auth-rsa.c

   1 /*
   2  *
   3  * auth-rsa.c
   4  *
   5  * Author: Tatu Ylonen <ylo@cs.hut.fi>
   6  *
   7  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
   8  *                    All rights reserved
   9  *
  10  * Created: Mon Mar 27 01:46:52 1995 ylo
  11  *
  12  * RSA-based authentication.  This code determines whether to admit a login
  13  * based on RSA authentication.  This file also contains functions to check
  14  * validity of the host key.
  15  *
  16  */
  17
  18 #include "includes.h"
  19 RCSID("$Id$");
  20
  21 #include "rsa.h"
  22 #include "packet.h"
  23 #include "xmalloc.h"
  24 #include "ssh.h"
  25 #include "mpaux.h"
  26 #include "uidswap.h"
  27 #include "match.h"
  28 #include "servconf.h"
  29
  30 #include <openssl/rsa.h>
  31 #include <openssl/md5.h>
  32
  33 /* Flags that may be set in authorized_keys options. */
  34 extern int no_port_forwarding_flag;
  35 extern int no_agent_forwarding_flag;
  36 extern int no_x11_forwarding_flag;
  37 extern int no_pty_flag;
  38 extern char *forced_command;
  39 extern struct envstring *custom_environment;
  40
  41 /*
  42  * Session identifier that is used to bind key exchange and authentication
  43  * responses to a particular session.
  44  */
  45 extern unsigned char session_id[16];
  46
  47 /*
  48  * The .ssh/authorized_keys file contains public keys, one per line, in the
  49  * following format:
  50  *   options bits e n comment
  51  * where bits, e and n are decimal numbers,
  52  * and comment is any string of characters up to newline.  The maximum
  53  * length of a line is 8000 characters.  See the documentation for a
  54  * description of the options.
  55  */
  56
  57 /*
  58  * Performs the RSA authentication challenge-response dialog with the client,
  59  * and returns true (non-zero) if the client gave the correct answer to
  60  * our challenge; returns zero if the client gives a wrong answer.
  61  */
  62
  63 int
  64 auth_rsa_challenge_dialog(RSA *pk)
  65 {
  66         BIGNUM *challenge, *encrypted_challenge;
  67         BN_CTX *ctx;
  68         unsigned char buf[32], mdbuf[16], response[16];
  69         MD5_CTX md;
  70         unsigned int i;
  71         int plen, len;
  72
  73         encrypted_challenge = BN_new();
  74         challenge = BN_new();
  75
  76         /* Generate a random challenge. */
  77         BN_rand(challenge, 256, 0, 0);
  78         ctx = BN_CTX_new();
  79         BN_mod(challenge, challenge, pk->n, ctx);
  80         BN_CTX_free(ctx);
  81
  82         /* Encrypt the challenge with the public key. */
  83         rsa_public_encrypt(encrypted_challenge, challenge, pk);
  84
  85         /* Send the encrypted challenge to the client. */
  86         packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
  87         packet_put_bignum(encrypted_challenge);
  88         packet_send();
  89         BN_clear_free(encrypted_challenge);
  90         packet_write_wait();
  91
  92         /* Wait for a response. */
  93         packet_read_expect(&plen, SSH_CMSG_AUTH_RSA_RESPONSE);
  94         packet_integrity_check(plen, 16, SSH_CMSG_AUTH_RSA_RESPONSE);
  95         for (i = 0; i < 16; i++)
  96                 response[i] = packet_get_char();
  97
  98         /* The response is MD5 of decrypted challenge plus session id. */
  99         len = BN_num_bytes(challenge);
 100         if (len <= 0 || len > 32)
 101                 fatal("auth_rsa_challenge_dialog: bad challenge length %d", len);
 102         memset(buf, 0, 32);
 103         BN_bn2bin(challenge, buf + 32 - len);
 104         MD5_Init(&md);
 105         MD5_Update(&md, buf, 32);
 106         MD5_Update(&md, session_id, 16);
 107         MD5_Final(mdbuf, &md);
 108         BN_clear_free(challenge);
 109
 110         /* Verify that the response is the original challenge. */
 111         if (memcmp(response, mdbuf, 16) != 0) {
 112                 /* Wrong answer. */
 113                 return 0;
 114         }
 115         /* Correct answer. */
 116         return 1;
 117 }
 118
 119 /*
 120  * Performs the RSA authentication dialog with the client.  This returns
 121  * 0 if the client could not be authenticated, and 1 if authentication was
 122  * successful.  This may exit if there is a serious protocol violation.
