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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: ssh_api.c,v 1.29 2024/05/17 00:30:24 djm Exp $ */
/*
 * Copyright (c) 2012 Markus Friedl.  All rights reserved.
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */

#include <sys/types.h>

#include <stdio.h>
#include <stdlib.h>

#include "ssh_api.h"
#include "compat.h"
#include "log.h"
#include "authfile.h"
#include "sshkey.h"
#include "dh.h"
#include "misc.h"
#include "ssh2.h"
#include "version.h"
#include "myproposal.h"
#include "ssherr.h"
#include "sshbuf.h"

#include <string.h>

int	_ssh_exchange_banner(struct ssh *);
int	_ssh_send_banner(struct ssh *, struct sshbuf *);
int	_ssh_read_banner(struct ssh *, struct sshbuf *);
int	_ssh_order_hostkeyalgs(struct ssh *);
int	_ssh_verify_host_key(struct sshkey *, struct ssh *);
struct sshkey *_ssh_host_public_key(int, int, struct ssh *);
struct sshkey *_ssh_host_private_key(int, int, struct ssh *);
int	_ssh_host_key_sign(struct ssh *, struct sshkey *, struct sshkey *,
    u_char **, size_t *, const u_char *, size_t, const char *);

/*
 * stubs for privsep calls in the server side implementation of kex.
 */
int	mm_sshkey_sign(struct sshkey *, u_char **, u_int *,
    const u_char *, u_int, const char *, const char *, const char *, u_int);

#ifdef WITH_OPENSSL
DH	*mm_choose_dh(int, int, int);
#endif

int
mm_sshkey_sign(struct sshkey *key, u_char **sigp, u_int *lenp,
    const u_char *data, u_int datalen, const char *alg,
    const char *sk_provider, const char *sk_pin, u_int compat)
{
	size_t slen = 0;
	int ret;

	ret = sshkey_sign(key, sigp, &slen, data, datalen, alg,
	    sk_provider, sk_pin, compat);
	*lenp = slen;
	return ret;
}

#ifdef WITH_OPENSSL
DH *
mm_choose_dh(int min, int nbits, int max)
{
	return choose_dh(min, nbits, max);
}
#endif

/* API */

int
ssh_init(struct ssh **sshp, int is_server, struct kex_params *kex_params)
{
	char *myproposal[PROPOSAL_MAX] = { KEX_CLIENT };
	char *populated[PROPOSAL_MAX];
	struct ssh *ssh;
	char **proposal;
	static int called;
	int r;

	if (!called) {
#ifdef WITH_OPENSSL
		OpenSSL_add_all_algorithms();
#endif
		called = 1;
	}

	if ((ssh = ssh_packet_set_connection(NULL, -1, -1)) == NULL)
		return SSH_ERR_ALLOC_FAIL;
	if (is_server)
		ssh_packet_set_server(ssh);

	/* Initialize key exchange */
	proposal = kex_params ? kex_params->proposal : myproposal;
	kex_proposal_populate_entries(ssh, populated,
	    proposal[PROPOSAL_KEX_ALGS],
	    proposal[PROPOSAL_ENC_ALGS_CTOS],
	    proposal[PROPOSAL_MAC_ALGS_CTOS],
	    proposal[PROPOSAL_COMP_ALGS_CTOS],
	    proposal[PROPOSAL_SERVER_HOST_KEY_ALGS]);
	r = kex_ready(ssh, populated);
	kex_proposal_free_entries(populated);
	if (r != 0) {
		ssh_free(ssh);
		return r;
	}

