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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: auth.c,v 1.161 2024/05/17 00:30:23 djm Exp $ */
/*
 * Copyright (c) 2000 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 <sys/types.h>
#include <sys/stat.h>
#include <sys/socket.h>
#include <sys/wait.h>

#include <stdlib.h>
#include <errno.h>
#include <fcntl.h>
#include <login_cap.h>
#include <paths.h>
#include <pwd.h>
#include <stdarg.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <limits.h>
#include <netdb.h>
#include <time.h>

#include "xmalloc.h"
#include "match.h"
#include "groupaccess.h"
#include "log.h"
#include "sshbuf.h"
#include "misc.h"
#include "servconf.h"
#include "sshkey.h"
#include "hostfile.h"
#include "auth.h"
#include "auth-options.h"
#include "canohost.h"
#include "uidswap.h"
#include "packet.h"
#ifdef GSSAPI
#include "ssh-gss.h"
#endif
#include "authfile.h"
#include "monitor_wrap.h"
#include "ssherr.h"
#include "channels.h"

/* import */
extern ServerOptions options;
extern struct include_list includes;
extern struct sshauthopt *auth_opts;

/* Debugging messages */
static struct sshbuf *auth_debug;

/*
 * Check if the user is allowed to log in via ssh. If user is listed
 * in DenyUsers or one of user's groups is listed in DenyGroups, false
 * will be returned. If AllowUsers isn't empty and user isn't listed
 * there, or if AllowGroups isn't empty and one of user's groups isn't
 * listed there, false will be returned.
 * If the user's shell is not executable, false will be returned.
 * Otherwise true is returned.
 */
int
allowed_user(struct ssh *ssh, struct passwd * pw)
{
	struct stat st;
	const char *hostname = NULL, *ipaddr = NULL;
	int r;
	u_int i;

	/* Shouldn't be called if pw is NULL, but better safe than sorry... */
	if (!pw || !pw->pw_name)
		return 0;

	/*
	 * Deny if shell does not exist or is not executable unless we
	 * are chrooting.
	 */
	if (options.chroot_directory == NULL ||
	    strcasecmp(options.chroot_directory, "none") == 0) {
		char *shell = xstrdup((pw->pw_shell[0] == '\0') ?
		    _PATH_BSHELL : pw->pw_shell); /* empty = /bin/sh */

		if (stat(shell, &st) == -1) {
			logit("User %.100s not allowed because shell %.100s "
			    "does not exist", pw->pw_name, shell);
			free(shell);
			return 0;
		}
		if (S_ISREG(st.st_mode) == 0 ||
		    (st.st_mode & (S_IXOTH|S_IXUSR|S_IXGRP)) == 0) {
			logit("User %.100s not allowed because shell %.100s "
			    "is not executable", pw->pw_name, shell);
			free(shell);
			return 0;
		}
		free(shell);
	}

	if (options.num_deny_users > 0 || options.num_allow_users > 0 ||
	    options.num_deny_groups > 0 || options.num_allow_groups > 0) {
		hostname = auth_get_canonical_hostname(ssh, options.use_dns);
		ipaddr = ssh_remote_ipaddr(ssh);
	}

	/* Return false if user is listed in DenyUsers */
	if (options.num_deny_users > 0) {
		for (i = 0; i < options.num_deny_users; i++) {
			r = match_user(pw->pw_name, hostname, ipaddr,
			    options.deny_users[i]);
			if (r < 0) {
				fatal("Invalid DenyUsers pattern \"%.100s\"",
				    options.deny_users[i]);
			} else if (r != 0) {
				logit("User %.100s from %.100s not allowed "
				    "because listed in DenyUsers",
				    pw->pw_name, hostname);
				return 0;
			}
		}
	}
	/* Return false if AllowUsers isn't empty and user isn't listed there */
	if (options.num_allow_users > 0) {
		for (i = 0; i < options.num_allow_users; i++) {
			r = match_user(pw->pw_name, hostname, ipaddr,
			    options.allow_users[i]);
			if (r < 0) {
				fatal("Invalid AllowUsers pattern \"%.100s\"",
				    options.allow_users[i]);
			} else if (r == 1)
				break;
		}
		/* i < options.num_allow_users iff we break for loop */
		if (i >= options.num_allow_users) {
			logit("User %.100s from %.100s not allowed because "
			    "not listed in AllowUsers", pw->pw_name, hostname);
			return 0;
		}
	}
	if (options.num_deny_groups > 0 || options.num_allow_groups > 0) {
		/* Get the user's group access list (primary and supplementary) */
		if (ga_init(pw->pw_name, pw->pw_gid) == 0) {
			logit("User %.100s from %.100s not allowed because "
			    "not in any group", pw->pw_name, hostname);
			return 0;
		}

