1 /* $NetBSD: pam_ssh.c,v 1.25 2018/04/07 19:28:32 christos 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 1.25 2018/04/07 19:28:32 christos 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 "key.h" 66 #include "buffer.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) 72 73 extern char **environ; 74 75 struct pam_ssh_key { 76 Key *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 * 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 char *comment; 107 Key *key; 108 109 if (snprintf(fn, sizeof(fn), "%s/%s", dir, kfn) > (int)sizeof(fn)) 110 return (NULL); 111 comment = NULL; 112 /* 113 * If the key is unencrypted, OpenSSL ignores the passphrase, so 114 * it will seem like the user typed in the right one. This allows 115 * a user to circumvent nullok by providing a dummy passphrase. 116 * Verify that the key really *is* encrypted by trying to load it 117 * with an empty passphrase, and if the key is not encrypted, 118 * accept only an empty passphrase. 119 */ 120 key = key_load_private(fn, "", &comment); 121 if (key != NULL && !(*passphrase == '\0' && nullok)) { 122 key_free(key); 123 free(comment); 124 return (NULL); 125 } 126 if (key == NULL) 127 key = key_load_private(fn, passphrase, &comment); 128 if (key == NULL) { 129 openpam_log(PAM_LOG_DEBUG, "failed to load key from %s", fn); 130 if (comment != NULL) 131 free(comment); 132 return (NULL); 133 } 134 135 openpam_log(PAM_LOG_DEBUG, "loaded '%s' from %s", comment, fn); 136 if ((psk = malloc(sizeof(*psk))) == NULL) { 137 key_free(key); 138 free(comment); 139 return (NULL); 140 } 141 psk->key = key; 142 psk->comment = comment; 143 return (psk); 144 } 145 146 /* 147 * Wipes a private key and frees the associated resources. 148 */ 149 static void 150 pam_ssh_free_key(pam_handle_t *pamh __unused, 151 void *data, int pam_err __unused) 152 { 153 struct pam_ssh_key *psk; 154 155 psk = data; 156 key_free(psk->key); 157 free(psk->comment); 158 free(psk); 159 } 160 161 PAM_EXTERN int 162 pam_sm_authenticate(pam_handle_t *pamh, int flags __unused, 163 int argc __unused, const char *argv[] __unused) 164 { 165 const char **kfn, *passphrase, *user; 166 const void *item; 167 struct passwd *pwd, pwres; 168 struct pam_ssh_key *psk; 169 int nkeys, nullok, pam_err, pass; 170 char pwbuf[1024]; 171 172 nullok = (openpam_get_option(pamh, "nullok") != NULL); 173 174 /* PEM is not loaded by default */ 175 OpenSSL_add_all_algorithms(); 176 177 /* get user name and home directory */ 178 pam_err = pam_get_user(pamh, &user, NULL); 179 if (pam_err != PAM_SUCCESS) 180 return (pam_err); 181 if (getpwnam_r(user, &pwres, pwbuf, sizeof(pwbuf), &pwd) != 0 || 182 pwd == NULL) 183 return (PAM_USER_UNKNOWN); 184 if (pwd->pw_dir == NULL) 185 return (PAM_AUTH_ERR); 186 187 nkeys = 0; 188 pass = (pam_get_item(pamh, PAM_AUTHTOK, &item) == PAM_SUCCESS && 189 item != NULL); 190 load_keys: 191 /* get passphrase */ 192 pam_err = pam_get_authtok(pamh, PAM_AUTHTOK, 193 &passphrase, pam_ssh_prompt); 194 if (pam_err != PAM_SUCCESS) 195 return (pam_err); 196 197 /* switch to user credentials */ 198 pam_err = openpam_borrow_cred(pamh, pwd); 199 if (pam_err != PAM_SUCCESS) 200 return (pam_err); 201 202 /* try to load keys from all keyfiles we know of */ 203 for (kfn = pam_ssh_keyfiles; *kfn != NULL; ++kfn) { 204 psk = pam_ssh_load_key(pwd->pw_dir, *kfn, passphrase, nullok); 205 if (psk != NULL) { 206 pam_set_data(pamh, *kfn, psk, pam_ssh_free_key); 207 ++nkeys; 208 } 209 } 210 211 /* switch back to arbitrator credentials */ 212 openpam_restore_cred(pamh); 213 214 /* 215 * If we tried an old token and didn't get anything, and 216 * try_first_pass was specified, try again after prompting the 217 * user for a new passphrase. 