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support for user-verified FIDO keys

FIDO2 supports a notion of "user verification" where the user is
required to demonstrate their identity to the token before particular
operations (e.g. signing). Typically this is done by authenticating
themselves using a PIN that has been set on the token.

This adds support for generating and using user verified keys where
the verification happens via PIN (other options might be added in the
future, but none are in common use now). Practically, this adds
another key generation option "verify-required" that yields a key that
requires a PIN before each authentication.

feedback markus@ and Pedro Martelletto; ok markus@

/* $OpenBSD: ssh-keysign.c,v 1.64 2020/08/27 01:06:18 djm Exp $ */
/*
 * Copyright (c) 2002 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>

#ifdef WITH_OPENSSL
#include <openssl/evp.h>
#endif

#include <fcntl.h>
#include <paths.h>
#include <pwd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <stdarg.h>
#include <unistd.h>
#include <errno.h>

#include "xmalloc.h"
#include "log.h"
#include "sshkey.h"
#include "ssh.h"
#include "ssh2.h"
#include "misc.h"
#include "sshbuf.h"
#include "authfile.h"
#include "msg.h"
#include "canohost.h"
#include "pathnames.h"
#include "readconf.h"
#include "uidswap.h"
#include "ssherr.h"

extern char *__progname;

static int
valid_request(struct passwd *pw, char *host, struct sshkey **ret,
    u_char *data, size_t datalen)
{
	struct sshbuf *b;
	struct sshkey *key = NULL;
	u_char type, *pkblob;
	char *p;
	size_t blen, len;
	char *pkalg, *luser;
	int r, pktype, fail;

	if (ret != NULL)
		*ret = NULL;
	fail = 0;

	if ((b = sshbuf_from(data, datalen)) == NULL)
		fatal("%s: sshbuf_from failed", __func__);

	/* session id, currently limited to SHA1 (20 bytes) or SHA256 (32) */
	if ((r = sshbuf_get_string(b, NULL, &len)) != 0)
		fatal("%s: buffer error: %s", __func__, ssh_err(r));
	if (len != 20 && len != 32)
		fail++;

	if ((r = sshbuf_get_u8(b, &type)) != 0)
		fatal("%s: buffer error: %s", __func__, ssh_err(r));
	if (type != SSH2_MSG_USERAUTH_REQUEST)
		fail++;

	/* server user */
	if ((r = sshbuf_skip_string(b)) != 0)
		fatal("%s: buffer error: %s", __func__, ssh_err(r));

	/* service */
	if ((r = sshbuf_get_cstring(b, &p, NULL)) != 0)
		fatal("%s: buffer error: %s", __func__, ssh_err(r));
	if (strcmp("ssh-connection", p) != 0)
		fail++;
	free(p);

	/* method */
	if ((r = sshbuf_get_cstring(b, &p, NULL)) != 0)
		fatal("%s: buffer error: %s", __func__, ssh_err(r));
	if (strcmp("hostbased", p) != 0)
		fail++;
	free(p);

	/* pubkey */
	if ((r = sshbuf_get_cstring(b, &pkalg, NULL)) != 0 ||
	    (r = sshbuf_get_string(b, &pkblob, &blen)) != 0)
		fatal("%s: buffer error: %s", __func__, ssh_err(r));

	pktype = sshkey_type_from_name(pkalg);
	if (pktype == KEY_UNSPEC)
		fail++;
	else if ((r = sshkey_from_blob(pkblob, blen, &key)) != 0) {
		error("%s: bad key blob: %s", __func__, ssh_err(r));
		fail++;
	} else if (key->type != pktype)
		fail++;
	free(pkalg);
	free(pkblob);

	/* client host name, handle trailing dot */
	if ((r = sshbuf_get_cstring(b, &p, &len)) != 0)
		fatal("%s: buffer error: %s", __func__, ssh_err(r));
	debug2("%s: check expect chost %s got %s", __func__, host, p);
	if (strlen(host) != len - 1)
		fail++;
	else if (p[len - 1] != '.')
		fail++;
	else if (strncasecmp(host, p, len - 1) != 0)
		fail++;
	free(p);

	/* local user */
	if ((r = sshbuf_get_cstring(b, &luser, NULL)) != 0)
		fatal("%s: buffer error: %s", __func__, ssh_err(r));

	if (strcmp(pw->pw_name, luser) != 0)
		fail++;
	free(luser);

	/* end of message */
	if (sshbuf_len(b) != 0)
		fail++;
	sshbuf_free(b);

	debug3("%s: fail %d", __func__, fail);

	if (fail)
		sshkey_free(key);
	else if (ret != NULL)
		*ret = key;

	return (fail ? -1 : 0);
}

int
main(int argc, char **argv)
{
	struct sshbuf *b;
	Options options;
#define NUM_KEYTYPES 5
	struct sshkey *keys[NUM_KEYTYPES], *key = NULL;
	struct passwd *pw;
	int r, key_fd[NUM_KEYTYPES], i, found, version = 2, fd;
	u_char *signature, *data, rver;
	char *host, *fp;
	size_t slen, dlen;

	if (pledge("stdio rpath getpw dns id", NULL) != 0)
		fatal("%s: pledge: %s", __progname, strerror(errno));

