src/src/tls.c

   1 /*************************************************
   2 *     Exim - an Internet mail transport agent    *
   3 *************************************************/
   4
   5 /* Copyright (c) University of Cambridge 1995 - 2018 */
   6 /* See the file NOTICE for conditions of use and distribution. */
   7
   8 /* This module provides TLS (aka SSL) support for Exim. The code for OpenSSL is
   9 based on a patch that was originally contributed by Steve Haslam. It was
  10 adapted from stunnel, a GPL program by Michal Trojnara. The code for GNU TLS is
  11 based on a patch contributed by Nikos Mavrogiannopoulos. Because these packages
  12 are so very different, the functions for each are kept in separate files. The
  13 relevant file is #included as required, after any any common functions.
  14
  15 No cryptographic code is included in Exim. All this module does is to call
  16 functions from the OpenSSL or GNU TLS libraries. */
  17
  18
  19 #include "exim.h"
  20 #include "transports/smtp.h"
  21
  22 #if !defined(DISABLE_TLS) && !defined(USE_OPENSSL) && !defined(USE_GNUTLS)
  23 # error One of USE_OPENSSL or USE_GNUTLS must be defined for a TLS build
  24 #endif
  25
  26
  27 #if defined(MACRO_PREDEF) && !defined(DISABLE_TLS)
  28 # include "macro_predef.h"
  29 # ifdef USE_GNUTLS
  30 #  include "tls-gnu.c"
  31 # else
  32 #  include "tls-openssl.c"
  33 # endif
  34 #endif
  35
  36 #ifndef MACRO_PREDEF
  37
  38 /* This module is compiled only when it is specifically requested in the
  39 build-time configuration. However, some compilers don't like compiling empty
  40 modules, so keep them happy with a dummy when skipping the rest. Make it
  41 reference itself to stop picky compilers complaining that it is unused, and put
  42 in a dummy argument to stop even pickier compilers complaining about infinite
  43 loops. */
  44
  45 #ifdef DISABLE_TLS
  46 static void dummy(int x) { dummy(x-1); }
  47 #else
  48
  49 /* Static variables that are used for buffering data by both sets of
  50 functions and the common functions below.
  51
  52 We're moving away from this; GnuTLS is already using a state, which
  53 can switch, so we can do TLS callouts during ACLs. */
  54
  55 static const int ssl_xfer_buffer_size = 4096;
  56 #ifdef USE_OPENSSL
  57 static uschar *ssl_xfer_buffer = NULL;
  58 static int ssl_xfer_buffer_lwm = 0;
  59 static int ssl_xfer_buffer_hwm = 0;
  60 static int ssl_xfer_eof = FALSE;
  61 static BOOL ssl_xfer_error = FALSE;
  62 #endif
  63
  64
  65 /*************************************************
  66 *       Expand string; give error on failure     *
  67 *************************************************/
  68
  69 /* If expansion is forced to fail, set the result NULL and return TRUE.
  70 Other failures return FALSE. For a server, an SMTP response is given.
  71
  72 Arguments:
  73   s         the string to expand; if NULL just return TRUE
  74   name      name of string being expanded (for error)
  75   result    where to put the result
  76
  77 Returns:    TRUE if OK; result may still be NULL after forced failure
  78 */
  79
  80 static BOOL
  81 expand_check(const uschar *s, const uschar *name, uschar **result, uschar ** errstr)
  82 {
  83 if (!s)
  84   *result = NULL;
  85 else if (  !(*result = expand_string(US s)) /* need to clean up const more */
  86         && !f.expand_string_forcedfail
  87         )
  88   {
  89   *errstr = US"Internal error";
  90   log_write(0, LOG_MAIN|LOG_PANIC, "expansion of %s failed: %s", name,
  91     expand_string_message);
  92   return FALSE;
  93   }
  94 return TRUE;
  95 }
  96
  97
  98 /*************************************************
  99 *        Timezone environment flipping           *
 100 *************************************************/
 101
 102 static uschar *
 103 to_tz(uschar * tz)
 104 {
 105 uschar * old = US getenv("TZ");
 106 (void) setenv("TZ", CCS tz, 1);
 107 tzset();
 108 return old;
 109 }
 110
 111 static void
 112 restore_tz(uschar * tz)
 113 {
 114 if (tz)
 115   (void) setenv("TZ", CCS tz, 1);
 116 else
 117   (void) os_unsetenv(US"TZ");
 118 tzset();
 119 }
 120
 121 /*************************************************
 122 *        Many functions are package-specific     *
 123 *************************************************/
 124
 125 #ifdef USE_GNUTLS
 126 # include "tls-gnu.c"
 127 # include "tlscert-gnu.c"
 128 # define ssl_xfer_buffer (state_server.xfer_buffer)
 129 # define ssl_xfer_buffer_lwm (state_server.xfer_buffer_lwm)
 130 # define ssl_xfer_buffer_hwm (state_server.xfer_buffer_hwm)
 131 # define ssl_xfer_eof (state_server.xfer_eof)
 132 # define ssl_xfer_error (state_server.xfer_error)
 133 #endif
 134
 135 #ifdef USE_OPENSSL
 136 # include "tls-openssl.c"
 137 # include "tlscert-openssl.c"
 138 #endif
 139
 140
 141
 142 /*************************************************
 143 *           TLS version of ungetc                *
 144 *************************************************/
 145
 146 /* Puts a character back in the input buffer. Only ever
 147 called once.
