ui/vnc-auth-sasl.c

   1 /*
   2  * QEMU VNC display driver: SASL auth protocol
   3  *
   4  * Copyright (C) 2009 Red Hat, Inc
   5  *
   6  * Permission is hereby granted, free of charge, to any person obtaining a copy
   7  * of this software and associated documentation files (the "Software"), to deal
   8  * in the Software without restriction, including without limitation the rights
   9  * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  10  * copies of the Software, and to permit persons to whom the Software is
  11  * furnished to do so, subject to the following conditions:
  12  *
  13  * The above copyright notice and this permission notice shall be included in
  14  * all copies or substantial portions of the Software.
  15  *
  16  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  17  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  18  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
  19  * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  20  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  21  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
  22  * THE SOFTWARE.
  23  */
  24
  25 #include "vnc.h"
  26
  27 /* Max amount of data we send/recv for SASL steps to prevent DOS */
  28 #define SASL_DATA_MAX_LEN (1024 * 1024)
  29
  30
  31 void vnc_sasl_client_cleanup(VncState *vs)
  32 {
  33     if (vs->sasl.conn) {
  34         vs->sasl.runSSF = vs->sasl.waitWriteSSF = vs->sasl.wantSSF = 0;
  35         vs->sasl.encodedLength = vs->sasl.encodedOffset = 0;
  36         vs->sasl.encoded = NULL;
  37         free(vs->sasl.username);
  38         free(vs->sasl.mechlist);
  39         vs->sasl.username = vs->sasl.mechlist = NULL;
  40         sasl_dispose(&vs->sasl.conn);
  41         vs->sasl.conn = NULL;
  42     }
  43 }
  44
  45
  46 long vnc_client_write_sasl(VncState *vs)
  47 {
  48     long ret;
  49
  50     VNC_DEBUG("Write SASL: Pending output %p size %zd offset %zd "
  51               "Encoded: %p size %d offset %d\n",
  52               vs->output.buffer, vs->output.capacity, vs->output.offset,
  53               vs->sasl.encoded, vs->sasl.encodedLength, vs->sasl.encodedOffset);
  54
  55     if (!vs->sasl.encoded) {
  56         int err;
  57         err = sasl_encode(vs->sasl.conn,
  58                           (char *)vs->output.buffer,
  59                           vs->output.offset,
  60                           (const char **)&vs->sasl.encoded,
  61                           &vs->sasl.encodedLength);
  62         if (err != SASL_OK)
  63             return vnc_client_io_error(vs, -1, EIO);
  64
  65         vs->sasl.encodedOffset = 0;
  66     }
  67
  68     ret = vnc_client_write_buf(vs,
  69                                vs->sasl.encoded + vs->sasl.encodedOffset,
  70                                vs->sasl.encodedLength - vs->sasl.encodedOffset);
  71     if (!ret)
  72         return 0;
  73
  74     vs->sasl.encodedOffset += ret;
  75     if (vs->sasl.encodedOffset == vs->sasl.encodedLength) {
  76         vs->output.offset = 0;
  77         vs->sasl.encoded = NULL;
  78         vs->sasl.encodedOffset = vs->sasl.encodedLength = 0;
  79     }
  80
  81     /* Can't merge this block with one above, because
  82      * someone might have written more unencrypted
  83      * data in vs->output while we were processing
  84      * SASL encoded output
  85      */
  86     if (vs->output.offset == 0) {
  87         qemu_set_fd_handler2(vs->csock, NULL, vnc_client_read, NULL, vs);
  88     }
  89
  90     return ret;
  91 }
  92
  93
  94 long vnc_client_read_sasl(VncState *vs)
  95 {
  96     long ret;
  97     uint8_t encoded[4096];
  98     const char *decoded;
