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Start the process of splitting sshd into separate binaries. This step
splits sshd into a listener and a session binary. More splits are
planned.

After this changes, the listener binary will validate the configuration,
load the hostkeys, listen on port 22 and manage MaxStartups only. All
session handling will be performed by a new sshd-session binary that the
listener fork+execs.

This reduces the listener process to the minimum necessary and sets us
up for future work on the sshd-session binary.

feedback/ok markus@ deraadt@

NB. if you're updating via source, please restart sshd after installing,
otherwise you run the risk of locking yourself out.

/* $OpenBSD: kexgexs.c,v 1.47 2024/05/17 00:30:23 djm Exp $ */
/*
 * Copyright (c) 2000 Niels Provos.  All rights reserved.
 * Copyright (c) 2001 Markus Friedl.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */


#include <stdio.h>
#include <string.h>
#include <signal.h>

#include <openssl/dh.h>

#include "sshkey.h"
#include "cipher.h"
#include "digest.h"
#include "kex.h"
#include "log.h"
#include "packet.h"
#include "dh.h"
#include "ssh2.h"
#ifdef GSSAPI
#include "ssh-gss.h"
#endif
#include "monitor_wrap.h"
#include "dispatch.h"
#include "ssherr.h"
#include "sshbuf.h"
#include "misc.h"

static int input_kex_dh_gex_request(int, u_int32_t, struct ssh *);
static int input_kex_dh_gex_init(int, u_int32_t, struct ssh *);

int
kexgex_server(struct ssh *ssh)
{
	ssh_dispatch_set(ssh, SSH2_MSG_KEX_DH_GEX_REQUEST,
	    &input_kex_dh_gex_request);
	debug("expecting SSH2_MSG_KEX_DH_GEX_REQUEST");
	return 0;
}

static int
input_kex_dh_gex_request(int type, u_int32_t seq, struct ssh *ssh)
{
	struct kex *kex = ssh->kex;
	int r;
	u_int min = 0, max = 0, nbits = 0;
	const BIGNUM *dh_p, *dh_g;

	debug("SSH2_MSG_KEX_DH_GEX_REQUEST received");
	ssh_dispatch_set(ssh, SSH2_MSG_KEX_DH_GEX_REQUEST, &kex_protocol_error);

	if ((r = sshpkt_get_u32(ssh, &min)) != 0 ||
	    (r = sshpkt_get_u32(ssh, &nbits)) != 0 ||
	    (r = sshpkt_get_u32(ssh, &max)) != 0 ||
	    (r = sshpkt_get_end(ssh)) != 0)
		goto out;
	kex->nbits = nbits;
	kex->min = min;
	kex->max = max;
	min = MAXIMUM(DH_GRP_MIN, min);
	max = MINIMUM(DH_GRP_MAX, max);
	nbits = MAXIMUM(DH_GRP_MIN, nbits);
	nbits = MINIMUM(DH_GRP_MAX, nbits);

	if (kex->max < kex->min || kex->nbits < kex->min ||
	    kex->max < kex->nbits || kex->max < DH_GRP_MIN) {
		r = SSH_ERR_DH_GEX_OUT_OF_RANGE;
		goto out;
	}

	/* Contact privileged parent */
	kex->dh = mm_choose_dh(min, nbits, max);
	if (kex->dh == NULL) {
		(void)sshpkt_disconnect(ssh, "no matching DH grp found");
		r = SSH_ERR_ALLOC_FAIL;
		goto out;
	}
	debug("SSH2_MSG_KEX_DH_GEX_GROUP sent");
	DH_get0_pqg(kex->dh, &dh_p, NULL, &dh_g);
	if ((r = sshpkt_start(ssh, SSH2_MSG_KEX_DH_GEX_GROUP)) != 0 ||
	    (r = sshpkt_put_bignum2(ssh, dh_p)) != 0 ||
	    (r = sshpkt_put_bignum2(ssh, dh_g)) != 0 ||
	    (r = sshpkt_send(ssh)) != 0)
		goto out;

