auth2-pubkey.c revision 262566
1/* $OpenBSD: auth2-pubkey.c,v 1.39 2013/12/30 23:52:27 djm Exp $ */
2/*
3 * Copyright (c) 2000 Markus Friedl.  All rights reserved.
4 *
5 * Redistribution and use in source and binary forms, with or without
6 * modification, are permitted provided that the following conditions
7 * are met:
8 * 1. Redistributions of source code must retain the above copyright
9 *    notice, this list of conditions and the following disclaimer.
10 * 2. Redistributions in binary form must reproduce the above copyright
11 *    notice, this list of conditions and the following disclaimer in the
12 *    documentation and/or other materials provided with the distribution.
13 *
14 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
15 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
16 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
17 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
18 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
19 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
20 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
21 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
22 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
23 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
24 */
25
26#include "includes.h"
27
28#include <sys/types.h>
29#include <sys/stat.h>
30#include <sys/wait.h>
31
32#include <errno.h>
33#include <fcntl.h>
34#ifdef HAVE_PATHS_H
35# include <paths.h>
36#endif
37#include <pwd.h>
38#include <signal.h>
39#include <stdio.h>
40#include <stdarg.h>
41#include <string.h>
42#include <time.h>
43#include <unistd.h>
44
45#include "xmalloc.h"
46#include "ssh.h"
47#include "ssh2.h"
48#include "packet.h"
49#include "buffer.h"
50#include "log.h"
51#include "servconf.h"
52#include "compat.h"
53#include "key.h"
54#include "hostfile.h"
55#include "auth.h"
56#include "pathnames.h"
57#include "uidswap.h"
58#include "auth-options.h"
59#include "canohost.h"
60#ifdef GSSAPI
61#include "ssh-gss.h"
62#endif
63#include "monitor_wrap.h"
64#include "misc.h"
65#include "authfile.h"
66#include "match.h"
67
68/* import */
69extern ServerOptions options;
70extern u_char *session_id2;
71extern u_int session_id2_len;
72
73static int
74userauth_pubkey(Authctxt *authctxt)
75{
76	Buffer b;
77	Key *key = NULL;
78	char *pkalg, *userstyle;
79	u_char *pkblob, *sig;
80	u_int alen, blen, slen;
81	int have_sig, pktype;
82	int authenticated = 0;
83
84	if (!authctxt->valid) {
85		debug2("userauth_pubkey: disabled because of invalid user");
86		return 0;
87	}
88	have_sig = packet_get_char();
89	if (datafellows & SSH_BUG_PKAUTH) {
90		debug2("userauth_pubkey: SSH_BUG_PKAUTH");
91		/* no explicit pkalg given */
92		pkblob = packet_get_string(&blen);
93		buffer_init(&b);
94		buffer_append(&b, pkblob, blen);
95		/* so we have to extract the pkalg from the pkblob */
96		pkalg = buffer_get_string(&b, &alen);
97		buffer_free(&b);
98	} else {
99		pkalg = packet_get_string(&alen);
100		pkblob = packet_get_string(&blen);
101	}
102	pktype = key_type_from_name(pkalg);
103	if (pktype == KEY_UNSPEC) {
104		/* this is perfectly legal */
105		logit("userauth_pubkey: unsupported public key algorithm: %s",
106		    pkalg);
107		goto done;
108	}
109	key = key_from_blob(pkblob, blen);
110	if (key == NULL) {
111		error("userauth_pubkey: cannot decode key: %s", pkalg);
112		goto done;
113	}
114	if (key->type != pktype) {
115		error("userauth_pubkey: type mismatch for decoded key "
116		    "(received %d, expected %d)", key->type, pktype);
117		goto done;
118	}
119	if (key_type_plain(key->type) == KEY_RSA &&
120	    (datafellows & SSH_BUG_RSASIGMD5) != 0) {
121		logit("Refusing RSA key because client uses unsafe "
122		    "signature scheme");
123		goto done;
124	}
125	if (have_sig) {
126		sig = packet_get_string(&slen);
127		packet_check_eom();
128		buffer_init(&b);
129		if (datafellows & SSH_OLD_SESSIONID) {
130			buffer_append(&b, session_id2, session_id2_len);
131		} else {
132			buffer_put_string(&b, session_id2, session_id2_len);
133		}
134		/* reconstruct packet */
135		buffer_put_char(&b, SSH2_MSG_USERAUTH_REQUEST);
136		xasprintf(&userstyle, "%s%s%s", authctxt->user,
137		    authctxt->style ? ":" : "",
138		    authctxt->style ? authctxt->style : "");
139		buffer_put_cstring(&b, userstyle);
140		free(userstyle);
141		buffer_put_cstring(&b,
142		    datafellows & SSH_BUG_PKSERVICE ?