  99     unsigned int decodedLen;
 100     int err;
 101
 102     ret = vnc_client_read_buf(vs, encoded, sizeof(encoded));
 103     if (!ret)
 104         return 0;
 105
 106     err = sasl_decode(vs->sasl.conn,
 107                       (char *)encoded, ret,
 108                       &decoded, &decodedLen);
 109
 110     if (err != SASL_OK)
 111         return vnc_client_io_error(vs, -1, -EIO);
 112     VNC_DEBUG("Read SASL Encoded %p size %ld Decoded %p size %d\n",
 113               encoded, ret, decoded, decodedLen);
 114     buffer_reserve(&vs->input, decodedLen);
 115     buffer_append(&vs->input, decoded, decodedLen);
 116     return decodedLen;
 117 }
 118
 119
 120 static int vnc_auth_sasl_check_access(VncState *vs)
 121 {
 122     const void *val;
 123     int err;
 124     int allow;
 125
 126     err = sasl_getprop(vs->sasl.conn, SASL_USERNAME, &val);
 127     if (err != SASL_OK) {
 128         VNC_DEBUG("cannot query SASL username on connection %d (%s), denying access\n",
 129                   err, sasl_errstring(err, NULL, NULL));
 130         return -1;
 131     }
 132     if (val == NULL) {
 133         VNC_DEBUG("no client username was found, denying access\n");
 134         return -1;
 135     }
 136     VNC_DEBUG("SASL client username %s\n", (const char *)val);
 137
 138     vs->sasl.username = g_strdup((const char*)val);
 139
 140     if (vs->vd->sasl.acl == NULL) {
 141         VNC_DEBUG("no ACL activated, allowing access\n");
 142         return 0;
 143     }
 144
 145     allow = qemu_acl_party_is_allowed(vs->vd->sasl.acl, vs->sasl.username);
 146
 147     VNC_DEBUG("SASL client %s %s by ACL\n", vs->sasl.username,
 148               allow ? "allowed" : "denied");
 149     return allow ? 0 : -1;
 150 }
 151
 152 static int vnc_auth_sasl_check_ssf(VncState *vs)
 153 {
 154     const void *val;
 155     int err, ssf;
 156
 157     if (!vs->sasl.wantSSF)
 158         return 1;
 159
 160     err = sasl_getprop(vs->sasl.conn, SASL_SSF, &val);
 161     if (err != SASL_OK)
 162         return 0;
 163
 164     ssf = *(const int *)val;
 165     VNC_DEBUG("negotiated an SSF of %d\n", ssf);
 166     if (ssf < 56)
 167         return 0; /* 56 is good for Kerberos */
 168
 169     /* Only setup for read initially, because we're about to send an RPC
 170      * reply which must be in plain text. When the next incoming RPC
 171      * arrives, we'll switch on writes too
 172      *
 173      * cf qemudClientReadSASL  in qemud.c
 174      */
 175     vs->sasl.runSSF = 1;
 176
 177     /* We have a SSF that's good enough */
 178     return 1;
 179 }
 180
 181 /*
 182  * Step Msg
 183  *
 184  * Input from client:
 185  *
 186  * u32 clientin-length
 187  * u8-array clientin-string
 188  *
 189  * Output to client:
 190  *
 191  * u32 serverout-length
 192  * u8-array serverout-strin
 193  * u8 continue
 194  */
 195
 196 static int protocol_client_auth_sasl_step_len(VncState *vs, uint8_t *data, size_t len);
 197
 198 static int protocol_client_auth_sasl_step(VncState *vs, uint8_t *data, size_t len)
 199 {
 200     uint32_t datalen = len;
 201     const char *serverout;
 202     unsigned int serveroutlen;
 203     int err;
 204     char *clientdata = NULL;
 205
 206     /* NB, distinction of NULL vs "" is *critical* in SASL */
 207     if (datalen) {
 208         clientdata = (char*)data;
 209         clientdata[datalen-1] = '\0'; /* Wire includes '\0', but make sure */
 210         datalen--; /* Don't count NULL byte when passing to _start() */
 211     }
 212
 213     VNC_DEBUG("Step using SASL Data %p (%d bytes)\n",
 214               clientdata, datalen);
 215     err = sasl_server_step(vs->sasl.conn,