		/* Return false if one of user's groups is listed in DenyGroups */
		if (options.num_deny_groups > 0)
			if (ga_match(options.deny_groups,
			    options.num_deny_groups)) {
				ga_free();
				logit("User %.100s from %.100s not allowed "
				    "because a group is listed in DenyGroups",
				    pw->pw_name, hostname);
				return 0;
			}
		/*
		 * Return false if AllowGroups isn't empty and one of user's groups
		 * isn't listed there
		 */
		if (options.num_allow_groups > 0)
			if (!ga_match(options.allow_groups,
			    options.num_allow_groups)) {
				ga_free();
				logit("User %.100s from %.100s not allowed "
				    "because none of user's groups are listed "
				    "in AllowGroups", pw->pw_name, hostname);
				return 0;
			}
		ga_free();
	}
	/* We found no reason not to let this user try to log on... */
	return 1;
}

/*
 * Formats any key left in authctxt->auth_method_key for inclusion in
 * auth_log()'s message. Also includes authxtct->auth_method_info if present.
 */
static char *
format_method_key(Authctxt *authctxt)
{
	const struct sshkey *key = authctxt->auth_method_key;
	const char *methinfo = authctxt->auth_method_info;
	char *fp, *cafp, *ret = NULL;

	if (key == NULL)
		return NULL;

	if (sshkey_is_cert(key)) {
		fp = sshkey_fingerprint(key,
		    options.fingerprint_hash, SSH_FP_DEFAULT);
		cafp = sshkey_fingerprint(key->cert->signature_key,
		    options.fingerprint_hash, SSH_FP_DEFAULT);
		xasprintf(&ret, "%s %s ID %s (serial %llu) CA %s %s%s%s",
		    sshkey_type(key), fp == NULL ? "(null)" : fp,
		    key->cert->key_id,
		    (unsigned long long)key->cert->serial,
		    sshkey_type(key->cert->signature_key),
		    cafp == NULL ? "(null)" : cafp,
		    methinfo == NULL ? "" : ", ",
		    methinfo == NULL ? "" : methinfo);
		free(fp);
		free(cafp);
	} else {
		fp = sshkey_fingerprint(key, options.fingerprint_hash,
		    SSH_FP_DEFAULT);
		xasprintf(&ret, "%s %s%s%s", sshkey_type(key),
		    fp == NULL ? "(null)" : fp,
		    methinfo == NULL ? "" : ", ",
		    methinfo == NULL ? "" : methinfo);
		free(fp);
	}
	return ret;
}

void
auth_log(struct ssh *ssh, int authenticated, int partial,
    const char *method, const char *submethod)
{
	Authctxt *authctxt = (Authctxt *)ssh->authctxt;
	int level = SYSLOG_LEVEL_VERBOSE;
	const char *authmsg;
	char *extra = NULL;

	if (!mm_is_monitor() && !authctxt->postponed)
		return;

	/* Raise logging level */
	if (authenticated == 1 ||
	    !authctxt->valid ||
	    authctxt->failures >= options.max_authtries / 2 ||
	    strcmp(method, "password") == 0)
		level = SYSLOG_LEVEL_INFO;

	if (authctxt->postponed)
		authmsg = "Postponed";
	else if (partial)
		authmsg = "Partial";
	else
		authmsg = authenticated ? "Accepted" : "Failed";

	if ((extra = format_method_key(authctxt)) == NULL) {
		if (authctxt->auth_method_info != NULL)
			extra = xstrdup(authctxt->auth_method_info);
	}

	do_log2(level, "%s %s%s%s for %s%.100s from %.200s port %d ssh2%s%s",
	    authmsg,
	    method,
	    submethod != NULL ? "/" : "", submethod == NULL ? "" : submethod,
	    authctxt->valid ? "" : "invalid user ",
	    authctxt->user,
	    ssh_remote_ipaddr(ssh),
	    ssh_remote_port(ssh),
	    extra != NULL ? ": " : "",
	    extra != NULL ? extra : "");

	free(extra);
}

void
auth_maxtries_exceeded(struct ssh *ssh)
{
	Authctxt *authctxt = (Authctxt *)ssh->authctxt;

	error("maximum authentication attempts exceeded for "
	    "%s%.100s from %.200s port %d ssh2",
	    authctxt->valid ? "" : "invalid user ",
	    authctxt->user,
	    ssh_remote_ipaddr(ssh),
	    ssh_remote_port(ssh));
	ssh_packet_disconnect(ssh, "Too many authentication failures");
	/* NOTREACHED */
}