	/* Ensure that stdin and stdout are connected */
	if ((fd = open(_PATH_DEVNULL, O_RDWR)) < 2)
		exit(1);
	/* Leave /dev/null fd iff it is attached to stderr */
	if (fd > 2)
		close(fd);

	i = 0;
	/* XXX This really needs to read sshd_config for the paths */
	key_fd[i++] = open(_PATH_HOST_DSA_KEY_FILE, O_RDONLY);
	key_fd[i++] = open(_PATH_HOST_ECDSA_KEY_FILE, O_RDONLY);
	key_fd[i++] = open(_PATH_HOST_ED25519_KEY_FILE, O_RDONLY);
	key_fd[i++] = open(_PATH_HOST_XMSS_KEY_FILE, O_RDONLY);
	key_fd[i++] = open(_PATH_HOST_RSA_KEY_FILE, O_RDONLY);

	if ((pw = getpwuid(getuid())) == NULL)
		fatal("getpwuid failed");
	pw = pwcopy(pw);

	permanently_set_uid(pw);

#ifdef DEBUG_SSH_KEYSIGN
	log_init("ssh-keysign", SYSLOG_LEVEL_DEBUG3, SYSLOG_FACILITY_AUTH, 0);
#endif

	/* verify that ssh-keysign is enabled by the admin */
	initialize_options(&options);
	(void)read_config_file(_PATH_HOST_CONFIG_FILE, pw, "", "",
	    &options, 0, NULL);
	fill_default_options(&options);
	if (options.enable_ssh_keysign != 1)
		fatal("ssh-keysign not enabled in %s",
		    _PATH_HOST_CONFIG_FILE);

	for (i = found = 0; i < NUM_KEYTYPES; i++) {
		if (key_fd[i] != -1)
			found = 1;
	}
	if (found == 0)
		fatal("could not open any host key");

#ifdef WITH_OPENSSL
	OpenSSL_add_all_algorithms();
#endif
	found = 0;
	for (i = 0; i < NUM_KEYTYPES; i++) {
		keys[i] = NULL;
		if (key_fd[i] == -1)
			continue;
		r = sshkey_load_private_type_fd(key_fd[i], KEY_UNSPEC,
		    NULL, &key, NULL);
		close(key_fd[i]);
		if (r != 0)
			debug("parse key %d: %s", i, ssh_err(r));
		else if (key != NULL) {
			keys[i] = key;
			found = 1;
		}
	}
	if (!found)
		fatal("no hostkey found");

	if (pledge("stdio dns", NULL) != 0)
		fatal("%s: pledge: %s", __progname, strerror(errno));

	if ((b = sshbuf_new()) == NULL)
		fatal("%s: sshbuf_new failed", __progname);
	if (ssh_msg_recv(STDIN_FILENO, b) < 0)
		fatal("ssh_msg_recv failed");
	if ((r = sshbuf_get_u8(b, &rver)) != 0)
		fatal("%s: buffer error: %s", __progname, ssh_err(r));
	if (rver != version)
		fatal("bad version: received %d, expected %d", rver, version);
	if ((r = sshbuf_get_u32(b, (u_int *)&fd)) != 0)
		fatal("%s: buffer error: %s", __progname, ssh_err(r));
	if (fd < 0 || fd == STDIN_FILENO || fd == STDOUT_FILENO)
		fatal("bad fd = %d", fd);
	if ((host = get_local_name(fd)) == NULL)
		fatal("cannot get local name for fd");

	if ((r = sshbuf_get_string(b, &data, &dlen)) != 0)
		fatal("%s: buffer error: %s", __progname, ssh_err(r));
	if (valid_request(pw, host, &key, data, dlen) < 0)
		fatal("not a valid request");
	free(host);

	found = 0;
	for (i = 0; i < NUM_KEYTYPES; i++) {
		if (keys[i] != NULL &&
		    sshkey_equal_public(key, keys[i])) {
			found = 1;
			break;
		}
	}
	if (!found) {
		if ((fp = sshkey_fingerprint(key, options.fingerprint_hash,
		    SSH_FP_DEFAULT)) == NULL)
			fatal("%s: sshkey_fingerprint failed", __progname);
		fatal("no matching hostkey found for key %s %s",
		    sshkey_type(key), fp ? fp : "");
	}

	if ((r = sshkey_sign(keys[i], &signature, &slen, data, dlen,
	    NULL, NULL, NULL, 0)) != 0)
		fatal("sshkey_sign failed: %s", ssh_err(r));
	free(data);

	/* send reply */
	sshbuf_reset(b);
	if ((r = sshbuf_put_string(b, signature, slen)) != 0)
		fatal("%s: buffer error: %s", __progname, ssh_err(r));
	if (ssh_msg_send(STDOUT_FILENO, version, b) == -1)
		fatal("ssh_msg_send failed");

	return (0);
}