 216                            clientdata,
 217                            datalen,
 218                            &serverout,
 219                            &serveroutlen);
 220     if (err != SASL_OK &&
 221         err != SASL_CONTINUE) {
 222         VNC_DEBUG("sasl step failed %d (%s)\n",
 223                   err, sasl_errdetail(vs->sasl.conn));
 224         sasl_dispose(&vs->sasl.conn);
 225         vs->sasl.conn = NULL;
 226         goto authabort;
 227     }
 228
 229     if (serveroutlen > SASL_DATA_MAX_LEN) {
 230         VNC_DEBUG("sasl step reply data too long %d\n",
 231                   serveroutlen);
 232         sasl_dispose(&vs->sasl.conn);
 233         vs->sasl.conn = NULL;
 234         goto authabort;
 235     }
 236
 237     VNC_DEBUG("SASL return data %d bytes, nil; %d\n",
 238               serveroutlen, serverout ? 0 : 1);
 239
 240     if (serveroutlen) {
 241         vnc_write_u32(vs, serveroutlen + 1);
 242         vnc_write(vs, serverout, serveroutlen + 1);
 243     } else {
 244         vnc_write_u32(vs, 0);
 245     }
 246
 247     /* Whether auth is complete */
 248     vnc_write_u8(vs, err == SASL_CONTINUE ? 0 : 1);
 249
 250     if (err == SASL_CONTINUE) {
 251         VNC_DEBUG("%s", "Authentication must continue\n");
 252         /* Wait for step length */
 253         vnc_read_when(vs, protocol_client_auth_sasl_step_len, 4);
 254     } else {
 255         if (!vnc_auth_sasl_check_ssf(vs)) {
 256             VNC_DEBUG("Authentication rejected for weak SSF %d\n", vs->csock);
 257             goto authreject;
 258         }
 259
 260         /* Check username whitelist ACL */
 261         if (vnc_auth_sasl_check_access(vs) < 0) {
 262             VNC_DEBUG("Authentication rejected for ACL %d\n", vs->csock);
 263             goto authreject;
 264         }
 265
 266         VNC_DEBUG("Authentication successful %d\n", vs->csock);
 267         vnc_write_u32(vs, 0); /* Accept auth */
 268         /*
 269          * Delay writing in SSF encoded mode until pending output
 270          * buffer is written
 271          */
 272         if (vs->sasl.runSSF)
 273             vs->sasl.waitWriteSSF = vs->output.offset;
 274         start_client_init(vs);
 275     }
 276
 277     return 0;
 278
 279  authreject:
 280     vnc_write_u32(vs, 1); /* Reject auth */
 281     vnc_write_u32(vs, sizeof("Authentication failed"));
 282     vnc_write(vs, "Authentication failed", sizeof("Authentication failed"));
 283     vnc_flush(vs);
 284     vnc_client_error(vs);
 285     return -1;
 286
 287  authabort:
 288     vnc_client_error(vs);
 289     return -1;
 290 }
 291
 292 static int protocol_client_auth_sasl_step_len(VncState *vs, uint8_t *data, size_t len)
 293 {
 294     uint32_t steplen = read_u32(data, 0);
 295     VNC_DEBUG("Got client step len %d\n", steplen);
 296     if (steplen > SASL_DATA_MAX_LEN) {
 297         VNC_DEBUG("Too much SASL data %d\n", steplen);
 298         vnc_client_error(vs);
 299         return -1;
 300     }
 301
 302     if (steplen == 0)
 303         return protocol_client_auth_sasl_step(vs, NULL, 0);
 304     else
 305         vnc_read_when(vs, protocol_client_auth_sasl_step, steplen);
 306     return 0;
 307 }
 308
 309 /*
 310  * Start Msg
 311  *
 312  * Input from client:
 313  *
 314  * u32 clientin-length
 315  * u8-array clientin-string
 316  *
 317  * Output to client:
 318  *
 319  * u32 serverout-length
 320  * u8-array serverout-strin
 321  * u8 continue
 322  */
 323
 324 #define SASL_DATA_MAX_LEN (1024 * 1024)
 325
 326 static int protocol_client_auth_sasl_start(VncState *vs, uint8_t *data, size_t len)
 327 {
 328     uint32_t datalen = len;
 329     const char *serverout;
 330     unsigned int serveroutlen;