143		    "ssh-userauth" :
144		    authctxt->service);
145		if (datafellows & SSH_BUG_PKAUTH) {
146			buffer_put_char(&b, have_sig);
147		} else {
148			buffer_put_cstring(&b, "publickey");
149			buffer_put_char(&b, have_sig);
150			buffer_put_cstring(&b, pkalg);
151		}
152		buffer_put_string(&b, pkblob, blen);
153#ifdef DEBUG_PK
154		buffer_dump(&b);
155#endif
156		pubkey_auth_info(authctxt, key, NULL);
157
158		/* test for correct signature */
159		authenticated = 0;
160		if (PRIVSEP(user_key_allowed(authctxt->pw, key)) &&
161		    PRIVSEP(key_verify(key, sig, slen, buffer_ptr(&b),
162		    buffer_len(&b))) == 1)
163			authenticated = 1;
164		buffer_free(&b);
165		free(sig);
166	} else {
167		debug("test whether pkalg/pkblob are acceptable");
168		packet_check_eom();
169
170		/* XXX fake reply and always send PK_OK ? */
171		/*
172		 * XXX this allows testing whether a user is allowed
173		 * to login: if you happen to have a valid pubkey this
174		 * message is sent. the message is NEVER sent at all
175		 * if a user is not allowed to login. is this an
176		 * issue? -markus
177		 */
178		if (PRIVSEP(user_key_allowed(authctxt->pw, key))) {
179			packet_start(SSH2_MSG_USERAUTH_PK_OK);
180			packet_put_string(pkalg, alen);
181			packet_put_string(pkblob, blen);
182			packet_send();
183			packet_write_wait();
184			authctxt->postponed = 1;
185		}
186	}
187	if (authenticated != 1)
188		auth_clear_options();
189done:
190	debug2("userauth_pubkey: authenticated %d pkalg %s", authenticated, pkalg);
191	if (key != NULL)
192		key_free(key);
193	free(pkalg);
194	free(pkblob);
195	return authenticated;
196}
197
198void
199pubkey_auth_info(Authctxt *authctxt, const Key *key, const char *fmt, ...)
