path: root/debian/patches/CVE-2023-48795.patch

From 9148d0a8031d89f53f045b63ac3a709611d94778 Mon Sep 17 00:00:00 2001
From: "djm@openbsd.org" <djm@openbsd.org>
Date: Mon, 18 Dec 2023 14:45:17 +0000
Subject: 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

Origin: backport, https://anongit.mindrot.org/openssh.git/commit/?id=1edb00c58f8a6875fad6a497aa2bacf37f9e6cd5
Last-Update: 2023-12-19

Patch-Name: CVE-2023-48795.patch
---
 PROTOCOL      | 26 +++++++++++++++++
 kex.c         | 72 +++++++++++++++++++++++++++++++----------------
 kex.h         |  1 +
 packet.c      | 78 ++++++++++++++++++++++++++++++++++++++-------------
 sshconnect2.c | 14 +++------
 sshd.c        |  9 ++++--
 6 files changed, 143 insertions(+), 57 deletions(-)

diff --git a/PROTOCOL b/PROTOCOL
index 27804d0ca..ad611a01a 100644
--- a/PROTOCOL
+++ b/PROTOCOL
@@ -104,6 +104,32 @@ http://git.libssh.org/users/aris/libssh.git/plain/doc/curve25519-sha256@libssh.o
 
 This is identical to curve25519-sha256 as later published in RFC8731.
 
+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"
diff --git a/kex.c b/kex.c
index e7b7316c7..0b4fc4767 100644
--- 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 *b)
 	}
 	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 seq, struct ssh *ssh)
 {
 	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 seq, struct ssh *ssh)
 	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 seq, struct ssh *ssh)
 		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 seq, struct ssh *ssh)
 	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], char *peer[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 */
diff --git a/kex.h b/kex.h
index e356f5ed0..99b47435f 100644
--- 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;
diff --git a/packet.c b/packet.c
index 3f64d2d32..25553fbb5 100644
--- 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_char *typep, u_int32_t *seqnr_p)
 	/* 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_char *typep, u_int32_t *seqnr_p)
 		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_char *typep, u_int32_t *seqnr_p)
 #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 *ssh, u_char *typep, u_int32_t *seqnr_p)
 		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 *ssh, u_char *typep, u_int32_t *seqnr_p)
 			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 *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 kex **kexp)
 	    (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 char *fmt,...)
 	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 ||
diff --git a/sshconnect2.c b/sshconnect2.c
index cb6a94e76..3e5f69470 100644
--- a/sshconnect2.c
+++ b/sshconnect2.c
@@ -250,7 +250,8 @@ ssh_kex2(struct ssh *ssh, char *host, struct sockaddr *hostaddr, u_short port,
 		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] = prop_kex = compat_kex_proposal(ssh, s);
 	myproposal[PROPOSAL_ENC_ALGS_CTOS] =
@@ -441,7 +442,6 @@ 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 *);
@@ -561,7 +561,7 @@ ssh_userauth2(struct ssh *ssh, const char *local_user,
 
 	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);
@@ -613,13 +613,6 @@ input_userauth_service_accept(int type, u_int32_t seq, struct ssh *ssh)
 	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)
 {
@@ -701,6 +694,7 @@ input_userauth_success(int type, u_int32_t seq, struct ssh *ssh)
 	free(authctxt->methoddata);
 	authctxt->methoddata = NULL;
 	authctxt->success = 1;			/* break out */
+	ssh_dispatch_set(ssh, SSH2_MSG_EXT_INFO, dispatch_protocol_error);
 	return 0;
 }
 
diff --git a/sshd.c b/sshd.c
index fc22fcb62..dd8fd94a3 100644
--- a/sshd.c
+++ b/sshd.c
@@ -2471,11 +2471,13 @@ do_ssh2_kex(struct ssh *ssh)
 {
 	char *myproposal[PROPOSAL_MAX] = { KEX_SERVER };
 	struct kex *kex;
-	char *prop_kex = NULL, *prop_enc = NULL, *prop_hostkey = NULL;
+	char *s, *prop_kex = NULL, *prop_enc = NULL, *prop_hostkey = NULL;
 	int r;
 
-	myproposal[PROPOSAL_KEX_ALGS] = prop_kex = 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] = prop_kex = compat_kex_proposal(ssh, s);
 	myproposal[PROPOSAL_ENC_ALGS_CTOS] =
 	    myproposal[PROPOSAL_ENC_ALGS_STOC] = prop_enc =
 	    compat_cipher_proposal(ssh, options.ciphers);
@@ -2583,6 +2585,7 @@ do_ssh2_kex(struct ssh *ssh)
 	free(prop_kex);
 	free(prop_enc);
 	free(prop_hostkey);
+	free(s);
 	debug("KEX done");
 }