 331     int err;
 332     char *clientdata = NULL;
 333
 334     /* NB, distinction of NULL vs "" is *critical* in SASL */
 335     if (datalen) {
 336         clientdata = (char*)data;
 337         clientdata[datalen-1] = '\0'; /* Should be on wire, but make sure */
 338         datalen--; /* Don't count NULL byte when passing to _start() */
 339     }
 340
 341     VNC_DEBUG("Start SASL auth with mechanism %s. Data %p (%d bytes)\n",
 342               vs->sasl.mechlist, clientdata, datalen);
 343     err = sasl_server_start(vs->sasl.conn,
 344                             vs->sasl.mechlist,
 345                             clientdata,
 346                             datalen,
 347                             &serverout,
 348                             &serveroutlen);
 349     if (err != SASL_OK &&
 350         err != SASL_CONTINUE) {
 351         VNC_DEBUG("sasl start failed %d (%s)\n",
 352                   err, sasl_errdetail(vs->sasl.conn));
 353         sasl_dispose(&vs->sasl.conn);
 354         vs->sasl.conn = NULL;
 355         goto authabort;
 356     }
 357     if (serveroutlen > SASL_DATA_MAX_LEN) {
 358         VNC_DEBUG("sasl start reply data too long %d\n",
 359                   serveroutlen);
 360         sasl_dispose(&vs->sasl.conn);
 361         vs->sasl.conn = NULL;
 362         goto authabort;
 363     }
 364
 365     VNC_DEBUG("SASL return data %d bytes, nil; %d\n",
 366               serveroutlen, serverout ? 0 : 1);
 367
 368     if (serveroutlen) {
 369         vnc_write_u32(vs, serveroutlen + 1);
 370         vnc_write(vs, serverout, serveroutlen + 1);
 371     } else {
 372         vnc_write_u32(vs, 0);
 373     }
 374
 375     /* Whether auth is complete */
 376     vnc_write_u8(vs, err == SASL_CONTINUE ? 0 : 1);
 377
 378     if (err == SASL_CONTINUE) {
 379         VNC_DEBUG("%s", "Authentication must continue\n");
 380         /* Wait for step length */
 381         vnc_read_when(vs, protocol_client_auth_sasl_step_len, 4);
 382     } else {
 383         if (!vnc_auth_sasl_check_ssf(vs)) {
 384             VNC_DEBUG("Authentication rejected for weak SSF %d\n", vs->csock);
 385             goto authreject;
 386         }
 387
 388         /* Check username whitelist ACL */
 389         if (vnc_auth_sasl_check_access(vs) < 0) {
 390             VNC_DEBUG("Authentication rejected for ACL %d\n", vs->csock);
 391             goto authreject;
 392         }
 393
 394         VNC_DEBUG("Authentication successful %d\n", vs->csock);
 395         vnc_write_u32(vs, 0); /* Accept auth */
 396         start_client_init(vs);
 397     }
 398
 399     return 0;
 400
 401  authreject:
 402     vnc_write_u32(vs, 1); /* Reject auth */
 403     vnc_write_u32(vs, sizeof("Authentication failed"));
 404     vnc_write(vs, "Authentication failed", sizeof("Authentication failed"));
 405     vnc_flush(vs);
 406     vnc_client_error(vs);
 407     return -1;
 408
 409  authabort:
 410     vnc_client_error(vs);
 411     return -1;
 412 }
 413
 414 static int protocol_client_auth_sasl_start_len(VncState *vs, uint8_t *data, size_t len)
 415 {
 416     uint32_t startlen = read_u32(data, 0);
 417     VNC_DEBUG("Got client start len %d\n", startlen);
 418     if (startlen > SASL_DATA_MAX_LEN) {
 419         VNC_DEBUG("Too much SASL data %d\n", startlen);
 420         vnc_client_error(vs);
 421         return -1;
 422     }
 423
 424     if (startlen == 0)
 425         return protocol_client_auth_sasl_start(vs, NULL, 0);
 426
 427     vnc_read_when(vs, protocol_client_auth_sasl_start, startlen);
 428     return 0;
 429 }
 430
 431 static int protocol_client_auth_sasl_mechname(VncState *vs, uint8_t *data, size_t len)
 432 {
