS_ENABLED(CONFIG_TCP_AUTHOPT)
+	tcp_rsk(req)->recv_rnextkeyid = tmp_opt.rnextkeyid;
+#endif
+
 	tmp_opt.tstamp_ok = tmp_opt.saw_tstamp;
 	tcp_openreq_init(req, &tmp_opt, skb, sk);
 	inet_rsk(req)->no_srccheck = inet_sk(sk)->transparent;
 
 	/* Note: tcp_v6_init_req() might override ir_iif for link locals */
-- 
2.25.1