 123  */
 124
 125 int
 126 auth_rsa(struct passwd *pw, BIGNUM *client_n)
 127 {
 128         extern ServerOptions options;
 129         char line[8192], file[1024];
 130         int authenticated;
 131         unsigned int bits;
 132         FILE *f;
 133         unsigned long linenum = 0;
 134         struct stat st;
 135         RSA *pk;
 136
 137         /* Temporarily use the user's uid. */
 138         temporarily_use_uid(pw->pw_uid);
 139
 140         /* The authorized keys. */
 141         snprintf(file, sizeof file, "%.500s/%.100s", pw->pw_dir,
 142                  SSH_USER_PERMITTED_KEYS);
 143
 144         /* Fail quietly if file does not exist */
 145         if (stat(file, &st) < 0) {
 146                 /* Restore the privileged uid. */
 147                 restore_uid();
 148                 return 0;
 149         }
 150         /* Open the file containing the authorized keys. */
 151         f = fopen(file, "r");
 152         if (!f) {
 153                 /* Restore the privileged uid. */
 154                 restore_uid();
 155                 packet_send_debug("Could not open %.900s for reading.", file);
 156                 packet_send_debug("If your home is on an NFS volume, it may need to be world-readable.");
 157                 return 0;
 158         }
 159         if (options.strict_modes) {
 160                 int fail = 0;
 161                 char buf[1024];
 162                 /* Check open file in order to avoid open/stat races */
 163                 if (fstat(fileno(f), &st) < 0 ||
 164                     (st.st_uid != 0 && st.st_uid != pw->pw_uid) ||
 165                     (st.st_mode & 022) != 0) {
 166                         snprintf(buf, sizeof buf, "RSA authentication refused for %.100s: "
 167                                  "bad ownership or modes for '%s'.", pw->pw_name, file);
 168                         fail = 1;
 169                 } else {
 170                         /* Check path to SSH_USER_PERMITTED_KEYS */
 171                         int i;
 172                         static const char *check[] = {
 173                                 "", SSH_USER_DIR, NULL
 174                         };
 175                         for (i = 0; check[i]; i++) {
 176                                 snprintf(line, sizeof line, "%.500s/%.100s", pw->pw_dir, check[i]);
 177                                 if (stat(line, &st) < 0 ||
 178                                     (st.st_uid != 0 && st.st_uid != pw->pw_uid) ||
 179                                     (st.st_mode & 022) != 0) {
 180                                         snprintf(buf, sizeof buf, "RSA authentication refused for %.100s: "
 181                                                  "bad ownership or modes for '%s'.", pw->pw_name, line);
 182                                         fail = 1;
 183                                         break;
 184                                 }
 185                         }
 186                 }
 187                 if (fail) {
 188                         fclose(f);
 189                         log(buf);
 190                         packet_send_debug(buf);
 191                         restore_uid();
 192                         return 0;
 193                 }
 194         }
 195         /* Flag indicating whether authentication has succeeded. */
 196         authenticated = 0;
 197
 198         pk = RSA_new();
 199         pk->e = BN_new();
 200         pk->n = BN_new();
 201
 202         /*
 203          * Go though the accepted keys, looking for the current key.  If
 204          * found, perform a challenge-response dialog to verify that the
 205          * user really has the corresponding private key.
 206          */
 207         while (fgets(line, sizeof(line), f)) {
 208                 char *cp;
 209                 char *options;
 210
 211                 linenum++;
 212
 213                 /* Skip leading whitespace, empty and comment lines. */
 214                 for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
 215                         ;
 216                 if (!*cp || *cp == '\n' || *cp == '#')
 217                         continue;
 218
 219                 /*
 220                  * Check if there are options for this key, and if so,
 221                  * save their starting address and skip the option part
 222                  * for now.  If there are no options, set the starting
 223                  * address to NULL.