 433     char *mechname = malloc(len + 1);
 434     if (!mechname) {
 435         VNC_DEBUG("Out of memory reading mechname\n");
 436         vnc_client_error(vs);
 437     }
 438     strncpy(mechname, (char*)data, len);
 439     mechname[len] = '\0';
 440     VNC_DEBUG("Got client mechname '%s' check against '%s'\n",
 441               mechname, vs->sasl.mechlist);
 442
 443     if (strncmp(vs->sasl.mechlist, mechname, len) == 0) {
 444         if (vs->sasl.mechlist[len] != '\0' &&
 445             vs->sasl.mechlist[len] != ',') {
 446             VNC_DEBUG("One %d", vs->sasl.mechlist[len]);
 447             goto fail;
 448         }
 449     } else {
 450         char *offset = strstr(vs->sasl.mechlist, mechname);
 451         VNC_DEBUG("Two %p\n", offset);
 452         if (!offset) {
 453             goto fail;
 454         }
 455         VNC_DEBUG("Two '%s'\n", offset);
 456         if (offset[-1] != ',' ||
 457             (offset[len] != '\0'&&
 458              offset[len] != ',')) {
 459             goto fail;
 460         }
 461     }
 462
 463     free(vs->sasl.mechlist);
 464     vs->sasl.mechlist = mechname;
 465
 466     VNC_DEBUG("Validated mechname '%s'\n", mechname);
 467     vnc_read_when(vs, protocol_client_auth_sasl_start_len, 4);
 468     return 0;
 469
 470  fail:
 471     vnc_client_error(vs);
 472     free(mechname);
 473     return -1;
 474 }
 475
 476 static int protocol_client_auth_sasl_mechname_len(VncState *vs, uint8_t *data, size_t len)
 477 {
 478     uint32_t mechlen = read_u32(data, 0);
 479     VNC_DEBUG("Got client mechname len %d\n", mechlen);
 480     if (mechlen > 100) {
 481         VNC_DEBUG("Too long SASL mechname data %d\n", mechlen);
 482         vnc_client_error(vs);
 483         return -1;
 484     }
 485     if (mechlen < 1) {
 486         VNC_DEBUG("Too short SASL mechname %d\n", mechlen);
 487         vnc_client_error(vs);
 488         return -1;
 489     }
 490     vnc_read_when(vs, protocol_client_auth_sasl_mechname,mechlen);
 491     return 0;
 492 }
 493
 494 void start_auth_sasl(VncState *vs)
 495 {
 496     const char *mechlist = NULL;
 497     sasl_security_properties_t secprops;
 498     int err;
 499     char *localAddr, *remoteAddr;
 500     int mechlistlen;
 501
 502     VNC_DEBUG("Initialize SASL auth %d\n", vs->csock);
 503
 504     /* Get local & remote client addresses in form  IPADDR;PORT */
 505     if (!(localAddr = vnc_socket_local_addr("%s;%s", vs->csock)))
 506         goto authabort;
 507
 508     if (!(remoteAddr = vnc_socket_remote_addr("%s;%s", vs->csock))) {
 509         free(localAddr);
 510         goto authabort;
 511     }
 512
 513     err = sasl_server_new("vnc",
 514                           NULL, /* FQDN - just delegates to gethostname */
 515                           NULL, /* User realm */
 516                           localAddr,
 517                           remoteAddr,
 518                           NULL, /* Callbacks, not needed */
 519                           SASL_SUCCESS_DATA,
 520                           &vs->sasl.conn);
 521     free(localAddr);
 522     free(remoteAddr);
 523     localAddr = remoteAddr = NULL;
 524
 525     if (err != SASL_OK) {
 526         VNC_DEBUG("sasl context setup failed %d (%s)",
 527                   err, sasl_errstring(err, NULL, NULL));
 528         vs->sasl.conn = NULL;
 529         goto authabort;
 530     }
 531
 532 #ifdef CONFIG_VNC_TLS
 533     /* Inform SASL that we've got an external SSF layer from TLS/x509 */
 534     if (vs->auth == VNC_AUTH_VENCRYPT &&
 535         vs->subauth == VNC_AUTH_VENCRYPT_X509SASL) {
 536         gnutls_cipher_algorithm_t cipher;
 537         sasl_ssf_t ssf;
 538
