xref: /netbsd-src/lib/libpam/modules/pam_ssh/pam_ssh.c (revision cefc293e00a80d48201bf14f466354a51d687319)
1 /*	$NetBSD: pam_ssh.c,v 1.30 2022/06/15 08:31:34 hannken Exp $	*/
2 
3 /*-
4  * Copyright (c) 2003 Networks Associates Technology, Inc.
5  * All rights reserved.
6  *
7  * This software was developed for the FreeBSD Project by ThinkSec AS and
8  * NAI Labs, the Security Research Division of Network Associates, Inc.
9  * under DARPA/SPAWAR contract N66001-01-C-8035 ("CBOSS"), as part of the
10  * DARPA CHATS research program.
11  *
12  * Redistribution and use in source and binary forms, with or without
13  * modification, are permitted provided that the following conditions
14  * are met:
15  * 1. Redistributions of source code must retain the above copyright
16  *    notice, this list of conditions and the following disclaimer.
17  * 2. Redistributions in binary form must reproduce the above copyright
18  *    notice, this list of conditions and the following disclaimer in the
19  *    documentation and/or other materials provided with the distribution.
20  * 3. The name of the author may not be used to endorse or promote
21  *    products derived from this software without specific prior written
22  *    permission.
23  *
24  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
25  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
26  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
27  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
28  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
29  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
30  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
31  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
32  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
33  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
34  * SUCH DAMAGE.
35  */
36 
37 #include <sys/cdefs.h>
38 #ifdef __FreeBSD__
39 __FBSDID("$FreeBSD: src/lib/libpam/modules/pam_ssh/pam_ssh.c,v 1.40 2004/02/10 10:13:21 des Exp $");
40 #else
41 __RCSID("$NetBSD: pam_ssh.c,v 1.30 2022/06/15 08:31:34 hannken Exp $");
42 #endif
43 
44 #include <sys/param.h>
45 #include <sys/wait.h>
46 
47 #include <errno.h>
48 #include <fcntl.h>
49 #include <paths.h>
50 #include <pwd.h>
51 #include <signal.h>
52 #include <stdio.h>
53 #include <string.h>
54 #include <unistd.h>
55 
56 #define PAM_SM_AUTH
57 #define PAM_SM_SESSION
58 
59 #include <security/pam_appl.h>
60 #include <security/pam_modules.h>
61 #include <security/openpam.h>
62 
63 #include <openssl/evp.h>
64 
65 #include "sshkey.h"
66 #include "sshbuf.h"
67 #include "authfd.h"
68 #include "authfile.h"
69 
70 #define ssh_add_identity(auth, key, comment) \
71     ssh_add_identity_constrained(auth, key, comment, 0, 0, 0, NULL, NULL, 00)
72 
73 extern char **environ;
74 
75 struct pam_ssh_key {
76 	struct sshkey	*key;
77 	char	*comment;
78 };
79 
80 static const char *pam_ssh_prompt = "SSH passphrase: ";
81 static const char *pam_ssh_have_keys = "pam_ssh_have_keys";
82 
83 static const char *pam_ssh_keyfiles[] = {
84 	".ssh/identity",	/* SSH1 RSA key */
85 	".ssh/id_rsa",		/* SSH2 RSA key */
86 	".ssh/id_dsa",		/* SSH2 DSA key */
87 	".ssh/id_ecdsa", 	/* SSH2 ECDSA key */
88 	NULL
89 };
90 
91 static const char *pam_ssh_agent = "/usr/bin/ssh-agent";
92 static const char *const pam_ssh_agent_argv[] = { "ssh_agent", "-s", NULL };
93 static const char *const pam_ssh_agent_envp[] = { NULL };
94 
95 /*
96  * Attempts to load a private key from the specified file in the specified
97  * directory, using the specified passphrase.  If successful, returns a
98  * struct pam_ssh_key containing the key and its comment.