/*
 * Check whether root logins are disallowed.
 */
int
auth_root_allowed(struct ssh *ssh, const char *method)
{
	switch (options.permit_root_login) {
	case PERMIT_YES:
		return 1;
	case PERMIT_NO_PASSWD:
		if (strcmp(method, "publickey") == 0 ||
		    strcmp(method, "hostbased") == 0 ||
		    strcmp(method, "gssapi-with-mic") == 0)
			return 1;
		break;
	case PERMIT_FORCED_ONLY:
		if (auth_opts->force_command != NULL) {
			logit("Root login accepted for forced command.");
			return 1;
		}
		break;
	}
	logit("ROOT LOGIN REFUSED FROM %.200s port %d",
	    ssh_remote_ipaddr(ssh), ssh_remote_port(ssh));
	return 0;
}


/*
 * Given a template and a passwd structure, build a filename
 * by substituting % tokenised options. Currently, %% becomes '%',
 * %h becomes the home directory and %u the username.
 *
 * This returns a buffer allocated by xmalloc.
 */
char *
expand_authorized_keys(const char *filename, struct passwd *pw)
{
	char *file, uidstr[32], ret[PATH_MAX];
	int i;

	snprintf(uidstr, sizeof(uidstr), "%llu",
	    (unsigned long long)pw->pw_uid);
	file = percent_expand(filename, "h", pw->pw_dir,
	    "u", pw->pw_name, "U", uidstr, (char *)NULL);

	/*
	 * Ensure that filename starts anchored. If not, be backward
	 * compatible and prepend the '%h/'
	 */
	if (path_absolute(file))
		return (file);

	i = snprintf(ret, sizeof(ret), "%s/%s", pw->pw_dir, file);
	if (i < 0 || (size_t)i >= sizeof(ret))
		fatal("expand_authorized_keys: path too long");
	free(file);
	return (xstrdup(ret));
}

char *
authorized_principals_file(struct passwd *pw)
{
	if (options.authorized_principals_file == NULL)
		return NULL;
	return expand_authorized_keys(options.authorized_principals_file, pw);
}

/* return ok if key exists in sysfile or userfile */
HostStatus
check_key_in_hostfiles(struct passwd *pw, struct sshkey *key, const char *host,
    const char *sysfile, const char *userfile)
{
	char *user_hostfile;
	struct stat st;
	HostStatus host_status;
	struct hostkeys *hostkeys;
	const struct hostkey_entry *found;

	hostkeys = init_hostkeys();
	load_hostkeys(hostkeys, host, sysfile, 0);
	if (userfile != NULL) {
		user_hostfile = tilde_expand_filename(userfile, pw->pw_uid);
		if (options.strict_modes &&
		    (stat(user_hostfile, &st) == 0) &&
		    ((st.st_uid != 0 && st.st_uid != pw->pw_uid) ||
		    (st.st_mode & 022) != 0)) {
			logit("Authentication refused for %.100s: "
			    "bad owner or modes for %.200s",
			    pw->pw_name, user_hostfile);
			auth_debug_add("Ignored %.200s: bad ownership or modes",
			    user_hostfile);
		} else {
			temporarily_use_uid(pw);
			load_hostkeys(hostkeys, host, user_hostfile, 0);
			restore_uid();
		}
		free(user_hostfile);
	}
	host_status = check_key_in_hostkeys(hostkeys, key, &found);
	if (host_status == HOST_REVOKED)
		error("WARNING: revoked key for %s attempted authentication",
		    host);
	else if (host_status == HOST_OK)
		debug_f("key for %s found at %s:%ld",
		    found->host, found->file, found->line);
	else
		debug_f("key for host %s not found", host);

	free_hostkeys(hostkeys);

	return host_status;
}

struct passwd *
getpwnamallow(struct ssh *ssh, const char *user)
{
	extern login_cap_t *lc;
	auth_session_t *as;
	struct passwd *pw;
	struct connection_info *ci;
	u_int i;