218 */ 219 if (nkeys == 0 && pass == 1 && 220 openpam_get_option(pamh, "try_first_pass") != NULL) { 221 pam_set_item(pamh, PAM_AUTHTOK, NULL); 222 pass = 0; 223 goto load_keys; 224 } 225 226 /* no keys? */ 227 if (nkeys == 0) 228 return (PAM_AUTH_ERR); 229 230 pam_set_data(pamh, pam_ssh_have_keys, NULL, NULL); 231 return (PAM_SUCCESS); 232 } 233 234 PAM_EXTERN int 235 pam_sm_setcred(pam_handle_t *pamh __unused, int flags __unused, 236 int argc __unused, const char *argv[] __unused) 237 { 238 239 return (PAM_SUCCESS); 240 } 241 242 /* 243 * Parses a line from ssh-agent's output. 244 */ 245 static void 246 pam_ssh_process_agent_output(pam_handle_t *pamh, FILE *f) 247 { 248 char *line, *p, *key, *val; 249 size_t len; 250 251 while ((line = fgetln(f, &len)) != NULL) { 252 if (len < 4 || strncmp(line, "SSH_", 4) != 0) 253 continue; 254 255 /* find equal sign at end of key */ 256 for (p = key = line; p < line + len; ++p) 257 if (*p == '=') 258 break; 259 if (p == line + len || *p != '=') 260 continue; 261 *p = '\0'; 262 263 /* find semicolon at end of value */ 264 for (val = ++p; p < line + len; ++p) 265 if (*p == ';') 266 break; 267 if (p == line + len || *p != ';') 268 continue; 269 *p = '\0'; 270 271 /* store key-value pair in environment */ 272 openpam_log(PAM_LOG_DEBUG, "got %s: %s", key, val); 273 pam_setenv(pamh, key, val, 1); 274 } 275 } 276 277 /* 278 * Starts an ssh agent and stores the environment variables derived from 279 * its output. 280 */ 281 static int 282 pam_ssh_start_agent(pam_handle_t *pamh, struct passwd *pwd) 283 { 284 int agent_pipe[2]; 285 pid_t pid; 286 FILE *f; 287 288 /* get a pipe which we will use to read the agent's output */ 289 if (pipe(agent_pipe) == -1) 290 return (PAM_SYSTEM_ERR); 291 292 /* start the agent */ 293 openpam_log(PAM_LOG_DEBUG, "starting an ssh agent"); 294 pid = fork(); 295 if (pid == (pid_t)-1) { 296 /* failed */ 297 close(agent_pipe[0]); 298 close(agent_pipe[1]); 299 return (PAM_SYSTEM_ERR); 300 } 301 if (pid == 0) { 302 #ifndef F_CLOSEM 303 int fd; 304 #endif 305 /* child: drop privs, close fds and start agent */ 306 if (setgid(pwd->pw_gid) == -1) { 307 openpam_log(PAM_LOG_DEBUG, "%s: Cannot setgid %d (%s)", 308 __func__, (int)pwd->pw_gid, strerror(errno)); 309 goto done; 310 } 311 if (initgroups(pwd->pw_name, pwd->pw_gid) == -1) { 312 openpam_log(PAM_LOG_DEBUG, 313 "%s: Cannot initgroups for %s (%s)", 314 __func__, pwd->pw_name, strerror(errno)); 315 goto done; 316 } 317 if (setuid(pwd->pw_uid) == -1) { 318 openpam_log(PAM_LOG_DEBUG, "%s: Cannot setuid %d (%s)", 319 __func__, (int)pwd->pw_uid, strerror(errno)); 320 goto done; 321 } 322 (void)close(STDIN_FILENO); 323 (void)open(_PATH_DEVNULL, O_RDONLY); 324 (void)dup2(agent_pipe[1], STDOUT_FILENO); 325 (void)dup2(agent_pipe[1], STDERR_FILENO); 326 #ifdef F_CLOSEM 327 (void)fcntl(3, F_CLOSEM, 0); 328 #else 329 for (fd = 3; fd < getdtablesize(); ++fd) 330 (void)close(fd); 331 #endif 332 (void)execve(pam_ssh_agent, 333 (char **)__UNCONST(pam_ssh_agent_argv), 334 (char **)__UNCONST(pam_ssh_agent_envp)); 335 done: 336 _exit(127); 337 } 338 339 /* parent */ 340 close(agent_pipe[1]); 341 if ((f = fdopen(agent_pipe[0], "r")) == NULL) 342 return (PAM_SYSTEM_ERR); 343 pam_ssh_process_agent_output(pamh, f); 344 fclose(f); 345 346 return (PAM_SUCCESS); 347 } 348 349 /* 350 * Adds previously stored keys to a running agent. 