 148 Only used by the server-side TLS.
 149
 150 Arguments:
 151   ch           the character
 152
 153 Returns:       the character
 154 */
 155
 156 int
 157 tls_ungetc(int ch)
 158 {
 159 ssl_xfer_buffer[--ssl_xfer_buffer_lwm] = ch;
 160 return ch;
 161 }
 162
 163
 164
 165 /*************************************************
 166 *           TLS version of feof                  *
 167 *************************************************/
 168
 169 /* Tests for a previous EOF
 170 Only used by the server-side TLS.
 171
 172 Arguments:     none
 173 Returns:       non-zero if the eof flag is set
 174 */
 175
 176 int
 177 tls_feof(void)
 178 {
 179 return (int)ssl_xfer_eof;
 180 }
 181
 182
 183
 184 /*************************************************
 185 *              TLS version of ferror             *
 186 *************************************************/
 187
 188 /* Tests for a previous read error, and returns with errno
 189 restored to what it was when the error was detected.
 190 Only used by the server-side TLS.
 191
 192 >>>>> Hmm. Errno not handled yet. Where do we get it from?  >>>>>
 193
 194 Arguments:     none
 195 Returns:       non-zero if the error flag is set
 196 */
 197
 198 int
 199 tls_ferror(void)
 200 {
 201 return (int)ssl_xfer_error;
 202 }
 203
 204
 205 /*************************************************
 206 *           TLS version of smtp_buffered         *
 207 *************************************************/
 208
 209 /* Tests for unused chars in the TLS input buffer.
 210 Only used by the server-side TLS.
 211
 212 Arguments:     none
 213 Returns:       TRUE/FALSE
 214 */
 215
 216 BOOL
 217 tls_smtp_buffered(void)
 218 {
 219 return ssl_xfer_buffer_lwm < ssl_xfer_buffer_hwm;
 220 }
 221
 222
 223 #endif  /*DISABLE_TLS*/
 224
 225 void
 226 tls_modify_variables(tls_support * dest_tsp)
 227 {
 228 modify_variable(US"tls_bits",                 &dest_tsp->bits);
 229 modify_variable(US"tls_certificate_verified", &dest_tsp->certificate_verified);
 230 modify_variable(US"tls_cipher",               &dest_tsp->cipher);
 231 modify_variable(US"tls_peerdn",               &dest_tsp->peerdn);
 232 #ifdef USE_OPENSSL
 233 modify_variable(US"tls_sni",                  &dest_tsp->sni);
 234 #endif
 235 }
 236
 237
 238 #ifndef DISABLE_TLS
 239 /************************************************
 240 *       TLS certificate name operations         *
 241 ************************************************/
 242
 243 /* Convert an rfc4514 DN to an exim comma-sep list.
 244 Backslashed commas need to be replaced by doublecomma
 245 for Exim's list quoting.  We modify the given string
 246 inplace.
 247 */
 248
 249 static void
 250 dn_to_list(uschar * dn)
 251 {
 252 for (uschar * cp = dn; *cp; cp++)
 253   if (cp[0] == '\\' && cp[1] == ',')
 254     *cp++ = ',';
 255 }
 256
 257
 258 /* Extract fields of a given type from an RFC4514-
 259 format Distinguished Name.  Return an Exim list.