	ci = server_get_connection_info(ssh, 1, options.use_dns);
	ci->user = user;
	parse_server_match_config(&options, &includes, ci);
	log_change_level(options.log_level);
	log_verbose_reset();
	for (i = 0; i < options.num_log_verbose; i++)
		log_verbose_add(options.log_verbose[i]);
	server_process_permitopen(ssh);

	pw = getpwnam(user);
	if (pw == NULL) {
		logit("Invalid user %.100s from %.100s port %d",
		    user, ssh_remote_ipaddr(ssh), ssh_remote_port(ssh));
		return (NULL);
	}
	if (!allowed_user(ssh, pw))
		return (NULL);
	if ((lc = login_getclass(pw->pw_class)) == NULL) {
		debug("unable to get login class: %s", user);
		return (NULL);
	}
	if ((as = auth_open()) == NULL || auth_setpwd(as, pw) != 0 ||
	    auth_approval(as, lc, pw->pw_name, "ssh") <= 0) {
		debug("Approval failure for %s", user);
		pw = NULL;
	}
	if (as != NULL)
		auth_close(as);
	if (pw != NULL)
		return (pwcopy(pw));
	return (NULL);
}

/* Returns 1 if key is revoked by revoked_keys_file, 0 otherwise */
int
auth_key_is_revoked(struct sshkey *key)
{
	char *fp = NULL;
	int r;

	if (options.revoked_keys_file == NULL)
		return 0;
	if ((fp = sshkey_fingerprint(key, options.fingerprint_hash,
	    SSH_FP_DEFAULT)) == NULL) {
		r = SSH_ERR_ALLOC_FAIL;
		error_fr(r, "fingerprint key");
		goto out;
	}

	r = sshkey_check_revoked(key, options.revoked_keys_file);
	switch (r) {
	case 0:
		break; /* not revoked */
	case SSH_ERR_KEY_REVOKED:
		error("Authentication key %s %s revoked by file %s",
		    sshkey_type(key), fp, options.revoked_keys_file);
		goto out;
	default:
		error_r(r, "Error checking authentication key %s %s in "
		    "revoked keys file %s", sshkey_type(key), fp,
		    options.revoked_keys_file);
		goto out;
	}

	/* Success */
	r = 0;

 out:
	free(fp);
	return r == 0 ? 0 : 1;
}

void
auth_debug_add(const char *fmt,...)
{
	char buf[1024];
	va_list args;
	int r;

	va_start(args, fmt);
	vsnprintf(buf, sizeof(buf), fmt, args);
	va_end(args);
	debug3("%s", buf);
	if (auth_debug != NULL)
		if ((r = sshbuf_put_cstring(auth_debug, buf)) != 0)
			fatal_fr(r, "sshbuf_put_cstring");
}

void
auth_debug_send(struct ssh *ssh)
{
	char *msg;
	int r;

	if (auth_debug == NULL)
		return;
	while (sshbuf_len(auth_debug) != 0) {
		if ((r = sshbuf_get_cstring(auth_debug, &msg, NULL)) != 0)
			fatal_fr(r, "sshbuf_get_cstring");
		ssh_packet_send_debug(ssh, "%s", msg);
		free(msg);
	}
}

void
auth_debug_reset(void)
{
	if (auth_debug != NULL)
		sshbuf_reset(auth_debug);
	else if ((auth_debug = sshbuf_new()) == NULL)
		fatal_f("sshbuf_new failed");
}

struct passwd *
fakepw(void)
{
	static int done = 0;
	static struct passwd fake;
	const char hashchars[] = "./ABCDEFGHIJKLMNOPQRSTUVWXYZ"
	    "abcdefghijklmnopqrstuvwxyz0123456789"; /* from bcrypt.c */
	char *cp;

	if (done)
		return (&fake);

	memset(&fake, 0, sizeof(fake));
	fake.pw_name = "NOUSER";
	fake.pw_passwd = xstrdup("$2a$10$"
	    "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx");
	for (cp = fake.pw_passwd + 7; *cp != '\0'; cp++)
		*cp = hashchars[arc4random_uniform(sizeof(hashchars) - 1)];
	fake.pw_gecos = "NOUSER";
	fake.pw_uid = (uid_t)-1;
	fake.pw_gid = (gid_t)-1;
	fake.pw_class = "";
	fake.pw_dir = "/nonexist";
	fake.pw_shell = "/nonexist";
	done = 1;

	return (&fake);
}

/*
 * Return the canonical name of the host in the other side of the current
 * connection.  The host name is cached, so it is efficient to call this
 * several times.
 */

const char *
auth_get_canonical_hostname(struct ssh *ssh, int use_dns)
{
	static char *dnsname;

	if (!use_dns)
		return ssh_remote_ipaddr(ssh);
	if (dnsname != NULL)
		return dnsname;
	dnsname = ssh_remote_hostname(ssh);
	return dnsname;
}