200{
201	char *fp, *extra;
202	va_list ap;
203	int i;
204
205	extra = NULL;
206	if (fmt != NULL) {
207		va_start(ap, fmt);
208		i = vasprintf(&extra, fmt, ap);
209		va_end(ap);
210		if (i < 0 || extra == NULL)
211			fatal("%s: vasprintf failed", __func__);
212	}
213
214	if (key_is_cert(key)) {
215		fp = key_fingerprint(key->cert->signature_key,
216		    SSH_FP_MD5, SSH_FP_HEX);
217		auth_info(authctxt, "%s ID %s (serial %llu) CA %s %s%s%s",
218		    key_type(key), key->cert->key_id,
219		    (unsigned long long)key->cert->serial,
220		    key_type(key->cert->signature_key), fp,
221		    extra == NULL ? "" : ", ", extra == NULL ? "" : extra);
222		free(fp);
223	} else {
224		fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
225		auth_info(authctxt, "%s %s%s%s", key_type(key), fp,
226		    extra == NULL ? "" : ", ", extra == NULL ? "" : extra);
227		free(fp);
228	}
229	free(extra);
230}
231
232static int
233match_principals_option(const char *principal_list, struct KeyCert *cert)
234{
235	char *result;
236	u_int i;
237
238	/* XXX percent_expand() sequences for authorized_principals? */
239
240	for (i = 0; i < cert->nprincipals; i++) {
241		if ((result = match_list(cert->principals[i],
242		    principal_list, NULL)) != NULL) {
243			debug3("matched principal from key options \"%.100s\"",
244			    result);
245			free(result);
246			return 1;
247		}
248	}
249	return 0;
250}
251
252static int
253match_principals_file(char *file, struct passwd *pw, struct KeyCert *cert)
254{
255	FILE *f;
256	char line[SSH_MAX_PUBKEY_BYTES], *cp, *ep, *line_opts;
257	u_long linenum = 0;
258	u_int i;
259
260	temporarily_use_uid(pw);
261	debug("trying authorized principals file %s", file);
262	if ((f = auth_openprincipals(file, pw, options.strict_modes)) == NULL) {
263		restore_uid();
264		return 0;
265	}
266	while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
267		/* Skip leading whitespace. */
268		for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
269			;
270		/* Skip blank and comment lines. */
271		if ((ep = strchr(cp, '#')) != NULL)
272			*ep = '\0';
273		if (!*cp || *cp == '\n')
274			continue;
275		/* Trim trailing whitespace. */
276		ep = cp + strlen(cp) - 1;
277		while (ep > cp && (*ep == '\n' || *ep == ' ' || *ep == '\t'))
278			*ep-- = '\0';
279		/*
280		 * If the line has internal whitespace then assume it has
281		 * key options.
282		 */
283		line_opts = NULL;
284		if ((ep = strrchr(cp, ' ')) != NULL ||
285		    (ep = strrchr(cp, '\t')) != NULL) {
286			for (; *ep == ' ' || *ep == '\t'; ep++)
287				;
288			line_opts = cp;
289			cp = ep;
290		}
291		for (i = 0; i < cert->nprincipals; i++) {
292			if (strcmp(cp, cert->principals[i]) == 0) {
293				debug3("matched principal \"%.100s\" "
294				    "from file \"%s\" on line %lu",
295				    cert->principals[i], file, linenum);
296				if (auth_parse_options(pw, line_opts,
297				    file, linenum) != 1)
298					continue;
299				fclose(f);
300				restore_uid();
301				return 1;
302			}
303		}
304	}
305	fclose(f);
306	restore_uid();
307	return 0;
308}
309
310/*
311 * Checks whether key is allowed in authorized_keys-format file,
312 * returns 1 if the key is allowed or 0 otherwise.