 224                  */
 225                 if (*cp < '0' || *cp > '9') {
 226                         int quoted = 0;
 227                         options = cp;
 228                         for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
 229                                 if (*cp == '\\' && cp[1] == '"')
 230                                         cp++;   /* Skip both */
 231                                 else if (*cp == '"')
 232                                         quoted = !quoted;
 233                         }
 234                 } else
 235                         options = NULL;
 236
 237                 /* Parse the key from the line. */
 238                 if (!auth_rsa_read_key(&cp, &bits, pk->e, pk->n)) {
 239                         debug("%.100s, line %lu: bad key syntax",
 240                               SSH_USER_PERMITTED_KEYS, linenum);
 241                         packet_send_debug("%.100s, line %lu: bad key syntax",
 242                                           SSH_USER_PERMITTED_KEYS, linenum);
 243                         continue;
 244                 }
 245                 /* cp now points to the comment part. */
 246
 247                 /* Check if the we have found the desired key (identified by its modulus). */
 248                 if (BN_cmp(pk->n, client_n) != 0)
 249                         continue;
 250
 251                 /* check the real bits  */
 252                 if (bits != BN_num_bits(pk->n))
 253                         log("Warning: %s, line %ld: keysize mismatch: "
 254                             "actual %d vs. announced %d.",
 255                             file, linenum, BN_num_bits(pk->n), bits);
 256
 257                 /* We have found the desired key. */
 258
 259
 260                 /* Perform the challenge-response dialog for this key. */
 261                 if (!auth_rsa_challenge_dialog(pk)) {
 262                         /* Wrong response. */
 263                         verbose("Wrong response to RSA authentication challenge.");
 264                         packet_send_debug("Wrong response to RSA authentication challenge.");
 265                         continue;
 266                 }
 267                 /*
 268                  * Correct response.  The client has been successfully
 269                  * authenticated. Note that we have not yet processed the
 270                  * options; this will be reset if the options cause the
 271                  * authentication to be rejected.
 272                  */
 273                 authenticated = 1;
 274
 275                 /* RSA part of authentication was accepted.  Now process the options. */
 276                 if (options) {
 277                         while (*options && *options != ' ' && *options != '\t') {
 278                                 cp = "no-port-forwarding";
 279                                 if (strncmp(options, cp, strlen(cp)) == 0) {
 280                                         packet_send_debug("Port forwarding disabled.");
 281                                         no_port_forwarding_flag = 1;
 282                                         options += strlen(cp);
 283                                         goto next_option;
 284                                 }
 285                                 cp = "no-agent-forwarding";
 286                                 if (strncmp(options, cp, strlen(cp)) == 0) {
 287                                         packet_send_debug("Agent forwarding disabled.");
 288                                         no_agent_forwarding_flag = 1;
 289                                         options += strlen(cp);
 290                                         goto next_option;
 291                                 }
 292                                 cp = "no-X11-forwarding";
 293                                 if (strncmp(options, cp, strlen(cp)) == 0) {
 294                                         packet_send_debug("X11 forwarding disabled.");
 295                                         no_x11_forwarding_flag = 1;
 296                                         options += strlen(cp);
 297                                         goto next_option;
 298                                 }
 299                                 cp = "no-pty";
 300                                 if (strncmp(options, cp, strlen(cp)) == 0) {
 301                                         packet_send_debug("Pty allocation disabled.");
 302                                         no_pty_flag = 1;
 303                                         options += strlen(cp);
 304                                         goto next_option;
 305                                 }
 306                                 cp = "command=\"";
 307                                 if (strncmp(options, cp, strlen(cp)) == 0) {
 308                                         int i;
 309                                         options += strlen(cp);
 310                                         forced_command = xmalloc(strlen(options) + 1);
 311                                         i = 0;
 312                                         while (*options) {
 313                                                 if (*options == '"')
 314                                                         break;
 315                                                 if (*options == '\\' && options[1] == '"') {
 316                                                         options += 2;
 317                                                         forced_command[i++] = '"';
 318                                                         continue;
 319                                                 }
 320                                                 forced_command[i++] = *options++;
 321                                         }
 322                                         if (!*options) {
 323                                                 debug("%.100s, line %lu: missing end quote",
 324                                                       SSH_USER_PERMITTED_KEYS, linenum);
 325                                                 packet_send_debug("%.100s, line %lu: missing end quote",
 326                                                                   SSH_USER_PERMITTED_KEYS, linenum);
 327                                                 continue;
 328                                         }
 329                                         forced_command[i] = 0;
 330                                         packet_send_debug("Forced command: %.900s", forced_command);
 331                                         options++;
 332                                         goto next_option;
 333                                 }
 334                                 cp = "environment=\"";
 335                                 if (strncmp(options, cp, strlen(cp)) == 0) {
 336                                         int i;
 337                                         char *s;
 338                                         struct envstring *new_envstring;
 339                                         options += strlen(cp);
 340                                         s = xmalloc(strlen(options) + 1);
 341                                         i = 0;
 342                                         while (*options) {
 343                                                 if (*options == '"')
 344                                                         break;
 345                                                 if (*options == '\\' && options[1] == '"') {
 346                                                         options += 2;
 347                                                         s[i++] = '"';
 348                                                         continue;
 349                                                 }