	ssh->kex->server = is_server;
	if (is_server) {
#ifdef WITH_OPENSSL
		ssh->kex->kex[KEX_DH_GRP1_SHA1] = kex_gen_server;
		ssh->kex->kex[KEX_DH_GRP14_SHA1] = kex_gen_server;
		ssh->kex->kex[KEX_DH_GRP14_SHA256] = kex_gen_server;
		ssh->kex->kex[KEX_DH_GRP16_SHA512] = kex_gen_server;
		ssh->kex->kex[KEX_DH_GRP18_SHA512] = kex_gen_server;
		ssh->kex->kex[KEX_DH_GEX_SHA1] = kexgex_server;
		ssh->kex->kex[KEX_DH_GEX_SHA256] = kexgex_server;
		ssh->kex->kex[KEX_ECDH_SHA2] = kex_gen_server;
#endif /* WITH_OPENSSL */
		ssh->kex->kex[KEX_C25519_SHA256] = kex_gen_server;
		ssh->kex->kex[KEX_KEM_SNTRUP761X25519_SHA512] = kex_gen_server;
		ssh->kex->load_host_public_key=&_ssh_host_public_key;
		ssh->kex->load_host_private_key=&_ssh_host_private_key;
		ssh->kex->sign=&_ssh_host_key_sign;
	} else {
#ifdef WITH_OPENSSL
		ssh->kex->kex[KEX_DH_GRP1_SHA1] = kex_gen_client;
		ssh->kex->kex[KEX_DH_GRP14_SHA1] = kex_gen_client;
		ssh->kex->kex[KEX_DH_GRP14_SHA256] = kex_gen_client;
		ssh->kex->kex[KEX_DH_GRP16_SHA512] = kex_gen_client;
		ssh->kex->kex[KEX_DH_GRP18_SHA512] = kex_gen_client;
		ssh->kex->kex[KEX_DH_GEX_SHA1] = kexgex_client;
		ssh->kex->kex[KEX_DH_GEX_SHA256] = kexgex_client;
		ssh->kex->kex[KEX_ECDH_SHA2] = kex_gen_client;
#endif /* WITH_OPENSSL */
		ssh->kex->kex[KEX_C25519_SHA256] = kex_gen_client;
		ssh->kex->kex[KEX_KEM_SNTRUP761X25519_SHA512] = kex_gen_client;
		ssh->kex->verify_host_key =&_ssh_verify_host_key;
	}
	*sshp = ssh;
	return 0;
}

void
ssh_free(struct ssh *ssh)
{
	struct key_entry *k;

	if (ssh == NULL)
		return;

	/*
	 * we've only created the public keys variants in case we
	 * are a acting as a server.
	 */
	while ((k = TAILQ_FIRST(&ssh->public_keys)) != NULL) {
		TAILQ_REMOVE(&ssh->public_keys, k, next);
		if (ssh->kex && ssh->kex->server)
			sshkey_free(k->key);
		free(k);
	}
	while ((k = TAILQ_FIRST(&ssh->private_keys)) != NULL) {
		TAILQ_REMOVE(&ssh->private_keys, k, next);
		free(k);
	}
	ssh_packet_close(ssh);
	free(ssh);
}

void
ssh_set_app_data(struct ssh *ssh, void *app_data)
{
	ssh->app_data = app_data;
}

void *
ssh_get_app_data(struct ssh *ssh)
{
	return ssh->app_data;
}

/* Returns < 0 on error, 0 otherwise */
int
ssh_add_hostkey(struct ssh *ssh, struct sshkey *key)
{
	struct sshkey *pubkey = NULL;
	struct key_entry *k = NULL, *k_prv = NULL;
	int r;

	if (ssh->kex->server) {
		if ((r = sshkey_from_private(key, &pubkey)) != 0)
			return r;
		if ((k = malloc(sizeof(*k))) == NULL ||
		    (k_prv = malloc(sizeof(*k_prv))) == NULL) {
			free(k);
			sshkey_free(pubkey);
			return SSH_ERR_ALLOC_FAIL;
		}
		k_prv->key = key;
		TAILQ_INSERT_TAIL(&ssh->private_keys, k_prv, next);