99  */
100 static struct pam_ssh_key *
pam_ssh_load_key(const char * dir,const char * kfn,const char * passphrase,int nullok)101 pam_ssh_load_key(const char *dir, const char *kfn, const char *passphrase,
102     int nullok)
103 {
104 	struct pam_ssh_key *psk;
105 	char fn[PATH_MAX];
106 	int r;
107 	char *comment;
108 	struct sshkey *key;
109 
110 	if (snprintf(fn, sizeof(fn), "%s/%s", dir, kfn) > (int)sizeof(fn))
111 		return (NULL);
112 	comment = NULL;
113 	/*
114 	 * If the key is unencrypted, OpenSSL ignores the passphrase, so
115 	 * it will seem like the user typed in the right one.  This allows
116 	 * a user to circumvent nullok by providing a dummy passphrase.
117 	 * Verify that the key really *is* encrypted by trying to load it
118 	 * with an empty passphrase, and if the key is not encrypted,
119 	 * accept only an empty passphrase.
120 	 */
121 	r = sshkey_load_private(fn, "", &key, &comment);
122 	if (r == 0 && !(*passphrase == '\0' && nullok)) {
123 		openpam_log(PAM_LOG_DEBUG, "rejected unencrypted key from %s", fn);
124 		sshkey_free(key);
125 		free(comment);
126 		return (NULL);
127 	}
128 	if (r)
129 		r = sshkey_load_private(fn, passphrase, &key, &comment);
130 	if (r) {
131 		openpam_log(PAM_LOG_DEBUG, "failed to load key from %s", fn);
132 		if (comment != NULL)
133 			free(comment);
134 		return (NULL);
135 	}
136 
137 	openpam_log(PAM_LOG_DEBUG, "loaded '%s' from %s", comment, fn);
138 	if ((psk = malloc(sizeof(*psk))) == NULL) {
139 		sshkey_free(key);
140 		free(comment);
141 		return (NULL);
142 	}
143 	psk->key = key;
144 	psk->comment = comment;
145 	return (psk);
146 }
147 
148 /*
149  * Wipes a private key and frees the associated resources.
150  */
151 static void
pam_ssh_free_key(pam_handle_t * pamh __unused,void * data,int pam_err __unused)152 pam_ssh_free_key(pam_handle_t *pamh __unused,
153     void *data, int pam_err __unused)
154 {
155 	struct pam_ssh_key *psk;
156 
157 	psk = data;
158 	sshkey_free(psk->key);
159 	free(psk->comment);
160 	free(psk);
161 }
162 
163 PAM_EXTERN int
pam_sm_authenticate(pam_handle_t * pamh,int flags __unused,int argc __unused,const char * argv[]__unused)164 pam_sm_authenticate(pam_handle_t *pamh, int flags __unused,
165     int argc __unused, const char *argv[] __unused)
166 {
167 	const char **kfn, *passphrase, *user;
168 	const void *item;
169 	struct passwd *pwd, pwres;
170 	struct pam_ssh_key *psk;
171 	int nkeys, nullok, pam_err, pass;
172 	char pwbuf[1024];
173 
174 	nullok = (openpam_get_option(pamh, "nullok") != NULL);
175 
176 	/* PEM is not loaded by default */
177 	OpenSSL_add_all_algorithms();
178 
179 	/* get user name and home directory */
180 	pam_err = pam_get_user(pamh, &user, NULL);
181 	if (pam_err != PAM_SUCCESS)
182 		return (pam_err);
183 	if (getpwnam_r(user, &pwres, pwbuf, sizeof(pwbuf), &pwd) != 0 ||
184 	    pwd == NULL)
185 		return (PAM_USER_UNKNOWN);
186 	if (pwd->pw_dir == NULL)
187 		return (PAM_AUTH_ERR);
188 
189 	nkeys = 0;
190 	pass = (pam_get_item(pamh, PAM_AUTHTOK, &item) == PAM_SUCCESS &&
191 	    item != NULL);
192  load_keys:
193 	/* get passphrase */
194 	pam_err = pam_get_authtok(pamh, PAM_AUTHTOK,
195 	    &passphrase, pam_ssh_prompt);
196 	if (pam_err != PAM_SUCCESS)
197 		return (pam_err);
198 
199 	/* switch to user credentials */
200 	pam_err = openpam_borrow_cred(pamh, pwd);
201 	if (pam_err != PAM_SUCCESS)
202 		return (pam_err);
203 
204 	/* try to load keys from all keyfiles we know of */
205 	for (kfn = pam_ssh_keyfiles; *kfn != NULL; ++kfn) {
206 		psk = pam_ssh_load_key(pwd->pw_dir, *kfn, passphrase, nullok);
207 		if (psk != NULL) {
208 			pam_set_data(pamh, *kfn, psk, pam_ssh_free_key);
209 			++nkeys;
210 		}
211 	}
212 
213 	/* switch back to arbitrator credentials */
214 	openpam_restore_cred(pamh);
215 
216 	/*
217 	 * If we tried an old token and didn't get anything, and
218 	 * try_first_pass was specified, try again after prompting the
219 	 * user for a new passphrase.
