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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
This commit is contained in:
djm@openbsd.org 2023-12-18 14:45:17 +00:00 committed by Damien Miller
parent 59d691b886
commit 1edb00c58f
No known key found for this signature in database
6 changed files with 148 additions and 85 deletions
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28
PROTOCOL
View File

@ -137,6 +137,32 @@ than as a named global or channel request to allow pings with very
short packet lengths, which would not be possible with other
approaches.
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"
@ -745,4 +771,4 @@ master instance and later clients.
OpenSSH extends the usual agent protocol. These changes are documented
in the PROTOCOL.agent file.
$OpenBSD: PROTOCOL,v 1.49 2023/08/28 03:28:43 djm Exp $
$OpenBSD: PROTOCOL,v 1.50 2023/12/18 14:45:17 djm Exp $

84
kex.c
View File

@ -1,4 +1,4 @@
/* $OpenBSD: kex.c,v 1.182 2023/10/11 04:46:29 djm Exp $ */
/* $OpenBSD: kex.c,v 1.183 2023/12/18 14:45:17 djm Exp $ */
/*
* Copyright (c) 2000, 2001 Markus Friedl. All rights reserved.
*
@ -65,7 +65,7 @@
#include "xmalloc.h"
/* 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] = {
@ -177,6 +177,18 @@ 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.
@ -184,7 +196,7 @@ 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')
@ -201,10 +213,8 @@ 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);
@ -334,15 +344,23 @@ kex_proposal_populate_entries(struct ssh *ssh, char *prop[PROPOSAL_MAX],
const char *defpropclient[PROPOSAL_MAX] = { KEX_CLIENT };
const char **defprop = ssh->kex->server ? defpropserver : defpropclient;
u_int i;
char *cp;
if (prop == NULL)
fatal_f("proposal missing");
/* Append EXT_INFO signalling to KexAlgorithms */
if (kexalgos == NULL)
kexalgos = defprop[PROPOSAL_KEX_ALGS];
if ((cp = kex_names_cat(kexalgos, ssh->kex->server ?
"kex-strict-s-v00@openssh.com" :
"ext-info-c,kex-strict-c-v00@openssh.com")) == NULL)
fatal_f("kex_names_cat");
for (i = 0; i < PROPOSAL_MAX; i++) {
switch(i) {
case PROPOSAL_KEX_ALGS:
prop[i] = compat_kex_proposal(ssh,
kexalgos ? kexalgos : defprop[i]);
prop[i] = compat_kex_proposal(ssh, cp);
break;
case PROPOSAL_ENC_ALGS_CTOS:
case PROPOSAL_ENC_ALGS_STOC:
@ -363,6 +381,7 @@ kex_proposal_populate_entries(struct ssh *ssh, char *prop[PROPOSAL_MAX],
prop[i] = xstrdup(defprop[i]);
}
}
free(cp);
}
void
@ -466,7 +485,12 @@ 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)
@ -563,7 +587,7 @@ kex_input_ext_info(int type, u_int32_t seq, struct ssh *ssh)
if (ninfo >= 1024) {
error("SSH2_MSG_EXT_INFO with too many entries, expected "
"<=1024, received %u", ninfo);
return SSH_ERR_INVALID_FORMAT;
return dispatch_protocol_error(type, seq, ssh);
}
for (i = 0; i < ninfo; i++) {
if ((r = sshpkt_get_cstring(ssh, &name, NULL)) != 0)
@ -681,7 +705,7 @@ 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;
@ -717,7 +741,7 @@ 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)
@ -981,20 +1005,14 @@ 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;
@ -1019,13 +1037,23 @@ 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 */

3
kex.h
View File

@ -1,4 +1,4 @@
/* $OpenBSD: kex.h,v 1.119 2023/08/28 03:28:43 djm Exp $ */
/* $OpenBSD: kex.h,v 1.120 2023/12/18 14:45:17 djm Exp $ */
/*
* Copyright (c) 2000, 2001 Markus Friedl. All rights reserved.
@ -149,6 +149,7 @@ 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;

99
packet.c
View File

@ -1,4 +1,4 @@
/* $OpenBSD: packet.c,v 1.312 2023/08/28 03:31:16 djm Exp $ */
/* $OpenBSD: packet.c,v 1.313 2023/12/18 14:45:17 djm Exp $ */
/*
* Author: Tatu Ylonen <ylo@cs.hut.fi>
* Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@ -1207,8 +1207,13 @@ 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;
@ -1216,6 +1221,11 @@ 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)
@ -1344,8 +1354,7 @@ 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)
@ -1416,29 +1425,6 @@ ssh_packet_read(struct ssh *ssh)
return type;
}
/*
* Waits until a packet has been received, verifies that its type matches
* that given, and gives a fatal error and exits if there is a mismatch.
*/
int
ssh_packet_read_expect(struct ssh *ssh, u_int expected_type)
{
int r;
u_char type;
if ((r = ssh_packet_read_seqnr(ssh, &type, NULL)) != 0)
return r;
if (type != expected_type) {
if ((r = sshpkt_disconnect(ssh,
"Protocol error: expected packet type %d, got %d",
expected_type, type)) != 0)
return r;
return SSH_ERR_PROTOCOL_ERROR;
}
return 0;
}
static int
ssh_packet_read_poll2_mux(struct ssh *ssh, u_char *typep, u_int32_t *seqnr_p)
{
@ -1629,10 +1615,16 @@ 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;
@ -1698,6 +1690,10 @@ 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;
@ -1720,10 +1716,39 @@ 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) {
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");
@ -1738,19 +1763,6 @@ 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;
@ -2242,6 +2254,7 @@ 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 ||
@ -2404,6 +2417,7 @@ 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 ||
@ -2732,6 +2746,7 @@ 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 ||

3
packet.h
View File

@ -1,4 +1,4 @@
/* $OpenBSD: packet.h,v 1.95 2023/08/28 03:31:16 djm Exp $ */
/* $OpenBSD: packet.h,v 1.96 2023/12/18 14:45:17 djm Exp $ */
/*
* Author: Tatu Ylonen <ylo@cs.hut.fi>
@ -124,7 +124,6 @@ int ssh_packet_send2_wrapped(struct ssh *);
int ssh_packet_send2(struct ssh *);
int ssh_packet_read(struct ssh *);
int ssh_packet_read_expect(struct ssh *, u_int type);
int ssh_packet_read_poll(struct ssh *);
int ssh_packet_read_poll2(struct ssh *, u_char *, u_int32_t *seqnr_p);
int ssh_packet_process_incoming(struct ssh *, const char *buf, u_int len);

12
sshconnect2.c
View File

@ -1,4 +1,4 @@
/* $OpenBSD: sshconnect2.c,v 1.369 2023/12/13 03:28:19 djm Exp $ */
/* $OpenBSD: sshconnect2.c,v 1.370 2023/12/18 14:45:17 djm Exp $ */
/*
* Copyright (c) 2000 Markus Friedl. All rights reserved.
* Copyright (c) 2008 Damien Miller. All rights reserved.
@ -358,7 +358,6 @@ 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 *);
@ -472,7 +471,7 @@ 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);
@ -531,12 +530,6 @@ input_userauth_service_accept(int type, u_int32_t seq, struct ssh *ssh)
return r;
}
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)
{
@ -615,6 +608,7 @@ 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;
}