		/* add the public key, too */
		k->key = pubkey;
		TAILQ_INSERT_TAIL(&ssh->public_keys, k, next);
		r = 0;
	} else {
		if ((k = malloc(sizeof(*k))) == NULL)
			return SSH_ERR_ALLOC_FAIL;
		k->key = key;
		TAILQ_INSERT_TAIL(&ssh->public_keys, k, next);
		r = 0;
	}

	return r;
}

int
ssh_set_verify_host_key_callback(struct ssh *ssh,
    int (*cb)(struct sshkey *, struct ssh *))
{
	if (cb == NULL || ssh->kex == NULL)
		return SSH_ERR_INVALID_ARGUMENT;

	ssh->kex->verify_host_key = cb;

	return 0;
}

int
ssh_input_append(struct ssh *ssh, const u_char *data, size_t len)
{
	return sshbuf_put(ssh_packet_get_input(ssh), data, len);
}

int
ssh_packet_next(struct ssh *ssh, u_char *typep)
{
	int r;
	u_int32_t seqnr;
	u_char type;

	/*
	 * Try to read a packet. Return SSH_MSG_NONE if no packet or not
	 * enough data.
	 */
	*typep = SSH_MSG_NONE;
	if (sshbuf_len(ssh->kex->client_version) == 0 ||
	    sshbuf_len(ssh->kex->server_version) == 0)
		return _ssh_exchange_banner(ssh);
	/*
	 * If we enough data and a dispatch function then
	 * call the function and get the next packet.
	 * Otherwise return the packet type to the caller so it
	 * can decide how to go on.
	 *
	 * We will only call the dispatch function for:
	 *     20-29    Algorithm negotiation
	 *     30-49    Key exchange method specific (numbers can be reused for
	 *              different authentication methods)
	 */
	for (;;) {
		if ((r = ssh_packet_read_poll2(ssh, &type, &seqnr)) != 0)
			return r;
		if (type > 0 && type < DISPATCH_MAX &&
		    type >= SSH2_MSG_KEXINIT && type <= SSH2_MSG_TRANSPORT_MAX &&
		    ssh->dispatch[type] != NULL) {
			if ((r = (*ssh->dispatch[type])(type, seqnr, ssh)) != 0)
				return r;
		} else {
			*typep = type;
			return 0;
		}
	}
}

const u_char *
ssh_packet_payload(struct ssh *ssh, size_t *lenp)
{
	return sshpkt_ptr(ssh, lenp);
}

int
ssh_packet_put(struct ssh *ssh, int type, const u_char *data, size_t len)
{
	int r;

	if ((r = sshpkt_start(ssh, type)) != 0 ||
	    (r = sshpkt_put(ssh, data, len)) != 0 ||
	    (r = sshpkt_send(ssh)) != 0)
		return r;
	return 0;
}

const u_char *
ssh_output_ptr(struct ssh *ssh, size_t *len)
{
	struct sshbuf *output = ssh_packet_get_output(ssh);

	*len = sshbuf_len(output);
	return sshbuf_ptr(output);
}

int
ssh_output_consume(struct ssh *ssh, size_t len)
{
	return sshbuf_consume(ssh_packet_get_output(ssh), len);
}

int
ssh_output_space(struct ssh *ssh, size_t len)
{
	return (0 == sshbuf_check_reserve(ssh_packet_get_output(ssh), len));
}

int
ssh_input_space(struct ssh *ssh, size_t len)
{
	return (0 == sshbuf_check_reserve(ssh_packet_get_input(ssh), len));
}

/* Read other side's version identification. */
int
_ssh_read_banner(struct ssh *ssh, struct sshbuf *banner)
{
	struct sshbuf *input = ssh_packet_get_input(ssh);
	const char *mismatch = "Protocol mismatch.\r\n";
	const u_char *s = sshbuf_ptr(input);
	u_char c;
	char *cp = NULL, *remote_version = NULL;
	int r = 0, remote_major, remote_minor, expect_nl;
	size_t n, j;