220 	 */
221 	if (nkeys == 0 && pass == 1 &&
222 	    openpam_get_option(pamh, "try_first_pass") != NULL) {
223 		pam_set_item(pamh, PAM_AUTHTOK, NULL);
224 		pass = 0;
225 		goto load_keys;
226 	}
227 
228 	/* no keys? */
229 	if (nkeys == 0)
230 		return (PAM_AUTH_ERR);
231 
232 	pam_set_data(pamh, pam_ssh_have_keys, NULL, NULL);
233 	return (PAM_SUCCESS);
234 }
235 
236 PAM_EXTERN int
pam_sm_setcred(pam_handle_t * pamh __unused,int flags __unused,int argc __unused,const char * argv[]__unused)237 pam_sm_setcred(pam_handle_t *pamh __unused, int flags __unused,
238     int argc __unused, const char *argv[] __unused)
239 {
240 
241 	return (PAM_SUCCESS);
242 }
243 
244 /*
245  * Parses a line from ssh-agent's output.
246  */
247 static void
pam_ssh_process_agent_output(pam_handle_t * pamh,FILE * f)248 pam_ssh_process_agent_output(pam_handle_t *pamh, FILE *f)
249 {
250 	char *line, *p, *key, *val;
251 	size_t len;
252 
253 	while ((line = fgetln(f, &len)) != NULL) {
254 		if (len < 4 || strncmp(line, "SSH_", 4) != 0)
255 			continue;
256 
257 		/* find equal sign at end of key */
258 		for (p = key = line; p < line + len; ++p)
259 			if (*p == '=')
260 				break;
261 		if (p == line + len || *p != '=')
262 			continue;
263 		*p = '\0';
264 
265 		/* find semicolon at end of value */
266 		for (val = ++p; p < line + len; ++p)
267 			if (*p == ';')
268 				break;
269 		if (p == line + len || *p != ';')
270 			continue;
271 		*p = '\0';
272 
273 		/* store key-value pair in environment */
274 		openpam_log(PAM_LOG_DEBUG, "got %s: %s", key, val);
275 		pam_setenv(pamh, key, val, 1);
276 	}
277 }
278 
279 /*
280  * Starts an ssh agent and stores the environment variables derived from
281  * its output.
