lib/pam_module/pam_ssh/pam_ssh.c

   1 /*-
   2  * Copyright (c) 2003 Networks Associates Technology, Inc.
   3  * Copyright (c) 2004-2011 Dag-Erling Smørgrav
   4  * All rights reserved.
   5  *
   6  * This software was developed for the FreeBSD Project by ThinkSec AS and
   7  * NAI Labs, the Security Research Division of Network Associates, Inc.
   8  * under DARPA/SPAWAR contract N66001-01-C-8035 ("CBOSS"), as part of the
   9  * DARPA CHATS research program.
  10  *
  11  * Redistribution and use in source and binary forms, with or without
  12  * modification, are permitted provided that the following conditions
  13  * are met:
  14  * 1. Redistributions of source code must retain the above copyright
  15  *    notice, this list of conditions and the following disclaimer.
  16  * 2. Redistributions in binary form must reproduce the above copyright
  17  *    notice, this list of conditions and the following disclaimer in the
  18  *    documentation and/or other materials provided with the distribution.
  19  * 3. The name of the author may not be used to endorse or promote
  20  *    products derived from this software without specific prior written
  21  *    permission.
  22  *
  23  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
  24  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  25  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  26  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
  27  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  28  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  29  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  30  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  31  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  32  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  33  * SUCH DAMAGE.
  34  *
  35  * $FreeBSD: head/lib/libpam/modules/pam_ssh/pam_ssh.c 296651 2016-03-11 11:38:31Z des $
  36  */
  37
  38 #include <sys/param.h>
  39 #include <sys/wait.h>
  40
  41 #include <errno.h>
  42 #include <fcntl.h>
  43 #include <paths.h>
  44 #include <pwd.h>
  45 #include <signal.h>
  46 #include <stdio.h>
  47 #include <string.h>
  48 #include <unistd.h>
  49
  50 #define PAM_SM_AUTH
  51 #define PAM_SM_SESSION
  52
  53 #include <security/pam_appl.h>
  54 #include <security/pam_modules.h>
  55 #include <security/openpam.h>
  56
  57 #include <openssl/evp.h>
  58
  59 #define __bounded__(x, y, z)
  60 #include "key.h"
  61 #include "buffer.h"
  62 #include "authfd.h"
  63 #include "authfile.h"
  64
  65 #define ssh_add_identity(auth, key, comment) \
  66         ssh_add_identity_constrained(auth, key, comment, 0, 0)
  67
  68 extern char **environ;
  69
  70 struct pam_ssh_key {
  71         Key     *key;
  72         char    *comment;
  73 };
  74
  75 static const char *pam_ssh_prompt = "SSH passphrase: ";
  76 static const char *pam_ssh_have_keys = "pam_ssh_have_keys";
  77
  78 static const char *pam_ssh_keyfiles[] = {
  79         ".ssh/identity",        /* SSH1 RSA key */
  80         ".ssh/id_rsa",          /* SSH2 RSA key */
  81         ".ssh/id_dsa",          /* SSH2 DSA key */
  82         ".ssh/id_ecdsa",        /* SSH2 ECDSA key */
  83         NULL
  84 };
  85
  86 static const char *pam_ssh_agent = "/usr/bin/ssh-agent";
  87 static char str_ssh_agent[] = "ssh-agent";
  88 static char str_dash_s[] = "-s";
  89 static char *const pam_ssh_agent_argv[] = { str_ssh_agent, str_dash_s, NULL };
  90 static char *const pam_ssh_agent_envp[] = { NULL };
  91
  92 /*
  93  * Attempts to load a private key from the specified file in the specified
  94  * directory, using the specified passphrase.  If successful, returns a
  95  * struct pam_ssh_key containing the key and its comment.
  96  */
  97 static struct pam_ssh_key *
  98 pam_ssh_load_key(const char *dir, const char *kfn, const char *passphrase,
  99     int nullok)
 100 {
 101         struct pam_ssh_key *psk;
 102         char fn[PATH_MAX];
 103         char *comment;
 104         Key *key;
 105
 106         if (snprintf(fn, sizeof(fn), "%s/%s", dir, kfn) > (int)sizeof(fn))
 107                 return (NULL);
 108         comment = NULL;
 109         /*
 110          * If the key is unencrypted, OpenSSL ignores the passphrase, so
 111          * it will seem like the user typed in the right one.  This allows
 112          * a user to circumvent nullok by providing a dummy passphrase.
