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                         log(buf);
 189                         packet_send_debug(buf);
 190                         restore_uid();
 191                         return 0;
 192                 }
 193         }
 194         /* Flag indicating whether authentication has succeeded. */
 195         authenticated = 0;
 196
 197         pk = RSA_new();
 198         pk->e = BN_new();
 199         pk->n = BN_new();
 200
 201         /*
 202          * Go though the accepted keys, looking for the current key.  If
 203          * found, perform a challenge-response dialog to verify that the
 204          * user really has the corresponding private key.
 205          */
 206         while (fgets(line, sizeof(line), f)) {
 207                 char *cp;
 208                 char *options;
 209
 210                 linenum++;
 211
 212                 /* Skip leading whitespace, empty and comment lines. */
 213                 for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
 214                         ;
 215                 if (!*cp || *cp == '\n' || *cp == '#')
 216                         continue;
 217
 218                 /*
 219                  * Check if there are options for this key, and if so,
 220                  * save their starting address and skip the option part
 221                  * for now.  If there are no options, set the starting
 222                  * address to NULL.
 223                  */
 224                 if (*cp < '0' || *cp > '9') {
 225                         int quoted = 0;
 226                         options = cp;
 227                         for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
 228                                 if (*cp == '\\' && cp[1] == '"')
 229                                         cp++;   /* Skip both */
 230                                 else if (*cp == '"')
 231                                         quoted = !quoted;
 232                         }
 233                 } else
 234                         options = NULL;
 235
 236                 /* Parse the key from the line. */
 237                 if (!auth_rsa_read_key(&cp, &bits, pk->e, pk->n)) {
 238                         debug("%.100s, line %lu: bad key syntax",
 239                               SSH_USER_PERMITTED_KEYS, linenum);
 240                         packet_send_debug("%.100s, line %lu: bad key syntax",
 241                                           SSH_USER_PERMITTED_KEYS, linenum);
 242                         continue;
 243                 }
 244                 /* cp now points to the comment part. */
 245
 246                 /* Check if the we have found the desired key (identified by its modulus). */
 247                 if (BN_cmp(pk->n, client_n) != 0)
 248                         continue;
 249
 250                 /* check the real bits  */
 251                 if (bits != BN_num_bits(pk->n))
 252                         log("Warning: %s, line %ld: keysize mismatch: "
 253                             "actual %d vs. announced %d.",
 254                             file, linenum, BN_num_bits(pk->n), bits);
 255
 256                 /* We have found the desired key. */
 257
 258
 259                 /* Perform the challenge-response dialog for this key. */
 260                 if (!auth_rsa_challenge_dialog(pk)) {
 261                         /* Wrong response. */
 262                         verbose("Wrong response to RSA authentication challenge.");
 263                         packet_send_debug("Wrong response to RSA authentication challenge.");
 264                         continue;
 265                 }
 266                 /*
 267                  * Correct response.  The client has been successfully
 268                  * authenticated. Note that we have not yet processed the
 269                  * options; this will be reset if the options cause the
 270                  * authentication to be rejected.
 271                  */
 272                 authenticated = 1;
 273
 274                 /* RSA part of authentication was accepted.  Now process the options. */
 275                 if (options) {
 276                         while (*options && *options != ' ' && *options != '\t') {
 277                                 cp = "no-port-forwarding";
 278                                 if (strncmp(options, cp, strlen(cp)) == 0) {
 279                                         packet_send_debug("Port forwarding disabled.");
 280                                         no_port_forwarding_flag = 1;
 281                                         options += strlen(cp);
 282                                         goto next_option;
 283                                 }
 284                                 cp = "no-agent-forwarding";
 285                                 if (strncmp(options, cp, strlen(cp)) == 0) {
 286                                         packet_send_debug("Agent forwarding disabled.");
 287                                         no_agent_forwarding_flag = 1;
 288                                         options += strlen(cp);
 289                                         goto next_option;
 290                                 }
 291                                 cp = "no-X11-forwarding";
 292                                 if (strncmp(options, cp, strlen(cp)) == 0) {
 293                                         packet_send_debug("X11 forwarding disabled.");
 294                                         no_x11_forwarding_flag = 1;
 295                                         options += strlen(cp);
 296                                         goto next_option;
 297                                 }
 298                                 cp = "no-pty";
 299                                 if (strncmp(options, cp, strlen(cp)) == 0) {
 300                                         packet_send_debug("Pty allocation disabled.");
 301                                         no_pty_flag = 1;
 302                                         options += strlen(cp);
 303                                         goto next_option;
 304                                 }
 305                                 cp = "command=\"";
 306                                 if (strncmp(options, cp, strlen(cp)) == 0) {
 307                                         int i;
 308                                         options += strlen(cp);
 309                                         forced_command = xmalloc(strlen(options) + 1);
 310                                         i = 0;
 311                                         while (*options) {
 312                                                 if (*options == '"')
 313                                                         break;
 314                                                 if (*options == '\\' && options[1] == '"') {
 315                                                         options += 2;
 316                                                         forced_command[i++] = '"';
 317                                                         continue;
 318                                                 }
 319                                                 forced_command[i++] = *options++;
 320                                         }
 321                                         if (!*options) {
 322                                                 debug("%.100s, line %lu: missing end quote",
 323                                                       SSH_USER_PERMITTED_KEYS, linenum);
 324                                                 packet_send_debug("%.100s, line %lu: missing end quote",
 325                                                                   SSH_USER_PERMITTED_KEYS, linenum);
 326                                                 continue;
 327                                         }
 328                                         forced_command[i] = 0;
 329                                         packet_send_debug("Forced command: %.900s", forced_command);
 330                                         options++;
 331                                         goto next_option;
 332                                 }
 333                                 cp = "environment=\"";
 334                                 if (strncmp(options, cp, strlen(cp)) == 0) {
 335                                         int i;
 336                                         char *s;
 337                                         struct envstring *new_envstring;
 338                                         options += strlen(cp);