282  */
283 static int
pam_ssh_start_agent(pam_handle_t * pamh,struct passwd * pwd)284 pam_ssh_start_agent(pam_handle_t *pamh, struct passwd *pwd)
285 {
286 	int agent_pipe[2];
287 	pid_t pid;
288 	FILE *f;
289 
290 	/* get a pipe which we will use to read the agent's output */
291 	if (pipe(agent_pipe) == -1)
292 		return (PAM_SYSTEM_ERR);
293 
294 	/* start the agent */
295 	openpam_log(PAM_LOG_DEBUG, "starting an ssh agent");
296 	pid = fork();
297 	if (pid == (pid_t)-1) {
298 		/* failed */
299 		close(agent_pipe[0]);
300 		close(agent_pipe[1]);
301 		return (PAM_SYSTEM_ERR);
302 	}
303 	if (pid == 0) {
304 #ifndef F_CLOSEM
305 		int fd;
306 #endif
307 		/* child: drop privs, close fds and start agent */
308 		if (setgid(pwd->pw_gid) == -1) {
309 			openpam_log(PAM_LOG_DEBUG, "%s: Cannot setgid %d (%s)",
310 			    __func__, (int)pwd->pw_gid, strerror(errno));
311 			goto done;
312 		}
313 		if (initgroups(pwd->pw_name, pwd->pw_gid) == -1) {
314 			openpam_log(PAM_LOG_DEBUG,
315 			    "%s: Cannot initgroups for %s (%s)",
316 			    __func__, pwd->pw_name, strerror(errno));
317 			goto done;
318 		}
319 		if (setuid(pwd->pw_uid) == -1) {
320 			openpam_log(PAM_LOG_DEBUG, "%s: Cannot setuid %d (%s)",
321 			    __func__, (int)pwd->pw_uid, strerror(errno));
322 			goto done;
323 		}
324 		(void)close(STDIN_FILENO);
325 		(void)open(_PATH_DEVNULL, O_RDONLY);
326 		(void)dup2(agent_pipe[1], STDOUT_FILENO);
327 		(void)dup2(agent_pipe[1], STDERR_FILENO);
328 #ifdef F_CLOSEM
329 		(void)fcntl(3, F_CLOSEM, 0);
330 #else
331 		for (fd = 3; fd < getdtablesize(); ++fd)
332 			(void)close(fd);
333 #endif
334 		(void)execve(pam_ssh_agent,
335 		    (char **)__UNCONST(pam_ssh_agent_argv),
336 		    (char **)__UNCONST(pam_ssh_agent_envp));
337 done:
338 		_exit(127);
339 	}
340 
341 	/* parent */
342 	close(agent_pipe[1]);
343 	if ((f = fdopen(agent_pipe[0], "r")) == NULL)
344 		return (PAM_SYSTEM_ERR);
345 	pam_ssh_process_agent_output(pamh, f);
346 	fclose(f);
347 
348 	return (PAM_SUCCESS);
349 }
350 
351 /*
352  * Adds previously stored keys to a running agent.
353  */
354 static int
pam_ssh_add_keys_to_agent(pam_handle_t * pamh)355 pam_ssh_add_keys_to_agent(pam_handle_t *pamh)
356 {
357 	const struct pam_ssh_key *psk;
358 	const char **kfn;
359 	char **envlist, **env;
360 	int pam_err;
361 	int agent_fd;
362 
363 	/* switch to PAM environment */
364 	envlist = environ;
365 	if ((environ = pam_getenvlist(pamh)) == NULL) {
366 		openpam_log(PAM_LOG_DEBUG, "%s: cannot get envlist",
367 		    __func__);
368 		environ = envlist;
369 		return (PAM_SYSTEM_ERR);
370 	}
371 
372 	/* get a connection to the agent */
373 	if (ssh_get_authentication_socket(&agent_fd) != 0) {
374 		openpam_log(PAM_LOG_DEBUG,
375 		    "%s: cannot get authentication connection",
376 		    __func__);
377 		pam_err = PAM_SYSTEM_ERR;
378 		agent_fd = -1;
379 		goto end;
380 	}
381 
382 	/* look for keys to add to it */
383 	for (kfn = pam_ssh_keyfiles; *kfn != NULL; ++kfn) {
384 		const void *vp;
385 		pam_err = pam_get_data(pamh, *kfn, &vp);
386 		psk = vp;
387 		if (pam_err == PAM_SUCCESS && psk != NULL) {