313 */
314static int
315check_authkeys_file(FILE *f, char *file, Key* key, struct passwd *pw)
316{
317	char line[SSH_MAX_PUBKEY_BYTES];
318	const char *reason;
319	int found_key = 0;
320	u_long linenum = 0;
321	Key *found;
322	char *fp;
323
324	found_key = 0;
325
326	found = NULL;
327	while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
328		char *cp, *key_options = NULL;
329		if (found != NULL)
330			key_free(found);
331		found = key_new(key_is_cert(key) ? KEY_UNSPEC : key->type);
332		auth_clear_options();
333
334		/* Skip leading whitespace, empty and comment lines. */
335		for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
336			;
337		if (!*cp || *cp == '\n' || *cp == '#')
338			continue;
339
340		if (key_read(found, &cp) != 1) {
341			/* no key?  check if there are options for this key */
342			int quoted = 0;
343			debug2("user_key_allowed: check options: '%s'", cp);
344			key_options = cp;
345			for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
346				if (*cp == '\\' && cp[1] == '"')
347					cp++;	/* Skip both */
348				else if (*cp == '"')
349					quoted = !quoted;
350			}
351			/* Skip remaining whitespace. */
352			for (; *cp == ' ' || *cp == '\t'; cp++)
353				;
354			if (key_read(found, &cp) != 1) {
355				debug2("user_key_allowed: advance: '%s'", cp);
356				/* still no key?  advance to next line*/
357				continue;
358			}
359		}
360		if (key_is_cert(key)) {
361			if (!key_equal(found, key->cert->signature_key))
362				continue;
363			if (auth_parse_options(pw, key_options, file,
364			    linenum) != 1)
365				continue;
366			if (!key_is_cert_authority)
367				continue;
368			fp = key_fingerprint(found, SSH_FP_MD5,
369			    SSH_FP_HEX);
370			debug("matching CA found: file %s, line %lu, %s %s",
371			    file, linenum, key_type(found), fp);
372			/*
373			 * If the user has specified a list of principals as
374			 * a key option, then prefer that list to matching
375			 * their username in the certificate principals list.
376			 */
377			if (authorized_principals != NULL &&
378			    !match_principals_option(authorized_principals,
379			    key->cert)) {
380				reason = "Certificate does not contain an "
381				    "authorized principal";
382 fail_reason:
383				free(fp);
384				error("%s", reason);
385				auth_debug_add("%s", reason);
386				continue;
387			}
388			if (key_cert_check_authority(key, 0, 0,
389			    authorized_principals == NULL ? pw->pw_name : NULL,
390			    &reason) != 0)
391				goto fail_reason;
392			if (auth_cert_options(key, pw) != 0) {
393				free(fp);
394				continue;
395			}
396			verbose("Accepted certificate ID \"%s\" "
397			    "signed by %s CA %s via %s", key->cert->key_id,
398			    key_type(found), fp, file);
399			free(fp);
400			found_key = 1;
401			break;
402		} else if (key_equal(found, key)) {
403			if (auth_parse_options(pw, key_options, file,
404			    linenum) != 1)
405				continue;
406			if (key_is_cert_authority)
407				continue;
408			found_key = 1;
409			fp = key_fingerprint(found, SSH_FP_MD5, SSH_FP_HEX);
410			debug("matching key found: file %s, line %lu %s %s",
411			    file, linenum, key_type(found), fp);
412			free(fp);
413			break;
414		}
415	}
416	if (found != NULL)
417		key_free(found);
418	if (!found_key)
419		debug2("key not found");
420	return found_key;
421}
422
423/* Authenticate a certificate key against TrustedUserCAKeys */
424static int
425user_cert_trusted_ca(struct passwd *pw, Key *key)
426{
427	char *ca_fp, *principals_file = NULL;
428	const char *reason;
429	int ret = 0;
430
431	if (!key_is_cert(key) || options.trusted_user_ca_keys == NULL)
432		return 0;
433
434	ca_fp = key_fingerprint(key->cert->signature_key,
435	    SSH_FP_MD5, SSH_FP_HEX);
436
437	if (key_in_file(key->cert->signature_key,
438	    options.trusted_user_ca_keys, 1) != 1) {
439		debug2("%s: CA %s %s is not listed in %s", __func__,
440		    key_type(key->cert->signature_key), ca_fp,
441		    options.trusted_user_ca_keys);
442		goto out;
443	}
444	/*
445	 * If AuthorizedPrincipals is in use, then compare the certificate
446	 * principals against the names in that file rather than matching
447	 * against the username.