 113          * Verify that the key really *is* encrypted by trying to load it
 114          * with an empty passphrase, and if the key is not encrypted,
 115          * accept only an empty passphrase.
 116          */
 117         key = key_load_private(fn, "", &comment);
 118         if (key != NULL && !(*passphrase == '\0' && nullok)) {
 119                 key_free(key);
 120                 return (NULL);
 121         }
 122         if (key == NULL)
 123                 key = key_load_private(fn, passphrase, &comment);
 124         if (key == NULL) {
 125                 openpam_log(PAM_LOG_DEBUG, "failed to load key from %s", fn);
 126                 return (NULL);
 127         }
 128
 129         openpam_log(PAM_LOG_DEBUG, "loaded '%s' from %s", comment, fn);
 130         if ((psk = malloc(sizeof(*psk))) == NULL) {
 131                 key_free(key);
 132                 free(comment);
 133                 return (NULL);
 134         }
 135         psk->key = key;
 136         psk->comment = comment;
 137         return (psk);
 138 }
 139
 140 /*
 141  * Wipes a private key and frees the associated resources.
 142  */
 143 static void
 144 pam_ssh_free_key(pam_handle_t *pamh __unused,
 145     void *data, int pam_err __unused)
 146 {
 147         struct pam_ssh_key *psk;
 148
 149         psk = data;
 150         key_free(psk->key);
 151         free(psk->comment);
 152         free(psk);
 153 }
 154
 155 PAM_EXTERN int
 156 pam_sm_authenticate(pam_handle_t *pamh, int flags __unused,
 157     int argc __unused, const char *argv[] __unused)
 158 {
 159         const char **kfn, *passphrase, *user;
 160         const void *item;
 161         struct passwd *pwd;
 162         struct pam_ssh_key *psk;
 163         int nkeys, nullok, pam_err, pass;
 164
 165         nullok = (openpam_get_option(pamh, "nullok") != NULL);
 166
 167         /* PEM is not loaded by default */
 168         OpenSSL_add_all_algorithms();
 169
 170         /* get user name and home directory */
 171         pam_err = pam_get_user(pamh, &user, NULL);
 172         if (pam_err != PAM_SUCCESS)
 173                 return (pam_err);
 174         pwd = getpwnam(user);
 175         if (pwd == NULL)
 176                 return (PAM_USER_UNKNOWN);
 177         if (pwd->pw_dir == NULL)
 178                 return (PAM_AUTH_ERR);
 179
 180         nkeys = 0;
 181         pass = (pam_get_item(pamh, PAM_AUTHTOK, &item) == PAM_SUCCESS &&
 182             item != NULL);
 183  load_keys:
 184         /* get passphrase */
 185         pam_err = pam_get_authtok(pamh, PAM_AUTHTOK,
 186             &passphrase, pam_ssh_prompt);
 187         if (pam_err != PAM_SUCCESS)
 188                 return (pam_err);
 189
 190         /* switch to user credentials */
 191         pam_err = openpam_borrow_cred(pamh, pwd);
 192         if (pam_err != PAM_SUCCESS)
 193                 return (pam_err);
 194
 195         /* try to load keys from all keyfiles we know of */
 196         for (kfn = pam_ssh_keyfiles; *kfn != NULL; ++kfn) {
 197                 psk = pam_ssh_load_key(pwd->pw_dir, *kfn, passphrase, nullok);
 198                 if (psk != NULL) {
 199                         pam_set_data(pamh, *kfn, psk, pam_ssh_free_key);
 200                         ++nkeys;
 201                 }
 202         }
 203
 204         /* switch back to arbitrator credentials */
 205         openpam_restore_cred(pamh);
 206
 207         /*
 208          * If we tried an old token and didn't get anything, and
 209          * try_first_pass was specified, try again after prompting the
 210          * user for a new passphrase.