 539         cipher = gnutls_cipher_get(vs->tls.session);
 540         if (!(ssf = (sasl_ssf_t)gnutls_cipher_get_key_size(cipher))) {
 541             VNC_DEBUG("%s", "cannot TLS get cipher size\n");
 542             sasl_dispose(&vs->sasl.conn);
 543             vs->sasl.conn = NULL;
 544             goto authabort;
 545         }
 546         ssf *= 8; /* tls key size is bytes, sasl wants bits */
 547
 548         err = sasl_setprop(vs->sasl.conn, SASL_SSF_EXTERNAL, &ssf);
 549         if (err != SASL_OK) {
 550             VNC_DEBUG("cannot set SASL external SSF %d (%s)\n",
 551                       err, sasl_errstring(err, NULL, NULL));
 552             sasl_dispose(&vs->sasl.conn);
 553             vs->sasl.conn = NULL;
 554             goto authabort;
 555         }
 556     } else
 557 #endif /* CONFIG_VNC_TLS */
 558         vs->sasl.wantSSF = 1;
 559
 560     memset (&secprops, 0, sizeof secprops);
 561     /* Inform SASL that we've got an external SSF layer from TLS */
 562     if (strncmp(vs->vd->display, "unix:", 5) == 0
 563 #ifdef CONFIG_VNC_TLS
 564         /* Disable SSF, if using TLS+x509+SASL only. TLS without x509
 565            is not sufficiently strong */
 566         || (vs->auth == VNC_AUTH_VENCRYPT &&
 567             vs->subauth == VNC_AUTH_VENCRYPT_X509SASL)
 568 #endif /* CONFIG_VNC_TLS */
 569         ) {
 570         /* If we've got TLS or UNIX domain sock, we don't care about SSF */
 571         secprops.min_ssf = 0;
 572         secprops.max_ssf = 0;
 573         secprops.maxbufsize = 8192;
 574         secprops.security_flags = 0;
 575     } else {
 576         /* Plain TCP, better get an SSF layer */
 577         secprops.min_ssf = 56; /* Good enough to require kerberos */
 578         secprops.max_ssf = 100000; /* Arbitrary big number */
 579         secprops.maxbufsize = 8192;
 580         /* Forbid any anonymous or trivially crackable auth */
 581         secprops.security_flags =
 582             SASL_SEC_NOANONYMOUS | SASL_SEC_NOPLAINTEXT;
 583     }
 584
 585     err = sasl_setprop(vs->sasl.conn, SASL_SEC_PROPS, &secprops);
 586     if (err != SASL_OK) {
 587         VNC_DEBUG("cannot set SASL security props %d (%s)\n",
 588                   err, sasl_errstring(err, NULL, NULL));
 589         sasl_dispose(&vs->sasl.conn);
 590         vs->sasl.conn = NULL;
 591         goto authabort;
 592     }
 593
 594     err = sasl_listmech(vs->sasl.conn,
 595                         NULL, /* Don't need to set user */
 596                         "", /* Prefix */
 597                         ",", /* Separator */
 598                         "", /* Suffix */
 599                         &mechlist,
 600                         NULL,
 601                         NULL);
 602     if (err != SASL_OK) {
 603         VNC_DEBUG("cannot list SASL mechanisms %d (%s)\n",
 604                   err, sasl_errdetail(vs->sasl.conn));
 605         sasl_dispose(&vs->sasl.conn);
 606         vs->sasl.conn = NULL;
 607         goto authabort;
 608     }
 609     VNC_DEBUG("Available mechanisms for client: '%s'\n", mechlist);
 610
 611     if (!(vs->sasl.mechlist = strdup(mechlist))) {
 612         VNC_DEBUG("Out of memory");
 613         sasl_dispose(&vs->sasl.conn);
 614         vs->sasl.conn = NULL;
 615         goto authabort;
 616     }
 617     mechlistlen = strlen(mechlist);
 618     vnc_write_u32(vs, mechlistlen);
 619     vnc_write(vs, mechlist, mechlistlen);
 620     vnc_flush(vs);
 621
 622     VNC_DEBUG("Wait for client mechname length\n");
 623     vnc_read_when(vs, protocol_client_auth_sasl_mechname_len, 4);
 624
 625     return;
 626
 627  authabort:
 628     vnc_client_error(vs);
 629     return;
 630 }
 631
 632