448	 */
449	if ((principals_file = authorized_principals_file(pw)) != NULL) {
450		if (!match_principals_file(principals_file, pw, key->cert)) {
451			reason = "Certificate does not contain an "
452			    "authorized principal";
453 fail_reason:
454			error("%s", reason);
455			auth_debug_add("%s", reason);
456			goto out;
457		}
458	}
459	if (key_cert_check_authority(key, 0, 1,
460	    principals_file == NULL ? pw->pw_name : NULL, &reason) != 0)
461		goto fail_reason;
462	if (auth_cert_options(key, pw) != 0)
463		goto out;
464
465	verbose("Accepted certificate ID \"%s\" signed by %s CA %s via %s",
466	    key->cert->key_id, key_type(key->cert->signature_key), ca_fp,
467	    options.trusted_user_ca_keys);
468	ret = 1;
469
470 out:
471	free(principals_file);
472	free(ca_fp);
473	return ret;
474}
475
476/*
477 * Checks whether key is allowed in file.
478 * returns 1 if the key is allowed or 0 otherwise.
479 */
480static int
481user_key_allowed2(struct passwd *pw, Key *key, char *file)
482{
483	FILE *f;
484	int found_key = 0;
485
486	/* Temporarily use the user's uid. */
487	temporarily_use_uid(pw);
488
489	debug("trying public key file %s", file);
490	if ((f = auth_openkeyfile(file, pw, options.strict_modes)) != NULL) {
491		found_key = check_authkeys_file(f, file, key, pw);
492		fclose(f);
493	}
494
495	restore_uid();
496	return found_key;
497}
498
499/*
500 * Checks whether key is allowed in output of command.
501 * returns 1 if the key is allowed or 0 otherwise.
502 */
503static int
504user_key_command_allowed2(struct passwd *user_pw, Key *key)
505{
506	FILE *f;
507	int ok, found_key = 0;
508	struct passwd *pw;
509	struct stat st;
510	int status, devnull, p[2], i;
511	pid_t pid;
512	char *username, errmsg[512];
513
514	if (options.authorized_keys_command == NULL ||
515	    options.authorized_keys_command[0] != '/')
516		return 0;
517
518	if (options.authorized_keys_command_user == NULL) {
519		error("No user for AuthorizedKeysCommand specified, skipping");
520		return 0;
521	}
522
523	username = percent_expand(options.authorized_keys_command_user,
524	    "u", user_pw->pw_name, (char *)NULL);
525	pw = getpwnam(username);
526	if (pw == NULL) {
527		error("AuthorizedKeysCommandUser \"%s\" not found: %s",
528		    username, strerror(errno));
529		free(username);
530		return 0;
531	}
532	free(username);
533
534	temporarily_use_uid(pw);
535
536	if (stat(options.authorized_keys_command, &st) < 0) {
537		error("Could not stat AuthorizedKeysCommand \"%s\": %s",
538		    options.authorized_keys_command, strerror(errno));
539		goto out;
540	}
541	if (auth_secure_path(options.authorized_keys_command, &st, NULL, 0,
542	    errmsg, sizeof(errmsg)) != 0) {
543		error("Unsafe AuthorizedKeysCommand: %s", errmsg);
544		goto out;
545	}
546
547	if (pipe(p) != 0) {
548		error("%s: pipe: %s", __func__, strerror(errno));
549		goto out;
550	}
551
552	debug3("Running AuthorizedKeysCommand: \"%s %s\" as \"%s\"",
553	    options.authorized_keys_command, user_pw->pw_name, pw->pw_name);
554
555	/*
556	 * Don't want to call this in the child, where it can fatal() and
557	 * run cleanup_exit() code.