/* These functions link key/cert options to the auth framework */

/* Log sshauthopt options locally and (optionally) for remote transmission */
void
auth_log_authopts(const char *loc, const struct sshauthopt *opts, int do_remote)
{
	int do_env = options.permit_user_env && opts->nenv > 0;
	int do_permitopen = opts->npermitopen > 0 &&
	    (options.allow_tcp_forwarding & FORWARD_LOCAL) != 0;
	int do_permitlisten = opts->npermitlisten > 0 &&
	    (options.allow_tcp_forwarding & FORWARD_REMOTE) != 0;
	size_t i;
	char msg[1024], buf[64];

	snprintf(buf, sizeof(buf), "%d", opts->force_tun_device);
	/* Try to keep this alphabetically sorted */
	snprintf(msg, sizeof(msg), "key options:%s%s%s%s%s%s%s%s%s%s%s%s%s%s%s",
	    opts->permit_agent_forwarding_flag ? " agent-forwarding" : "",
	    opts->force_command == NULL ? "" : " command",
	    do_env ?  " environment" : "",
	    opts->valid_before == 0 ? "" : "expires",
	    opts->no_require_user_presence ? " no-touch-required" : "",
	    do_permitopen ?  " permitopen" : "",
	    do_permitlisten ?  " permitlisten" : "",
	    opts->permit_port_forwarding_flag ? " port-forwarding" : "",
	    opts->cert_principals == NULL ? "" : " principals",
	    opts->permit_pty_flag ? " pty" : "",
	    opts->require_verify ? " uv" : "",
	    opts->force_tun_device == -1 ? "" : " tun=",
	    opts->force_tun_device == -1 ? "" : buf,
	    opts->permit_user_rc ? " user-rc" : "",
	    opts->permit_x11_forwarding_flag ? " x11-forwarding" : "");

	debug("%s: %s", loc, msg);
	if (do_remote)
		auth_debug_add("%s: %s", loc, msg);

	if (options.permit_user_env) {
		for (i = 0; i < opts->nenv; i++) {
			debug("%s: environment: %s", loc, opts->env[i]);
			if (do_remote) {
				auth_debug_add("%s: environment: %s",
				    loc, opts->env[i]);
			}
		}
	}

	/* Go into a little more details for the local logs. */
	if (opts->valid_before != 0) {
		format_absolute_time(opts->valid_before, buf, sizeof(buf));
		debug("%s: expires at %s", loc, buf);
	}
	if (opts->cert_principals != NULL) {
		debug("%s: authorized principals: \"%s\"",
		    loc, opts->cert_principals);
	}
	if (opts->force_command != NULL)
		debug("%s: forced command: \"%s\"", loc, opts->force_command);
	if (do_permitopen) {
		for (i = 0; i < opts->npermitopen; i++) {
			debug("%s: permitted open: %s",
			    loc, opts->permitopen[i]);
		}
	}
	if (do_permitlisten) {
		for (i = 0; i < opts->npermitlisten; i++) {
			debug("%s: permitted listen: %s",
			    loc, opts->permitlisten[i]);
		}
	}
}

/* Activate a new set of key/cert options; merging with what is there. */
int
auth_activate_options(struct ssh *ssh, struct sshauthopt *opts)
{
	struct sshauthopt *old = auth_opts;
	const char *emsg = NULL;

	debug_f("setting new authentication options");
	if ((auth_opts = sshauthopt_merge(old, opts, &emsg)) == NULL) {
		error("Inconsistent authentication options: %s", emsg);
		return -1;
	}
	return 0;
}

/* Disable forwarding, etc for the session */
void
auth_restrict_session(struct ssh *ssh)
{
	struct sshauthopt *restricted;

	debug_f("restricting session");

	/* A blank sshauthopt defaults to permitting nothing */
	if ((restricted = sshauthopt_new()) == NULL)
		fatal_f("sshauthopt_new failed");
	restricted->permit_pty_flag = 1;
	restricted->restricted = 1;

	if (auth_activate_options(ssh, restricted) != 0)
		fatal_f("failed to restrict session");
	sshauthopt_free(restricted);
}