(modified to not remove ssh_packet_read_expect() and to add to
KexAlgorithms in sshd.c and sshconnect2.c as this version pre-dates
kex_proposal_populate_entries())

Backport of:

From 1edb00c58f8a6875fad6a497aa2bacf37f9e6cd5 Mon Sep 17 00:00:00 2001
From: "djm@openbsd.org" <djm@openbsd.org>
Date: Mon, 18 Dec 2023 14:45:17 +0000
Subject: [PATCH] upstream: implement "strict key exchange" in ssh and sshd

This adds a protocol extension to improve the integrity of the SSH
transport protocol, particular in and around the initial key exchange
(KEX) phase.

Full details of the extension are in the PROTOCOL file.

with markus@

OpenBSD-Commit-ID: 2a66ac962f0a630d7945fee54004ed9e9c439f14
---
 PROTOCOL      |  28 +++++++++++++-
 kex.c         |  84 ++++++++++++++++++++++++++--------------
 kex.h         |   3 +-
 packet.c      | 103 +++++++++++++++++++++++++++++---------------------
 packet.h      |   3 +-
 sshconnect2.c |  12 ++----
 6 files changed, 148 insertions(+), 85 deletions(-)

--- a/PROTOCOL
+++ b/PROTOCOL
@@ -102,6 +102,32 @@ OpenSSH supports the use of ECDH in Curv
 described at:
 http://git.libssh.org/users/aris/libssh.git/plain/doc/curve25519-sha256@libssh.org.txt?h=curve25519
 
+1.9 transport: strict key exchange extension
+
+OpenSSH supports a number of transport-layer hardening measures under
+a "strict KEX" feature. This feature is signalled similarly to the
+RFC8308 ext-info feature: by including a additional algorithm in the
+initiial SSH2_MSG_KEXINIT kex_algorithms field. The client may append
+"kex-strict-c-v00@openssh.com" to its kex_algorithms and the server
+may append "kex-strict-s-v00@openssh.com". These pseudo-algorithms
+are only valid in the initial SSH2_MSG_KEXINIT and MUST be ignored
+if they are present in subsequent SSH2_MSG_KEXINIT packets.
+
+When an endpoint that supports this extension observes this algorithm
+name in a peer's KEXINIT packet, it MUST make the following changes to
+the the protocol:
+
+a) During initial KEX, terminate the connection if any unexpected or
+   out-of-sequence packet is received. This includes terminating the
+   connection if the first packet received is not SSH2_MSG_KEXINIT.
+   Unexpected packets for the purpose of strict KEX include messages
+   that are otherwise valid at any time during the connection such as
+   SSH2_MSG_DEBUG and SSH2_MSG_IGNORE.
+b) After sending or receiving a SSH2_MSG_NEWKEYS message, reset the
+   packet sequence number to zero. This behaviour persists for the
+   duration of the connection (i.e. not just the first
+   SSH2_MSG_NEWKEYS).
+
 2. Connection protocol changes
 
 2.1. connection: Channel write close extension "eow@openssh.com"
--- a/kex.c
+++ b/kex.c
@@ -68,7 +68,7 @@
 #endif
 
 /* prototype */
-static int kex_choose_conf(struct ssh *);
+static int kex_choose_conf(struct ssh *, uint32_t seq);
 static int kex_input_newkeys(int, u_int32_t, struct ssh *);
 
 static const char * const proposal_names[PROPOSAL_MAX] = {
@@ -209,6 +209,18 @@ kex_names_valid(const char *names)
 	return 1;
 }
 
+/* returns non-zero if proposal contains any algorithm from algs */
+static int
+has_any_alg(const char *proposal, const char *algs)
+{
+	char *cp;
+
+	if ((cp = match_list(proposal, algs, NULL)) == NULL)
+		return 0;
+	free(cp);
+	return 1;
+}
+
 /*
  * Concatenate algorithm names, avoiding duplicates in the process.
  * Caller must free returned string.
@@ -216,7 +228,7 @@ kex_names_valid(const char *names)
 char *
 kex_names_cat(const char *a, const char *b)
 {
-	char *ret = NULL, *tmp = NULL, *cp, *p, *m;
+	char *ret = NULL, *tmp = NULL, *cp, *p;
 	size_t len;
 
 	if (a == NULL || *a == '\0')
@@ -233,10 +245,8 @@ kex_names_cat(const char *a, const char
 	}
 	strlcpy(ret, a, len);
 	for ((p = strsep(&cp, ",")); p && *p != '\0'; (p = strsep(&cp, ","))) {
-		if ((m = match_list(ret, p, NULL)) != NULL) {
-			free(m);
+		if (has_any_alg(ret, p))
 			continue; /* Algorithm already present */
-		}
 		if (strlcat(ret, ",", len) >= len ||
 		    strlcat(ret, p, len) >= len) {
 			free(tmp);
@@ -467,7 +477,12 @@ kex_protocol_error(int type, u_int32_t s
 {
 	int r;
 