	for (j = n = 0;;) {
		sshbuf_reset(banner);
		expect_nl = 0;
		for (;;) {
			if (j >= sshbuf_len(input))
				return 0; /* insufficient data in input buf */
			c = s[j++];
			if (c == '\r') {
				expect_nl = 1;
				continue;
			}
			if (c == '\n')
				break;
			if (expect_nl)
				goto bad;
			if ((r = sshbuf_put_u8(banner, c)) != 0)
				return r;
			if (sshbuf_len(banner) > SSH_MAX_BANNER_LEN)
				goto bad;
		}
		if (sshbuf_len(banner) >= 4 &&
		    memcmp(sshbuf_ptr(banner), "SSH-", 4) == 0)
			break;
		debug_f("%.*s", (int)sshbuf_len(banner),
		    sshbuf_ptr(banner));
		/* Accept lines before banner only on client */
		if (ssh->kex->server || ++n > SSH_MAX_PRE_BANNER_LINES) {
  bad:
			if ((r = sshbuf_put(ssh_packet_get_output(ssh),
			    mismatch, strlen(mismatch))) != 0)
				return r;
			return SSH_ERR_NO_PROTOCOL_VERSION;
		}
	}
	if ((r = sshbuf_consume(input, j)) != 0)
		return r;

	/* XXX remote version must be the same size as banner for sscanf */
	if ((cp = sshbuf_dup_string(banner)) == NULL ||
	    (remote_version = calloc(1, sshbuf_len(banner))) == NULL) {
		r = SSH_ERR_ALLOC_FAIL;
		goto out;
	}

	/*
	 * Check that the versions match.  In future this might accept
	 * several versions and set appropriate flags to handle them.
	 */
	if (sscanf(cp, "SSH-%d.%d-%[^\n]\n",
	    &remote_major, &remote_minor, remote_version) != 3) {
		r = SSH_ERR_INVALID_FORMAT;
		goto out;
	}
	debug("Remote protocol version %d.%d, remote software version %.100s",
	    remote_major, remote_minor, remote_version);

	compat_banner(ssh, remote_version);
	if  (remote_major == 1 && remote_minor == 99) {
		remote_major = 2;
		remote_minor = 0;
	}
	if (remote_major != 2)
		r = SSH_ERR_PROTOCOL_MISMATCH;

	debug("Remote version string %.100s", cp);
 out:
	free(cp);
	free(remote_version);
	return r;
}

/* Send our own protocol version identification. */
int
_ssh_send_banner(struct ssh *ssh, struct sshbuf *banner)
{
	char *cp;
	int r;

	if ((r = sshbuf_putf(banner, "SSH-2.0-%.100s\r\n", SSH_VERSION)) != 0)
		return r;
	if ((r = sshbuf_putb(ssh_packet_get_output(ssh), banner)) != 0)
		return r;
	/* Remove trailing \r\n */
	if ((r = sshbuf_consume_end(banner, 2)) != 0)
		return r;
	if ((cp = sshbuf_dup_string(banner)) == NULL)
		return SSH_ERR_ALLOC_FAIL;
	debug("Local version string %.100s", cp);
	free(cp);
	return 0;
}

int
_ssh_exchange_banner(struct ssh *ssh)
{
	struct kex *kex = ssh->kex;
	int r;

	/*
	 * if _ssh_read_banner() cannot parse a full version string
	 * it will return NULL and we end up calling it again.
	 */

	r = 0;
	if (kex->server) {
		if (sshbuf_len(ssh->kex->server_version) == 0)
			r = _ssh_send_banner(ssh, ssh->kex->server_version);
		if (r == 0 &&
		    sshbuf_len(ssh->kex->server_version) != 0 &&
		    sshbuf_len(ssh->kex->client_version) == 0)
			r = _ssh_read_banner(ssh, ssh->kex->client_version);
	} else {
		if (sshbuf_len(ssh->kex->server_version) == 0)
			r = _ssh_read_banner(ssh, ssh->kex->server_version);
		if (r == 0 &&
		    sshbuf_len(ssh->kex->server_version) != 0 &&
		    sshbuf_len(ssh->kex->client_version) == 0)
			r = _ssh_send_banner(ssh, ssh->kex->client_version);
	}
	if (r != 0)
		return r;
	/* start initial kex as soon as we have exchanged the banners */
	if (sshbuf_len(ssh->kex->server_version) != 0 &&
	    sshbuf_len(ssh->kex->client_version) != 0) {
		if ((r = _ssh_order_hostkeyalgs(ssh)) != 0 ||
		    (r = kex_send_kexinit(ssh)) != 0)
			return r;
	}
	return 0;
}