	/* Compute our exchange value in parallel with the client */
	if ((r = dh_gen_key(kex->dh, kex->we_need * 8)) != 0)
		goto out;

	debug("expecting SSH2_MSG_KEX_DH_GEX_INIT");
	ssh_dispatch_set(ssh, SSH2_MSG_KEX_DH_GEX_INIT, &input_kex_dh_gex_init);
	r = 0;
 out:
	return r;
}

static int
input_kex_dh_gex_init(int type, u_int32_t seq, struct ssh *ssh)
{
	struct kex *kex = ssh->kex;
	BIGNUM *dh_client_pub = NULL;
	const BIGNUM *pub_key, *dh_p, *dh_g;
	struct sshbuf *shared_secret = NULL;
	struct sshbuf *server_host_key_blob = NULL;
	struct sshkey *server_host_public, *server_host_private;
	u_char *signature = NULL;
	u_char hash[SSH_DIGEST_MAX_LENGTH];
	size_t slen, hashlen;
	int r;

	debug("SSH2_MSG_KEX_DH_GEX_INIT received");
	ssh_dispatch_set(ssh, SSH2_MSG_KEX_DH_GEX_INIT, &kex_protocol_error);

	if ((r = kex_load_hostkey(ssh, &server_host_private,
	    &server_host_public)) != 0)
		goto out;

	/* key, cert */
	if ((r = sshpkt_get_bignum2(ssh, &dh_client_pub)) != 0 ||
	    (r = sshpkt_get_end(ssh)) != 0)
		goto out;
	if ((shared_secret = sshbuf_new()) == NULL) {
		r = SSH_ERR_ALLOC_FAIL;
		goto out;
	}
	if ((r = kex_dh_compute_key(kex, dh_client_pub, shared_secret)) != 0)
		goto out;
	if ((server_host_key_blob = sshbuf_new()) == NULL) {
		r = SSH_ERR_ALLOC_FAIL;
		goto out;
	}
	if ((r = sshkey_putb(server_host_public, server_host_key_blob)) != 0)
		goto out;

	/* calc H */
	DH_get0_key(kex->dh, &pub_key, NULL);
	DH_get0_pqg(kex->dh, &dh_p, NULL, &dh_g);
	hashlen = sizeof(hash);
	if ((r = kexgex_hash(
	    kex->hash_alg,
	    kex->client_version,
	    kex->server_version,
	    kex->peer,
	    kex->my,
	    server_host_key_blob,
	    kex->min, kex->nbits, kex->max,
	    dh_p, dh_g,
	    dh_client_pub,
	    pub_key,
	    sshbuf_ptr(shared_secret), sshbuf_len(shared_secret),
	    hash, &hashlen)) != 0)
		goto out;

	/* sign H */
	if ((r = kex->sign(ssh, server_host_private, server_host_public,
	    &signature, &slen, hash, hashlen, kex->hostkey_alg)) < 0)
		goto out;

	/* send server hostkey, DH pubkey 'f' and signed H */
	if ((r = sshpkt_start(ssh, SSH2_MSG_KEX_DH_GEX_REPLY)) != 0 ||
	    (r = sshpkt_put_stringb(ssh, server_host_key_blob)) != 0 ||
	    (r = sshpkt_put_bignum2(ssh, pub_key)) != 0 ||     /* f */
	    (r = sshpkt_put_string(ssh, signature, slen)) != 0 ||
	    (r = sshpkt_send(ssh)) != 0)
		goto out;

	if ((r = kex_derive_keys(ssh, hash, hashlen, shared_secret)) != 0 ||
	    (r = kex_send_newkeys(ssh)) != 0)
		goto out;

	/* retain copy of hostkey used at initial KEX */
	if (kex->initial_hostkey == NULL &&
	    (r = sshkey_from_private(server_host_public,
	    &kex->initial_hostkey)) != 0)
		goto out;
	/* success */
 out:
	explicit_bzero(hash, sizeof(hash));
	DH_free(kex->dh);
	kex->dh = NULL;
	BN_clear_free(dh_client_pub);
	sshbuf_free(shared_secret);
	sshbuf_free(server_host_key_blob);
	free(signature);
	return r;
}