-	error("kex protocol error: type %d seq %u", type, seq);
+	/* If in strict mode, any unexpected message is an error */
+	if ((ssh->kex->flags & KEX_INITIAL) && ssh->kex->kex_strict) {
+		ssh_packet_disconnect(ssh, "strict KEX violation: "
+		    "unexpected packet type %u (seqnr %u)", type, seq);
+	}
+	error_f("type %u seq %u", type, seq);
 	if ((r = sshpkt_start(ssh, SSH2_MSG_UNIMPLEMENTED)) != 0 ||
 	    (r = sshpkt_put_u32(ssh, seq)) != 0 ||
 	    (r = sshpkt_send(ssh)) != 0)
@@ -542,6 +557,11 @@ kex_input_ext_info(int type, u_int32_t s
 	ssh_dispatch_set(ssh, SSH2_MSG_EXT_INFO, &kex_protocol_error);
 	if ((r = sshpkt_get_u32(ssh, &ninfo)) != 0)
 		return r;
+	if (ninfo >= 1024) {
+		error("SSH2_MSG_EXT_INFO with too many entries, expected "
+		    "<=1024, received %u", ninfo);
+		return dispatch_protocol_error(type, seq, ssh);
+	}
 	for (i = 0; i < ninfo; i++) {
 		if ((r = sshpkt_get_cstring(ssh, &name, NULL)) != 0)
 			return r;
@@ -657,7 +677,7 @@ kex_input_kexinit(int type, u_int32_t se
 		error_f("no kex");
 		return SSH_ERR_INTERNAL_ERROR;
 	}
-	ssh_dispatch_set(ssh, SSH2_MSG_KEXINIT, NULL);
+	ssh_dispatch_set(ssh, SSH2_MSG_KEXINIT, &kex_protocol_error);
 	ptr = sshpkt_ptr(ssh, &dlen);
 	if ((r = sshbuf_put(kex->peer, ptr, dlen)) != 0)
 		return r;
@@ -693,7 +713,7 @@ kex_input_kexinit(int type, u_int32_t se
 	if (!(kex->flags & KEX_INIT_SENT))
 		if ((r = kex_send_kexinit(ssh)) != 0)
 			return r;
-	if ((r = kex_choose_conf(ssh)) != 0)
+	if ((r = kex_choose_conf(ssh, seq)) != 0)
 		return r;
 
 	if (kex->kex_type < KEX_MAX && kex->kex[kex->kex_type] != NULL)
@@ -960,20 +980,14 @@ proposals_match(char *my[PROPOSAL_MAX],
 	return (1);
 }
 
-/* returns non-zero if proposal contains any algorithm from algs */
 static int
-has_any_alg(const char *proposal, const char *algs)
+kexalgs_contains(char **peer, const char *ext)
 {
-	char *cp;
-
-	if ((cp = match_list(proposal, algs, NULL)) == NULL)
-		return 0;
-	free(cp);
-	return 1;
+	return has_any_alg(peer[PROPOSAL_KEX_ALGS], ext);
 }
 
 static int
-kex_choose_conf(struct ssh *ssh)
+kex_choose_conf(struct ssh *ssh, uint32_t seq)
 {
 	struct kex *kex = ssh->kex;
 	struct newkeys *newkeys;
@@ -998,13 +1012,23 @@ kex_choose_conf(struct ssh *ssh)
 		sprop=peer;
 	}
 
-	/* Check whether client supports ext_info_c */
-	if (kex->server && (kex->flags & KEX_INITIAL)) {
-		char *ext;
-
-		ext = match_list("ext-info-c", peer[PROPOSAL_KEX_ALGS], NULL);
-		kex->ext_info_c = (ext != NULL);
-		free(ext);
+	/* Check whether peer supports ext_info/kex_strict */
+	if ((kex->flags & KEX_INITIAL) != 0) {
+		if (kex->server) {
+			kex->ext_info_c = kexalgs_contains(peer, "ext-info-c");
+			kex->kex_strict = kexalgs_contains(peer,
+			    "kex-strict-c-v00@openssh.com");
+		} else {
+			kex->kex_strict = kexalgs_contains(peer,
+			    "kex-strict-s-v00@openssh.com");
+		}
+		if (kex->kex_strict) {
+			debug3_f("will use strict KEX ordering");
+			if (seq != 0)
+				ssh_packet_disconnect(ssh,
+				    "strict KEX violation: "
+				    "KEXINIT was not the first packet");
+		}
 	}
 