558	 */
559	restore_uid();
560
561	switch ((pid = fork())) {
562	case -1: /* error */
563		error("%s: fork: %s", __func__, strerror(errno));
564		close(p[0]);
565		close(p[1]);
566		return 0;
567	case 0: /* child */
568		for (i = 0; i < NSIG; i++)
569			signal(i, SIG_DFL);
570
571		if ((devnull = open(_PATH_DEVNULL, O_RDWR)) == -1) {
572			error("%s: open %s: %s", __func__, _PATH_DEVNULL,
573			    strerror(errno));
574			_exit(1);
575		}
576		/* Keep stderr around a while longer to catch errors */
577		if (dup2(devnull, STDIN_FILENO) == -1 ||
578		    dup2(p[1], STDOUT_FILENO) == -1) {
579			error("%s: dup2: %s", __func__, strerror(errno));
580			_exit(1);
581		}
582		closefrom(STDERR_FILENO + 1);
583
584		/* Don't use permanently_set_uid() here to avoid fatal() */
585		if (setresgid(pw->pw_gid, pw->pw_gid, pw->pw_gid) != 0) {
586			error("setresgid %u: %s", (u_int)pw->pw_gid,
587			    strerror(errno));
588			_exit(1);
589		}
590		if (setresuid(pw->pw_uid, pw->pw_uid, pw->pw_uid) != 0) {
591			error("setresuid %u: %s", (u_int)pw->pw_uid,
592			    strerror(errno));
593			_exit(1);
594		}
595		/* stdin is pointed to /dev/null at this point */
596		if (dup2(STDIN_FILENO, STDERR_FILENO) == -1) {
597			error("%s: dup2: %s", __func__, strerror(errno));
598			_exit(1);
599		}
600
601		execl(options.authorized_keys_command,
602		    options.authorized_keys_command, user_pw->pw_name, NULL);
603
604		error("AuthorizedKeysCommand %s exec failed: %s",
605		    options.authorized_keys_command, strerror(errno));
606		_exit(127);
607	default: /* parent */
608		break;
609	}
610
611	temporarily_use_uid(pw);
612
613	close(p[1]);
614	if ((f = fdopen(p[0], "r")) == NULL) {
615		error("%s: fdopen: %s", __func__, strerror(errno));
616		close(p[0]);
617		/* Don't leave zombie child */
618		kill(pid, SIGTERM);
619		while (waitpid(pid, NULL, 0) == -1 && errno == EINTR)
620			;
621		goto out;
622	}
623	ok = check_authkeys_file(f, options.authorized_keys_command, key, pw);
624	fclose(f);
625
626	while (waitpid(pid, &status, 0) == -1) {
627		if (errno != EINTR) {
628			error("%s: waitpid: %s", __func__, strerror(errno));
629			goto out;
630		}
631	}
632	if (WIFSIGNALED(status)) {
633		error("AuthorizedKeysCommand %s exited on signal %d",
634		    options.authorized_keys_command, WTERMSIG(status));
635		goto out;
636	} else if (WEXITSTATUS(status) != 0) {
637		error("AuthorizedKeysCommand %s returned status %d",
638		    options.authorized_keys_command, WEXITSTATUS(status));
639		goto out;
640	}
641	found_key = ok;
642 out:
643	restore_uid();
644	return found_key;
645}
646
647/*
648 * Check whether key authenticates and authorises the user.
649 */
650int
651user_key_allowed(struct passwd *pw, Key *key)
652{
653	u_int success, i;
654	char *file;
655
656	if (auth_key_is_revoked(key))
657		return 0;
658	if (key_is_cert(key) && auth_key_is_revoked(key->cert->signature_key))
659		return 0;
660
661	success = user_cert_trusted_ca(pw, key);
662	if (success)
663		return success;
664
665	success = user_key_command_allowed2(pw, key);
666	if (success > 0)
667		return success;
668
669	for (i = 0; !success && i < options.num_authkeys_files; i++) {
670
671		if (strcasecmp(options.authorized_keys_files[i], "none") == 0)
672			continue;
673		file = expand_authorized_keys(
674		    options.authorized_keys_files[i], pw);
675
676		success = user_key_allowed2(pw, key, file);
677		free(file);
678	}
679
680	return success;
681}
682
683Authmethod method_pubkey = {
684	"publickey",
685	userauth_pubkey,
686	&options.pubkey_authentication
687};
688