 211          */
 212         if (nkeys == 0 && pass == 1 &&
 213             openpam_get_option(pamh, "try_first_pass") != NULL) {
 214                 pam_set_item(pamh, PAM_AUTHTOK, NULL);
 215                 pass = 0;
 216                 goto load_keys;
 217         }
 218
 219         /* no keys? */
 220         if (nkeys == 0)
 221                 return (PAM_AUTH_ERR);
 222
 223         pam_set_data(pamh, pam_ssh_have_keys, NULL, NULL);
 224         return (PAM_SUCCESS);
 225 }
 226
 227 PAM_EXTERN int
 228 pam_sm_setcred(pam_handle_t *pamh __unused, int flags __unused,
 229     int argc __unused, const char *argv[] __unused)
 230 {
 231
 232         return (PAM_SUCCESS);
 233 }
 234
 235 /*
 236  * Parses a line from ssh-agent's output.
 237  */
 238 static void
 239 pam_ssh_process_agent_output(pam_handle_t *pamh, FILE *f)
 240 {
 241         char *line, *p, *key, *val;
 242         size_t len;
 243
 244         while ((line = fgetln(f, &len)) != NULL) {
 245                 if (len < 4 || strncmp(line, "SSH_", 4) != 0)
 246                         continue;
 247
 248                 /* find equal sign at end of key */
 249                 for (p = key = line; p < line + len; ++p)
 250                         if (*p == '=')
 251                                 break;
 252                 if (p == line + len || *p != '=')
 253                         continue;
 254                 *p = '\0';
 255
 256                 /* find semicolon at end of value */
 257                 for (val = ++p; p < line + len; ++p)
 258                         if (*p == ';')
 259                                 break;
 260                 if (p == line + len || *p != ';')
 261                         continue;
 262                 *p = '\0';
 263
 264                 /* store key-value pair in environment */
 265                 openpam_log(PAM_LOG_DEBUG, "got %s: %s", key, val);
 266                 pam_setenv(pamh, key, val, 1);
 267         }
 268 }
 269
 270 /*
 271  * Starts an ssh agent and stores the environment variables derived from
 272  * its output.
 273  */
 274 static int
 275 pam_ssh_start_agent(pam_handle_t *pamh)
 276 {
 277         int agent_pipe[2];
 278         pid_t pid;
 279         FILE *f;
 280
 281         /* get a pipe which we will use to read the agent's output */
 282         if (pipe(agent_pipe) == -1)
 283                 return (PAM_SYSTEM_ERR);
 284
 285         /* start the agent */
 286         openpam_log(PAM_LOG_DEBUG, "starting an ssh agent");
 287         pid = fork();
 288         if (pid == (pid_t)-1) {
 289                 /* failed */
 290                 close(agent_pipe[0]);
 291                 close(agent_pipe[1]);
 292                 return (PAM_SYSTEM_ERR);
 293         }
 294         if (pid == 0) {
 295                 int fd;
 296
 297                 /* child: drop privs, close fds and start agent */
 298                 setgid(getegid());
 299                 setuid(geteuid());
 300                 close(STDIN_FILENO);
 301                 open(_PATH_DEVNULL, O_RDONLY);
 302                 dup2(agent_pipe[1], STDOUT_FILENO);
 303                 dup2(agent_pipe[1], STDERR_FILENO);
 304                 for (fd = 3; fd < getdtablesize(); ++fd)
 305                         close(fd);
 306                 execve(pam_ssh_agent, pam_ssh_agent_argv, pam_ssh_agent_envp);
 307                 _exit(127);
 308         }
 309
 310         /* parent */
 311         close(agent_pipe[1]);
 312         if ((f = fdopen(agent_pipe[0], "r")) == NULL)
 313                 return (PAM_SYSTEM_ERR);
 314         pam_ssh_process_agent_output(pamh, f);
 315         fclose(f);
 316
 317         return (PAM_SUCCESS);
 318 }
 319
 320 /*
 321  * Adds previously stored keys to a running agent.