 	/* Check whether client supports rsa-sha2 algorithms */
--- a/kex.h
+++ b/kex.h
@@ -157,6 +157,7 @@ struct kex {
 	u_int	kex_type;
 	char	*server_sig_algs;
 	int	ext_info_c;
+	int	kex_strict;
 	struct sshbuf *my;
 	struct sshbuf *peer;
 	struct sshbuf *client_version;
--- a/packet.c
+++ b/packet.c
@@ -1205,8 +1205,13 @@ ssh_packet_send2_wrapped(struct ssh *ssh
 	sshbuf_dump(state->output, stderr);
 #endif
 	/* increment sequence number for outgoing packets */
-	if (++state->p_send.seqnr == 0)
+	if (++state->p_send.seqnr == 0) {
+		if ((ssh->kex->flags & KEX_INITIAL) != 0) {
+			ssh_packet_disconnect(ssh, "outgoing sequence number "
+			    "wrapped during initial key exchange");
+		}
 		logit("outgoing seqnr wraps around");
+	}
 	if (++state->p_send.packets == 0)
 		if (!(ssh->compat & SSH_BUG_NOREKEY))
 			return SSH_ERR_NEED_REKEY;
@@ -1214,6 +1219,11 @@ ssh_packet_send2_wrapped(struct ssh *ssh
 	state->p_send.bytes += len;
 	sshbuf_reset(state->outgoing_packet);
 
+	if (type == SSH2_MSG_NEWKEYS && ssh->kex->kex_strict) {
+		debug_f("resetting send seqnr %u", state->p_send.seqnr);
+		state->p_send.seqnr = 0;
+	}
+
 	if (type == SSH2_MSG_NEWKEYS)
 		r = ssh_set_newkeys(ssh, MODE_OUT);
 	else if (type == SSH2_MSG_USERAUTH_SUCCESS && state->server_side)
@@ -1342,8 +1352,7 @@ ssh_packet_read_seqnr(struct ssh *ssh, u
 	/* Stay in the loop until we have received a complete packet. */
 	for (;;) {
 		/* Try to read a packet from the buffer. */
-		r = ssh_packet_read_poll_seqnr(ssh, typep, seqnr_p);
-		if (r != 0)
+		if ((r = ssh_packet_read_poll_seqnr(ssh, typep, seqnr_p)) != 0)
 			break;
 		/* If we got a packet, return it. */
 		if (*typep != SSH_MSG_NONE)
@@ -1627,10 +1636,16 @@ ssh_packet_read_poll2(struct ssh *ssh, u
 		if ((r = sshbuf_consume(state->input, mac->mac_len)) != 0)
 			goto out;
 	}
+
 	if (seqnr_p != NULL)
 		*seqnr_p = state->p_read.seqnr;
-	if (++state->p_read.seqnr == 0)
+	if (++state->p_read.seqnr == 0) {
+		if ((ssh->kex->flags & KEX_INITIAL) != 0) {
+			ssh_packet_disconnect(ssh, "incoming sequence number "
+			    "wrapped during initial key exchange");
+		}
 		logit("incoming seqnr wraps around");
+	}
 	if (++state->p_read.packets == 0)
 		if (!(ssh->compat & SSH_BUG_NOREKEY))
 			return SSH_ERR_NEED_REKEY;
@@ -1696,6 +1711,10 @@ ssh_packet_read_poll2(struct ssh *ssh, u
 #endif
 	/* reset for next packet */
 	state->packlen = 0;
+	if (*typep == SSH2_MSG_NEWKEYS && ssh->kex->kex_strict) {
+		debug_f("resetting read seqnr %u", state->p_read.seqnr);
+		state->p_read.seqnr = 0;
+	}
 