351 */ 352 static int 353 pam_ssh_add_keys_to_agent(pam_handle_t *pamh) 354 { 355 const struct pam_ssh_key *psk; 356 const char **kfn; 357 char **envlist, **env; 358 int pam_err; 359 int agent_fd; 360 361 /* switch to PAM environment */ 362 envlist = environ; 363 if ((environ = pam_getenvlist(pamh)) == NULL) { 364 openpam_log(PAM_LOG_DEBUG, "%s: cannot get envlist", 365 __func__); 366 environ = envlist; 367 return (PAM_SYSTEM_ERR); 368 } 369 370 /* get a connection to the agent */ 371 if (ssh_get_authentication_socket(&agent_fd) != 0) { 372 openpam_log(PAM_LOG_DEBUG, 373 "%s: cannot get authentication connection", 374 __func__); 375 pam_err = PAM_SYSTEM_ERR; 376 agent_fd = -1; 377 goto end; 378 } 379 380 /* look for keys to add to it */ 381 for (kfn = pam_ssh_keyfiles; *kfn != NULL; ++kfn) { 382 const void *vp; 383 pam_err = pam_get_data(pamh, *kfn, &vp); 384 psk = vp; 385 if (pam_err == PAM_SUCCESS && psk != NULL) { 386 if (ssh_add_identity(agent_fd, psk->key, psk->comment)) 387 openpam_log(PAM_LOG_DEBUG, 388 "added %s to ssh agent", psk->comment); 389 else 390 openpam_log(PAM_LOG_DEBUG, "failed " 391 "to add %s to ssh agent", psk->comment); 392 /* we won't need the key again, so wipe it */ 393 pam_set_data(pamh, *kfn, NULL, NULL); 394 } 395 } 396 pam_err = PAM_SUCCESS; 397 end: 398 /* disconnect from agent */ 399 if (agent_fd != -1) 400 ssh_close_authentication_socket(agent_fd); 401 402 /* switch back to original environment */ 403 for (env = environ; *env != NULL; ++env) 404 free(*env); 405 free(environ); 406 environ = envlist; 407 408 return (pam_err); 409 } 410 411 PAM_EXTERN int 412 pam_sm_open_session(pam_handle_t *pamh, int flags __unused, 413 int argc __unused, const char *argv[] __unused) 414 { 415 struct passwd *pwd, pwres; 416 const char *user; 417 const void *data; 418 int pam_err = PAM_SUCCESS; 419 char pwbuf[1024]; 420 421 /* no keys, no work */ 422 if (pam_get_data(pamh, pam_ssh_have_keys, &data) != PAM_SUCCESS && 423 openpam_get_option(pamh, "want_agent") == NULL) 424 return (PAM_SUCCESS); 425 426 /* switch to user credentials */ 427 pam_err = pam_get_user(pamh, &user, NULL); 428 if (pam_err != PAM_SUCCESS) 429 return (pam_err); 430 if (getpwnam_r(user, &pwres, pwbuf, sizeof(pwbuf), &pwd) != 0 || 431 pwd == NULL) 432 return (PAM_USER_UNKNOWN); 433 434 /* start the agent */ 435 pam_err = pam_ssh_start_agent(pamh, pwd); 436 if (pam_err != PAM_SUCCESS) 437 return pam_err; 438 439 pam_err = openpam_borrow_cred(pamh, pwd); 440 if (pam_err != PAM_SUCCESS) 441 return pam_err; 442 443 /* we have an agent, see if we can add any keys to it */ 444 pam_err = pam_ssh_add_keys_to_agent(pamh); 445 if (pam_err != PAM_SUCCESS) { 446 /* XXX ignore failures */ 447 openpam_log(PAM_LOG_DEBUG, "failed adding keys to ssh agent"); 448 pam_err = PAM_SUCCESS; 449 } 450 451 openpam_restore_cred(pamh); 452 return pam_err; 453 } 454 455 PAM_EXTERN int 456 pam_sm_close_session(pam_handle_t *pamh, int flags __unused, 457 int argc __unused, const char *argv[] __unused) 458 { 459 const char *ssh_agent_pid; 460 char *end; 461 int status; 462 pid_t pid; 463 464 if ((ssh_agent_pid = pam_getenv(pamh, "SSH_AGENT_PID")) == NULL) { 465 openpam_log(PAM_LOG_DEBUG, "no ssh agent"); 466 return (PAM_SUCCESS); 467 } 468 pid = (pid_t)strtol(ssh_agent_pid, &end, 10); 469 if (*ssh_agent_pid == '\0' || *end != '\0') { 470 openpam_log(PAM_LOG_DEBUG, "invalid ssh agent pid"); 471 return (PAM_SESSION_ERR); 472 } 473 openpam_log(PAM_LOG_DEBUG, "killing ssh agent %d", (int)pid); 474 if (kill(pid, SIGTERM) == -1 || 475 (waitpid(pid, &status, 0) == -1 && errno != ECHILD)) 476 return (PAM_SYSTEM_ERR); 477 return (PAM_SUCCESS); 478 } 479 480 PAM_MODULE_ENTRY("pam_ssh"); 481