 260 NOTE: We modify the supplied dn string during operation.
 261
 262 Arguments:
 263         dn      Distinguished Name string
 264         mod     list containing optional output list-sep and
 265                 field selector match, comma-separated
 266 Return:
 267         allocated string with list of matching fields,
 268         field type stripped
 269 */
 270
 271 uschar *
 272 tls_field_from_dn(uschar * dn, const uschar * mod)
 273 {
 274 int insep = ',';
 275 uschar outsep = '\n';
 276 uschar * ele;
 277 uschar * match = NULL;
 278 int len;
 279 gstring * list = NULL;
 280
 281 while ((ele = string_nextinlist(&mod, &insep, NULL, 0)))
 282   if (ele[0] != '>')
 283     match = ele;        /* field tag to match */
 284   else if (ele[1])
 285     outsep = ele[1];    /* nondefault output separator */
 286
 287 dn_to_list(dn);
 288 insep = ',';
 289 len = match ? Ustrlen(match) : -1;
 290 while ((ele = string_nextinlist(CUSS &dn, &insep, NULL, 0)))
 291   if (  !match
 292      || Ustrncmp(ele, match, len) == 0 && ele[len] == '='
 293      )
 294     list = string_append_listele(list, outsep, ele+len+1);
 295 return string_from_gstring(list);
 296 }
 297
 298
 299 /* Compare a domain name with a possibly-wildcarded name. Wildcards
 300 are restricted to a single one, as the first element of patterns
 301 having at least three dot-separated elements.  Case-independent.
 302 Return TRUE for a match
 303 */
 304 static BOOL
 305 is_name_match(const uschar * name, const uschar * pat)
 306 {
 307 uschar * cp;
 308 return *pat == '*'              /* possible wildcard match */
 309   ?    *++pat == '.'            /* starts star, dot              */
 310     && !Ustrchr(++pat, '*')     /* has no more stars             */
 311     && Ustrchr(pat, '.')        /* and has another dot.          */
 312     && (cp = Ustrchr(name, '.'))/* The name has at least one dot */
 313     && strcmpic(++cp, pat) == 0 /* and we only compare after it. */
 314   :    !Ustrchr(pat+1, '*')
 315     && strcmpic(name, pat) == 0;
 316 }
 317
 318 /* Compare a list of names with the dnsname elements
 319 of the Subject Alternate Name, if any, and the
 320 Subject otherwise.
 321
 322 Arguments:
 323         namelist names to compare
 324         cert     certificate
 325
 326 Returns:
 327         TRUE/FALSE
 328 */
 329
 330 BOOL
 331 tls_is_name_for_cert(const uschar * namelist, void * cert)
 332 {
 333 uschar * altnames = tls_cert_subject_altname(cert, US"dns");
 334 uschar * subjdn;
 335 uschar * certname;
 336 int cmp_sep = 0;
 337 uschar * cmpname;
 338
 339 if ((altnames = tls_cert_subject_altname(cert, US"dns")))
 340   {
 341   int alt_sep = '\n';
 342   while ((cmpname = string_nextinlist(&namelist, &cmp_sep, NULL, 0)))
 343     {
 344     const uschar * an = altnames;
 345     while ((certname = string_nextinlist(&an, &alt_sep, NULL, 0)))
 346       if (is_name_match(cmpname, certname))
 347         return TRUE;
 348     }
 349   }
 350
 351 else if ((subjdn = tls_cert_subject(cert, NULL)))
 352   {
 353   int sn_sep = ',';
 354
 355   dn_to_list(subjdn);
 356   while ((cmpname = string_nextinlist(&namelist, &cmp_sep, NULL, 0)))
 357     {
 358     const uschar * sn = subjdn;
 359     while ((certname = string_nextinlist(&sn, &sn_sep, NULL, 0)))
 360       if (  *certname++ == 'C'
 361          && *certname++ == 'N'
 362          && *certname++ == '='
 363          && is_name_match(cmpname, certname)
 364          )
 365         return TRUE;
 366     }
 367   }
 368 return FALSE;
 369 }
 370
 371
 372 /* Environment cleanup: The GnuTLS library uses SSLKEYLOGFILE in the environment
 373 and writes a file by that name.  Our OpenSSL code does the same, using keying
 374 info from the library API.