 	if ((r = ssh_packet_check_rekey(ssh)) != 0)
 		return r;
@@ -1716,10 +1735,39 @@ ssh_packet_read_poll_seqnr(struct ssh *s
 		r = ssh_packet_read_poll2(ssh, typep, seqnr_p);
 		if (r != 0)
 			return r;
-		if (*typep) {
-			state->keep_alive_timeouts = 0;
-			DBG(debug("received packet type %d", *typep));
+		if (*typep == 0) {
+			/* no message ready */
+			return 0;
+		}
+		state->keep_alive_timeouts = 0;
+		DBG(debug("received packet type %d", *typep));
+
+		/* Always process disconnect messages */
+		if (*typep == SSH2_MSG_DISCONNECT) {
+			if ((r = sshpkt_get_u32(ssh, &reason)) != 0 ||
+			    (r = sshpkt_get_string(ssh, &msg, NULL)) != 0)
+				return r;
+			/* Ignore normal client exit notifications */
+			do_log2(ssh->state->server_side &&
+			    reason == SSH2_DISCONNECT_BY_APPLICATION ?
+			    SYSLOG_LEVEL_INFO : SYSLOG_LEVEL_ERROR,
+			    "Received disconnect from %s port %d:"
+			    "%u: %.400s", ssh_remote_ipaddr(ssh),
+			    ssh_remote_port(ssh), reason, msg);
+			free(msg);
+			return SSH_ERR_DISCONNECTED;
 		}
+
+		/*
+		 * Do not implicitly handle any messages here during initial
+		 * KEX when in strict mode. They will be need to be allowed
+		 * explicitly by the KEX dispatch table or they will generate
+		 * protocol errors.
+		 */
+		if (ssh->kex != NULL &&
+		    (ssh->kex->flags & KEX_INITIAL) && ssh->kex->kex_strict)
+			return 0;
+		/* Implicitly handle transport-level messages */
 		switch (*typep) {
 		case SSH2_MSG_IGNORE:
 			debug3("Received SSH2_MSG_IGNORE");
@@ -1734,19 +1782,6 @@ ssh_packet_read_poll_seqnr(struct ssh *s
 			debug("Remote: %.900s", msg);
 			free(msg);
 			break;
-		case SSH2_MSG_DISCONNECT:
-			if ((r = sshpkt_get_u32(ssh, &reason)) != 0 ||
-			    (r = sshpkt_get_string(ssh, &msg, NULL)) != 0)
-				return r;
-			/* Ignore normal client exit notifications */
-			do_log2(ssh->state->server_side &&
-			    reason == SSH2_DISCONNECT_BY_APPLICATION ?
-			    SYSLOG_LEVEL_INFO : SYSLOG_LEVEL_ERROR,
-			    "Received disconnect from %s port %d:"
-			    "%u: %.400s", ssh_remote_ipaddr(ssh),
-			    ssh_remote_port(ssh), reason, msg);
-			free(msg);
-			return SSH_ERR_DISCONNECTED;
 		case SSH2_MSG_UNIMPLEMENTED:
 			if ((r = sshpkt_get_u32(ssh, &seqnr)) != 0)
 				return r;
@@ -2211,6 +2246,7 @@ kex_to_blob(struct sshbuf *m, struct kex
 	    (r = sshbuf_put_u32(m, kex->hostkey_type)) != 0 ||
 	    (r = sshbuf_put_u32(m, kex->hostkey_nid)) != 0 ||
 	    (r = sshbuf_put_u32(m, kex->kex_type)) != 0 ||
+	    (r = sshbuf_put_u32(m, kex->kex_strict)) != 0 ||
 	    (r = sshbuf_put_stringb(m, kex->my)) != 0 ||
 	    (r = sshbuf_put_stringb(m, kex->peer)) != 0 ||
 	    (r = sshbuf_put_stringb(m, kex->client_version)) != 0 ||
@@ -2373,6 +2409,7 @@ kex_from_blob(struct sshbuf *m, struct k
 	    (r = sshbuf_get_u32(m, (u_int *)&kex->hostkey_type)) != 0 ||
 	    (r = sshbuf_get_u32(m, (u_int *)&kex->hostkey_nid)) != 0 ||
 	    (r = sshbuf_get_u32(m, &kex->kex_type)) != 0 ||
+	    (r = sshbuf_get_u32(m, &kex->kex_strict)) != 0 ||
 	    (r = sshbuf_get_stringb(m, kex->my)) != 0 ||
 	    (r = sshbuf_get_stringb(m, kex->peer)) != 0 ||
 	    (r = sshbuf_get_stringb(m, kex->client_version)) != 0 ||
@@ -2701,6 +2738,7 @@ sshpkt_disconnect(struct ssh *ssh, const
 	vsnprintf(buf, sizeof(buf), fmt, args);
 	va_end(args);
 