 350                                                 s[i++] = *options++;
 351                                         }
 352                                         if (!*options) {
 353                                                 debug("%.100s, line %lu: missing end quote",
 354                                                       SSH_USER_PERMITTED_KEYS, linenum);
 355                                                 packet_send_debug("%.100s, line %lu: missing end quote",
 356                                                                   SSH_USER_PERMITTED_KEYS, linenum);
 357                                                 continue;
 358                                         }
 359                                         s[i] = 0;
 360                                         packet_send_debug("Adding to environment: %.900s", s);
 361                                         debug("Adding to environment: %.900s", s);
 362                                         options++;
 363                                         new_envstring = xmalloc(sizeof(struct envstring));
 364                                         new_envstring->s = s;
 365                                         new_envstring->next = custom_environment;
 366                                         custom_environment = new_envstring;
 367                                         goto next_option;
 368                                 }
 369                                 cp = "from=\"";
 370                                 if (strncmp(options, cp, strlen(cp)) == 0) {
 371                                         char *patterns = xmalloc(strlen(options) + 1);
 372                                         int i;
 373                                         options += strlen(cp);
 374                                         i = 0;
 375                                         while (*options) {
 376                                                 if (*options == '"')
 377                                                         break;
 378                                                 if (*options == '\\' && options[1] == '"') {
 379                                                         options += 2;
 380                                                         patterns[i++] = '"';
 381                                                         continue;
 382                                                 }
 383                                                 patterns[i++] = *options++;
 384                                         }
 385                                         if (!*options) {
 386                                                 debug("%.100s, line %lu: missing end quote",
 387                                                       SSH_USER_PERMITTED_KEYS, linenum);
 388                                                 packet_send_debug("%.100s, line %lu: missing end quote",
 389                                                                   SSH_USER_PERMITTED_KEYS, linenum);
 390                                                 continue;
 391                                         }
 392                                         patterns[i] = 0;
 393                                         options++;
 394                                         if (!match_hostname(get_canonical_hostname(), patterns,
 395                                                      strlen(patterns)) &&
 396                                             !match_hostname(get_remote_ipaddr(), patterns,
 397                                                      strlen(patterns))) {
 398                                                 log("RSA authentication tried for %.100s with correct key but not from a permitted host (host=%.200s, ip=%.200s).",
 399                                                     pw->pw_name, get_canonical_hostname(),
 400                                                     get_remote_ipaddr());
 401                                                 packet_send_debug("Your host '%.200s' is not permitted to use this key for login.",
 402                                                 get_canonical_hostname());
 403                                                 xfree(patterns);
 404                                                 /* key invalid for this host, reset flags */
 405                                                 authenticated = 0;
 406                                                 no_agent_forwarding_flag = 0;
 407                                                 no_port_forwarding_flag = 0;
 408                                                 no_pty_flag = 0;
 409                                                 no_x11_forwarding_flag = 0;
 410                                                 while (custom_environment) {
 411                                                         struct envstring *ce = custom_environment;
 412                                                         custom_environment = ce->next;
 413                                                         xfree(ce->s);
 414                                                         xfree(ce);
 415                                                 }
 416                                                 if (forced_command) {
 417                                                         xfree(forced_command);
 418                                                         forced_command = NULL;
 419                                                 }
 420                                                 break;
 421                                         }
 422                                         xfree(patterns);
 423                                         /* Host name matches. */
 424                                         goto next_option;
 425                                 }
 426                 bad_option:
 427                                 log("Bad options in %.100s file, line %lu: %.50s",
 428                                     SSH_USER_PERMITTED_KEYS, linenum, options);
 429                                 packet_send_debug("Bad options in %.100s file, line %lu: %.50s",
 430                                                   SSH_USER_PERMITTED_KEYS, linenum, options);
 431                                 authenticated = 0;
 432                                 break;
 433
 434                 next_option:
 435                                 /*
 436                                  * Skip the comma, and move to the next option
 437                                  * (or break out if there are no more).
 438                                  */
 439                                 if (!*options)
 440                                         fatal("Bugs in auth-rsa.c option processing.");
 441                                 if (*options == ' ' || *options == '\t')
 442                                         break;          /* End of options. */
 443                                 if (*options != ',')
 444                                         goto bad_option;
 445                                 options++;
 446                                 /* Process the next option. */
 447                                 continue;
 448                         }
 449                 }
 450                 /*
 451                  * Break out of the loop if authentication was successful;
 452                  * otherwise continue searching.
 453                  */
 454                 if (authenticated)
 455                         break;
 456         }
 457
 458         /* Restore the privileged uid. */
 459         restore_uid();
 460
 461         /* Close the file. */
 462         fclose(f);
 463
 464         RSA_free(pk);
 465
 466         if (authenticated)
 467                 packet_send_debug("RSA authentication accepted.");
 468
 469         /* Return authentication result. */
 470         return authenticated;
 471 }