 339                                         s = xmalloc(strlen(options) + 1);
 340                                         i = 0;
 341                                         while (*options) {
 342                                                 if (*options == '"')
 343                                                         break;
 344                                                 if (*options == '\\' && options[1] == '"') {
 345                                                         options += 2;
 346                                                         s[i++] = '"';
 347                                                         continue;
 348                                                 }
 349                                                 s[i++] = *options++;
 350                                         }
 351                                         if (!*options) {
 352                                                 debug("%.100s, line %lu: missing end quote",
 353                                                       SSH_USER_PERMITTED_KEYS, linenum);
 354                                                 packet_send_debug("%.100s, line %lu: missing end quote",
 355                                                                   SSH_USER_PERMITTED_KEYS, linenum);
 356                                                 continue;
 357                                         }
 358                                         s[i] = 0;
 359                                         packet_send_debug("Adding to environment: %.900s", s);
 360                                         debug("Adding to environment: %.900s", s);
 361                                         options++;
 362                                         new_envstring = xmalloc(sizeof(struct envstring));
 363                                         new_envstring->s = s;
 364                                         new_envstring->next = custom_environment;
 365                                         custom_environment = new_envstring;
 366                                         goto next_option;
 367                                 }
 368                                 cp = "from=\"";
 369                                 if (strncmp(options, cp, strlen(cp)) == 0) {
 370                                         char *patterns = xmalloc(strlen(options) + 1);
 371                                         int i;
 372                                         options += strlen(cp);
 373                                         i = 0;
 374                                         while (*options) {
 375                                                 if (*options == '"')
 376                                                         break;
 377                                                 if (*options == '\\' && options[1] == '"') {
 378                                                         options += 2;
 379                                                         patterns[i++] = '"';
 380                                                         continue;
 381                                                 }
 382                                                 patterns[i++] = *options++;
 383                                         }
 384                                         if (!*options) {
 385                                                 debug("%.100s, line %lu: missing end quote",
 386                                                       SSH_USER_PERMITTED_KEYS, linenum);
 387                                                 packet_send_debug("%.100s, line %lu: missing end quote",
 388                                                                   SSH_USER_PERMITTED_KEYS, linenum);
 389                                                 continue;
 390                                         }
 391                                         patterns[i] = 0;
 392                                         options++;
 393                                         if (!match_hostname(get_canonical_hostname(), patterns,
 394                                                      strlen(patterns)) &&
 395                                             !match_hostname(get_remote_ipaddr(), patterns,
 396                                                      strlen(patterns))) {
 397                                                 log("RSA authentication tried for %.100s with correct key but not from a permitted host (host=%.200s, ip=%.200s).",
 398                                                     pw->pw_name, get_canonical_hostname(),
 399                                                     get_remote_ipaddr());
 400                                                 packet_send_debug("Your host '%.200s' is not permitted to use this key for login.",
 401                                                 get_canonical_hostname());
 402                                                 xfree(patterns);
 403                                                 /* key invalid for this host, reset flags */
 404                                                 authenticated = 0;
 405                                                 no_agent_forwarding_flag = 0;
 406                                                 no_port_forwarding_flag = 0;
 407                                                 no_pty_flag = 0;
 408                                                 no_x11_forwarding_flag = 0;
 409                                                 while (custom_environment) {
 410                                                         struct envstring *ce = custom_environment;
 411                                                         custom_environment = ce->next;
 412                                                         xfree(ce->s);
 413                                                         xfree(ce);
 414                                                 }
 415                                                 if (forced_command) {
 416                                                         xfree(forced_command);
 417                                                         forced_command = NULL;
 418                                                 }
 419                                                 break;
 420                                         }
 421                                         xfree(patterns);
 422                                         /* Host name matches. */
 423                                         goto next_option;
 424                                 }
 425                 bad_option:
 426                                 log("Bad options in %.100s file, line %lu: %.50s",
 427                                     SSH_USER_PERMITTED_KEYS, linenum, options);
 428                                 packet_send_debug("Bad options in %.100s file, line %lu: %.50s",
 429                                                   SSH_USER_PERMITTED_KEYS, linenum, options);
 430                                 authenticated = 0;
 431                                 break;
 432
 433                 next_option:
 434                                 /*
 435                                  * Skip the comma, and move to the next option
 436                                  * (or break out if there are no more).
 437                                  */
 438                                 if (!*options)
 439                                         fatal("Bugs in auth-rsa.c option processing.");
 440                                 if (*options == ' ' || *options == '\t')
 441                                         break;          /* End of options. */
 442                                 if (*options != ',')
 443                                         goto bad_option;
 444                                 options++;
 445                                 /* Process the next option. */
 446                                 continue;
 447                         }
 448                 }
 449                 /*
 450                  * Break out of the loop if authentication was successful;
 451                  * otherwise continue searching.
 452                  */
 453                 if (authenticated)
 454                         break;
 455         }
 456
 457         /* Restore the privileged uid. */
 458         restore_uid();
 459
 460         /* Close the file. */
 461         fclose(f);
 462
 463         RSA_free(pk);
 464
 465         if (authenticated)
 466                 packet_send_debug("RSA authentication accepted.");
 467
 468         /* Return authentication result. */
 469         return authenticated;
 470 }