 322  */
 323 static int
 324 pam_ssh_add_keys_to_agent(pam_handle_t *pamh)
 325 {
 326         const struct pam_ssh_key *psk;
 327         const char **kfn;
 328         const void *item;
 329         char **envlist, **env;
 330         int fd, pam_err;
 331
 332         /* switch to PAM environment */
 333         envlist = environ;
 334         if ((environ = pam_getenvlist(pamh)) == NULL) {
 335                 environ = envlist;
 336                 return (PAM_SYSTEM_ERR);
 337         }
 338
 339         /* get a connection to the agent */
 340         if (ssh_get_authentication_socket(&fd) != 0) {
 341                 openpam_log(PAM_LOG_DEBUG, "failed to connect to the agent");
 342                 pam_err = PAM_SYSTEM_ERR;
 343                 goto end;
 344         }
 345
 346         /* look for keys to add to it */
 347         for (kfn = pam_ssh_keyfiles; *kfn != NULL; ++kfn) {
 348                 pam_err = pam_get_data(pamh, *kfn, &item);
 349                 if (pam_err == PAM_SUCCESS && item != NULL) {
 350                         psk = item;
 351                         if (ssh_add_identity(fd, psk->key, psk->comment) == 0)
 352                                 openpam_log(PAM_LOG_DEBUG,
 353                                     "added %s to ssh agent", psk->comment);
 354                         else
 355                                 openpam_log(PAM_LOG_DEBUG, "failed "
 356                                     "to add %s to ssh agent", psk->comment);
 357                         /* we won't need the key again, so wipe it */
 358                         pam_set_data(pamh, *kfn, NULL, NULL);
 359                 }
 360         }
 361         pam_err = PAM_SUCCESS;
 362
 363         /* disconnect from agent */
 364         ssh_close_authentication_socket(fd);
 365
 366  end:
 367         /* switch back to original environment */
 368         for (env = environ; *env != NULL; ++env)
 369                 free(*env);
 370         free(environ);
 371         environ = envlist;
 372
 373         return (pam_err);
 374 }
 375
 376 PAM_EXTERN int
 377 pam_sm_open_session(pam_handle_t *pamh, int flags __unused,
 378     int argc __unused, const char *argv[] __unused)
 379 {
 380         struct passwd *pwd;
 381         const char *user;
 382         const void *data;
 383         int pam_err;
 384
 385         /* no keys, no work */
 386         if (pam_get_data(pamh, pam_ssh_have_keys, &data) != PAM_SUCCESS &&
 387             openpam_get_option(pamh, "want_agent") == NULL)
 388                 return (PAM_SUCCESS);
 389
 390         /* switch to user credentials */
 391         pam_err = pam_get_user(pamh, &user, NULL);
 392         if (pam_err != PAM_SUCCESS)
 393                 return (pam_err);
 394         pwd = getpwnam(user);
 395         if (pwd == NULL)
 396                 return (PAM_USER_UNKNOWN);
 397         pam_err = openpam_borrow_cred(pamh, pwd);
 398         if (pam_err != PAM_SUCCESS)
 399                 return (pam_err);
 400
 401         /* start the agent */
 402         pam_err = pam_ssh_start_agent(pamh);
 403         if (pam_err != PAM_SUCCESS) {
 404                 openpam_restore_cred(pamh);
 405                 return (pam_err);
 406         }
 407
 408         /* we have an agent, see if we can add any keys to it */
 409         pam_err = pam_ssh_add_keys_to_agent(pamh);
 410         if (pam_err != PAM_SUCCESS) {
 411                 /* XXX ignore failures */
 412         }
 413
 414         openpam_restore_cred(pamh);
 415         return (PAM_SUCCESS);
 416 }
 417
 418 PAM_EXTERN int
 419 pam_sm_close_session(pam_handle_t *pamh, int flags __unused,
 420     int argc __unused, const char *argv[] __unused)
 421 {
 422         const char *ssh_agent_pid;
 423         char *end;
 424         int status;
 425         pid_t pid;
 426
 427         if ((ssh_agent_pid = pam_getenv(pamh, "SSH_AGENT_PID")) == NULL) {
 428                 openpam_log(PAM_LOG_DEBUG, "no ssh agent");
 429                 return (PAM_SUCCESS);
 430         }
 431         pid = (pid_t)strtol(ssh_agent_pid, &end, 10);
 432         if (*ssh_agent_pid == '\0' || *end != '\0') {
 433                 openpam_log(PAM_LOG_DEBUG, "invalid ssh agent pid");
 434                 return (PAM_SESSION_ERR);
 435         }
 436         openpam_log(PAM_LOG_DEBUG, "killing ssh agent %d", (int)pid);
 437         if (kill(pid, SIGTERM) == -1 ||
 438             (waitpid(pid, &status, 0) == -1 && errno != ECHILD))
 439                 return (PAM_SYSTEM_ERR);
 440         return (PAM_SUCCESS);
 441 }
 442
 443 PAM_MODULE_ENTRY("pam_ssh");