struct sshkey *
_ssh_host_public_key(int type, int nid, struct ssh *ssh)
{
	struct key_entry *k;

	debug3_f("need %d", type);
	TAILQ_FOREACH(k, &ssh->public_keys, next) {
		debug3_f("check %s", sshkey_type(k->key));
		if (k->key->type == type &&
		    (type != KEY_ECDSA || k->key->ecdsa_nid == nid))
			return (k->key);
	}
	return (NULL);
}

struct sshkey *
_ssh_host_private_key(int type, int nid, struct ssh *ssh)
{
	struct key_entry *k;

	debug3_f("need %d", type);
	TAILQ_FOREACH(k, &ssh->private_keys, next) {
		debug3_f("check %s", sshkey_type(k->key));
		if (k->key->type == type &&
		    (type != KEY_ECDSA || k->key->ecdsa_nid == nid))
			return (k->key);
	}
	return (NULL);
}

int
_ssh_verify_host_key(struct sshkey *hostkey, struct ssh *ssh)
{
	struct key_entry *k;

	debug3_f("need %s", sshkey_type(hostkey));
	TAILQ_FOREACH(k, &ssh->public_keys, next) {
		debug3_f("check %s", sshkey_type(k->key));
		if (sshkey_equal_public(hostkey, k->key))
			return (0);	/* ok */
	}
	return (-1);	/* failed */
}

/* offer hostkey algorithms in kexinit depending on registered keys */
int
_ssh_order_hostkeyalgs(struct ssh *ssh)
{
	struct key_entry *k;
	char *orig, *avail, *oavail = NULL, *alg, *replace = NULL;
	char **proposal;
	size_t maxlen;
	int ktype, r;

	/* XXX we de-serialize ssh->kex->my, modify it, and change it */
	if ((r = kex_buf2prop(ssh->kex->my, NULL, &proposal)) != 0)
		return r;
	orig = proposal[PROPOSAL_SERVER_HOST_KEY_ALGS];
	if ((oavail = avail = strdup(orig)) == NULL) {
		r = SSH_ERR_ALLOC_FAIL;
		goto out;
	}
	maxlen = strlen(avail) + 1;
	if ((replace = calloc(1, maxlen)) == NULL) {
		r = SSH_ERR_ALLOC_FAIL;
		goto out;
	}
	*replace = '\0';
	while ((alg = strsep(&avail, ",")) && *alg != '\0') {
		if ((ktype = sshkey_type_from_name(alg)) == KEY_UNSPEC)
			continue;
		TAILQ_FOREACH(k, &ssh->public_keys, next) {
			if (k->key->type == ktype ||
			    (sshkey_is_cert(k->key) && k->key->type ==
			    sshkey_type_plain(ktype))) {
				if (*replace != '\0')
					strlcat(replace, ",", maxlen);
				strlcat(replace, alg, maxlen);
				break;
			}
		}
	}
	if (*replace != '\0') {
		debug2_f("orig/%d    %s", ssh->kex->server, orig);
		debug2_f("replace/%d %s", ssh->kex->server, replace);
		free(orig);
		proposal[PROPOSAL_SERVER_HOST_KEY_ALGS] = replace;
		replace = NULL;	/* owned by proposal */
		r = kex_prop2buf(ssh->kex->my, proposal);
	}
 out:
	free(oavail);
	free(replace);
	kex_prop_free(proposal);
	return r;
}

int
_ssh_host_key_sign(struct ssh *ssh, struct sshkey *privkey,
    struct sshkey *pubkey, u_char **signature, size_t *slen,
    const u_char *data, size_t dlen, const char *alg)
{
	return sshkey_sign(privkey, signature, slen, data, dlen,
	    alg, NULL, NULL, ssh->compat);
}