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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.

OpenBSD-Commit-ID: 2a66ac962f0a630d7945fee54004ed9e9c439f14

Approved by:	so (implicit)
Obtained from:	https://anongit.mindrot.org/openssh.git/patch/?id=1edb00c58f8a6875fad6a497aa2bacf37f9e6cd5
Security:	CVE-2023-48795
Security:	FreeBSD-SA-23:19.openssh

(cherry picked from commit 92f58c69a14c0afe910145f177c0e8aeaf9c7da4)
(cherry picked from commit 3bafcb9744c966ff7750357b24fc7942f2d928d4)
This commit is contained in:
Gordon Tetlow 2023-12-18 08:22:22 -08:00 committed by Franco Fichtner
parent ce33e9170e
commit 888363ef25
6 changed files with 142 additions and 79 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

26
crypto/openssh/PROTOCOL
View File

@ -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"

82
crypto/openssh/kex.c
View File

@ -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)
@ -544,7 +568,7 @@ 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)
@ -660,7 +684,7 @@ 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;
@ -696,7 +720,7 @@ 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 +984,14 @@ 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 +1016,23 @@ 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 */

1
crypto/openssh/kex.h
View File

@ -148,6 +148,7 @@ 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;

101
crypto/openssh/packet.c
View File

@ -1206,8 +1206,13 @@ 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;
@ -1215,6 +1220,11 @@ 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)
@ -1343,8 +1353,7 @@ 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)
@ -1415,29 +1424,6 @@ 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)
{
@ -1628,10 +1614,16 @@ 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;
@ -1697,6 +1689,10 @@ 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;
@ -1717,10 +1713,39 @@ 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");
@ -1735,19 +1760,6 @@ 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;
@ -2213,6 +2225,7 @@ 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 ||
@ -2375,6 +2388,7 @@ 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 ||
@ -2703,6 +2717,7 @@ 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 ||

1
crypto/openssh/packet.h
View File

@ -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);

10
crypto/openssh/sshconnect2.c
View File

@ -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);
@ -523,12 +522,6 @@ 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)
{
@ -607,6 +600,7 @@ 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;
}