+	debug2_f("sending SSH2_MSG_DISCONNECT: %s", buf);
 	if ((r = sshpkt_start(ssh, SSH2_MSG_DISCONNECT)) != 0 ||
 	    (r = sshpkt_put_u32(ssh, SSH2_DISCONNECT_PROTOCOL_ERROR)) != 0 ||
 	    (r = sshpkt_put_cstring(ssh, buf)) != 0 ||
--- a/sshconnect2.c
+++ b/sshconnect2.c
@@ -244,7 +244,8 @@ ssh_kex2(struct ssh *ssh, char *host, st
 		fatal_fr(r, "kex_assemble_namelist");
 	free(all_key);
 
-	if ((s = kex_names_cat(options.kex_algorithms, "ext-info-c")) == NULL)
+	if ((s = kex_names_cat(options.kex_algorithms,
+	    "ext-info-c,kex-strict-c-v00@openssh.com")) == NULL)
 		fatal_f("kex_names_cat");
 	myproposal[PROPOSAL_KEX_ALGS] = compat_kex_proposal(ssh, s);
 	myproposal[PROPOSAL_ENC_ALGS_CTOS] =
@@ -432,7 +433,6 @@ struct cauthmethod {
 };
 
 static int input_userauth_service_accept(int, u_int32_t, struct ssh *);
-static int input_userauth_ext_info(int, u_int32_t, struct ssh *);
 static int input_userauth_success(int, u_int32_t, struct ssh *);
 static int input_userauth_failure(int, u_int32_t, struct ssh *);
 static int input_userauth_banner(int, u_int32_t, struct ssh *);
@@ -552,7 +552,7 @@ ssh_userauth2(struct ssh *ssh, const cha
 
 	ssh->authctxt = &authctxt;
 	ssh_dispatch_init(ssh, &input_userauth_error);
-	ssh_dispatch_set(ssh, SSH2_MSG_EXT_INFO, &input_userauth_ext_info);
+	ssh_dispatch_set(ssh, SSH2_MSG_EXT_INFO, kex_input_ext_info);
 	ssh_dispatch_set(ssh, SSH2_MSG_SERVICE_ACCEPT, &input_userauth_service_accept);
 	ssh_dispatch_run_fatal(ssh, DISPATCH_BLOCK, &authctxt.success);	/* loop until success */
 	pubkey_cleanup(ssh);
@@ -604,13 +604,6 @@ input_userauth_service_accept(int type,
 	return r;
 }
 
-/* ARGSUSED */
-static int
-input_userauth_ext_info(int type, u_int32_t seqnr, struct ssh *ssh)
-{
-	return kex_input_ext_info(type, seqnr, ssh);
-}
-
 void
 userauth(struct ssh *ssh, char *authlist)
 {
@@ -692,6 +685,7 @@ input_userauth_success(int type, u_int32
 	free(authctxt->methoddata);
 	authctxt->methoddata = NULL;
 	authctxt->success = 1;			/* break out */
+	ssh_dispatch_set(ssh, SSH2_MSG_EXT_INFO, dispatch_protocol_error);
 	return 0;
 }
 
--- a/sshd.c
+++ b/sshd.c
@@ -2389,11 +2389,13 @@ static void
 do_ssh2_kex(struct ssh *ssh)
 {
 	char *myproposal[PROPOSAL_MAX] = { KEX_SERVER };
+	char *s;
 	struct kex *kex;
 	int r;
 
-	myproposal[PROPOSAL_KEX_ALGS] = compat_kex_proposal(ssh,
-	    options.kex_algorithms);
+	if ((s = kex_names_cat(options.kex_algorithms, "kex-strict-s-v00@openssh.com")) == NULL)
+		fatal_f("kex_names_cat");
+	myproposal[PROPOSAL_KEX_ALGS] = compat_kex_proposal(ssh, s);
 	myproposal[PROPOSAL_ENC_ALGS_CTOS] = compat_cipher_proposal(ssh,
 	    options.ciphers);
 	myproposal[PROPOSAL_ENC_ALGS_STOC] = compat_cipher_proposal(ssh,
@@ -2499,6 +2501,7 @@ do_ssh2_kex(struct ssh *ssh)
 	    (r = ssh_packet_write_wait(ssh)) != 0)
 		fatal_fr(r, "send test");
 #endif
+	free(s);
 	debug("KEX done");
 }
 
