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
73extern char **environ;
74
75struct pam_ssh_key {
76	struct sshkey	*key;
77	char	*comment;
78};
79
80static const char *pam_ssh_prompt = "SSH passphrase: ";
81static const char *pam_ssh_have_keys = "pam_ssh_have_keys";
82
83static 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
91static const char *pam_ssh_agent = "/usr/bin/ssh-agent";
92static const char *const pam_ssh_agent_argv[] = { "ssh_agent", "-s", NULL };
93static 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 */
100static struct pam_ssh_key *
101pam_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 */
151static void
152pam_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
163PAM_EXTERN int
164pam_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
236PAM_EXTERN int
237pam_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 */
247static void
248pam_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 */
283static int
284pam_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));
337done:
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 */
354static int
355pam_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
413PAM_EXTERN int
414pam_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
457PAM_EXTERN int
458pam_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
482PAM_MODULE_ENTRY("pam_ssh");
483