388 			if (ssh_add_identity(agent_fd, psk->key, psk->comment))
389 				openpam_log(PAM_LOG_DEBUG,
390 				    "added %s to ssh agent", psk->comment);
391 			else
392 				openpam_log(PAM_LOG_DEBUG, "failed "
393 				    "to add %s to ssh agent", psk->comment);
394 			/* we won't need the key again, so wipe it */
395 			pam_set_data(pamh, *kfn, NULL, NULL);
396 		}
397 	}
398 	pam_err = PAM_SUCCESS;
399  end:
400 	/* disconnect from agent */
401 	if (agent_fd != -1)
402 		ssh_close_authentication_socket(agent_fd);
403 
404 	/* switch back to original environment */
405 	for (env = environ; *env != NULL; ++env)
406 		free(*env);
407 	free(environ);
408 	environ = envlist;
409 
410 	return (pam_err);
411 }
412 
413 PAM_EXTERN int
pam_sm_open_session(pam_handle_t * pamh,int flags __unused,int argc __unused,const char * argv[]__unused)414 pam_sm_open_session(pam_handle_t *pamh, int flags __unused,
415     int argc __unused, const char *argv[] __unused)
416 {
417 	struct passwd *pwd, pwres;
418 	const char *user;
419 	const void *data;
420 	int pam_err = PAM_SUCCESS;
421 	char pwbuf[1024];
422 
423 	/* no keys, no work */
424 	if (pam_get_data(pamh, pam_ssh_have_keys, &data) != PAM_SUCCESS &&
425 	    openpam_get_option(pamh, "want_agent") == NULL)
426 		return (PAM_SUCCESS);
427 
428 	/* switch to user credentials */
429 	pam_err = pam_get_user(pamh, &user, NULL);
430 	if (pam_err != PAM_SUCCESS)
431 		return (pam_err);
432 	if (getpwnam_r(user, &pwres, pwbuf, sizeof(pwbuf), &pwd) != 0 ||
433 	    pwd == NULL)
434 		return (PAM_USER_UNKNOWN);
435 
436 	/* start the agent */
437 	pam_err = pam_ssh_start_agent(pamh, pwd);
438 	if (pam_err != PAM_SUCCESS)
439 		return pam_err;
440 
441 	pam_err = openpam_borrow_cred(pamh, pwd);
442 	if (pam_err != PAM_SUCCESS)
443 		return pam_err;
444 
445 	/* we have an agent, see if we can add any keys to it */
446 	pam_err = pam_ssh_add_keys_to_agent(pamh);
447 	if (pam_err != PAM_SUCCESS) {
448 		/* XXX ignore failures */
449 		openpam_log(PAM_LOG_DEBUG, "failed adding keys to ssh agent");
450 		pam_err = PAM_SUCCESS;
451 	}
452 
453 	openpam_restore_cred(pamh);
454 	return pam_err;
455 }
456 
457 PAM_EXTERN int
pam_sm_close_session(pam_handle_t * pamh,int flags __unused,int argc __unused,const char * argv[]__unused)458 pam_sm_close_session(pam_handle_t *pamh, int flags __unused,
459     int argc __unused, const char *argv[] __unused)
460 {
461 	const char *ssh_agent_pid;
462 	char *end;
463 	int status;
464 	pid_t pid;
465 
466 	if ((ssh_agent_pid = pam_getenv(pamh, "SSH_AGENT_PID")) == NULL) {
467 		openpam_log(PAM_LOG_DEBUG, "no ssh agent");
468 		return (PAM_SUCCESS);
469 	}
470 	pid = (pid_t)strtol(ssh_agent_pid, &end, 10);
471 	if (*ssh_agent_pid == '\0' || *end != '\0') {
472 		openpam_log(PAM_LOG_DEBUG, "invalid ssh agent pid");
473 		return (PAM_SESSION_ERR);
474 	}
475 	openpam_log(PAM_LOG_DEBUG, "killing ssh agent %d", (int)pid);
476 	if (kill(pid, SIGTERM) == -1 ||
477 	    (waitpid(pid, &status, 0) == -1 && errno != ECHILD))
478 		return (PAM_SYSTEM_ERR);
479 	return (PAM_SUCCESS);
480 }
481 
482 PAM_MODULE_ENTRY("pam_ssh");
483