 375 The GnuTLS support only works if exim is run by root, not taking advantage of
 376 the setuid bit.
 377 You can use either the external environment (modulo the keep_environment config)
 378 or the add_environment config option for SSLKEYLOGFILE; the latter takes
 379 precedence.
 380
 381 If the path is absolute, require it starts with the spooldir; otherwise delete
 382 the env variable.  If relative, prefix the spooldir.
 383 */
 384 void
 385 tls_clean_env(void)
 386 {
 387 uschar * path = US getenv("SSLKEYLOGFILE");
 388 if (path)
 389   if (!*path)
 390     unsetenv("SSLKEYLOGFILE");
 391   else if (*path != '/')
 392     {
 393     DEBUG(D_tls)
 394       debug_printf("prepending spooldir to  env SSLKEYLOGFILE\n");
 395     setenv("SSLKEYLOGFILE", CCS string_sprintf("%s/%s", spool_directory, path), 1);
 396     }
 397   else if (Ustrncmp(path, spool_directory, Ustrlen(spool_directory)) != 0)
 398     {
 399     DEBUG(D_tls)
 400       debug_printf("removing env SSLKEYLOGFILE=%s: not under spooldir\n", path);
 401     unsetenv("SSLKEYLOGFILE");
 402     }
 403 }
 404
 405 /*************************************************
 406 *       Drop privs for checking TLS config      *
 407 *************************************************/
 408
 409 /* We want to validate TLS options during readconf, but do not want to be
 410 root when we call into the TLS library, in case of library linkage errors
 411 which cause segfaults; before this check, those were always done as the Exim
 412 runtime user and it makes sense to continue with that.
 413
 414 Assumes:  tls_require_ciphers has been set, if it will be
 415           exim_user has been set, if it will be
 416           exim_group has been set, if it will be
 417
 418 Returns:  bool for "okay"; false will cause caller to immediately exit.
 419 */
 420
 421 BOOL
 422 tls_dropprivs_validate_require_cipher(BOOL nowarn)
 423 {
 424 const uschar *errmsg;
 425 pid_t pid;
 426 int rc, status;
 427 void (*oldsignal)(int);
 428
 429 /* If TLS will never be used, no point checking ciphers */
 430
 431 if (  !tls_advertise_hosts
 432    || !*tls_advertise_hosts
 433    || Ustrcmp(tls_advertise_hosts, ":") == 0
 434    )
 435   return TRUE;
 436 else if (!nowarn && !tls_certificate)
 437   log_write(0, LOG_MAIN,
 438     "Warning: No server certificate defined; will use a selfsigned one.\n"
 439     " Suggested action: either install a certificate or change tls_advertise_hosts option");
 440
 441 oldsignal = signal(SIGCHLD, SIG_DFL);
 442
 443 fflush(NULL);
 444 if ((pid = fork()) < 0)
 445   log_write(0, LOG_MAIN|LOG_PANIC_DIE, "fork failed for TLS check");
 446
 447 if (pid == 0)
 448   {
 449   /* in some modes, will have dropped privilege already */
 450   if (!geteuid())
 451     exim_setugid(exim_uid, exim_gid, FALSE,
 452         US"calling tls_validate_require_cipher");
 453
 454   if ((errmsg = tls_validate_require_cipher()))
 455     log_write(0, LOG_PANIC_DIE|LOG_CONFIG,
 456         "tls_require_ciphers invalid: %s", errmsg);
 457   fflush(NULL);
 458   exim_underbar_exit(0);
 459   }
 460
 461 do {
 462   rc = waitpid(pid, &status, 0);
 463 } while (rc < 0 && errno == EINTR);
 464
 465 DEBUG(D_tls)
 466   debug_printf("tls_validate_require_cipher child %d ended: status=0x%x\n",
 467       (int)pid, status);
 468
 469 signal(SIGCHLD, oldsignal);
 470
 471 return status == 0;
 472 }
 473
 474
 475
 476
 477 #endif  /*!DISABLE_TLS*/
 478 #endif  /*!MACRO_PREDEF*/
 479
 480 /* vi: aw ai sw=2
 481 */
 482 /* End of tls.c */