aboutsummaryrefslogtreecommitdiff
path: root/ui/vnc-auth-sasl.c
blob: 13a59f500e839016a940f792e3424c50d3582612 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
/*
 * QEMU VNC display driver: SASL auth protocol
 *
 * Copyright (C) 2009 Red Hat, Inc
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
 * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */

#include "qemu/osdep.h"
#include "vnc.h"

/* Max amount of data we send/recv for SASL steps to prevent DOS */
#define SASL_DATA_MAX_LEN (1024 * 1024)


void vnc_sasl_client_cleanup(VncState *vs)
{
    if (vs->sasl.conn) {
        vs->sasl.runSSF = false;
        vs->sasl.wantSSF = false;
        vs->sasl.waitWriteSSF = 0;
        vs->sasl.encodedLength = vs->sasl.encodedOffset = 0;
        vs->sasl.encoded = NULL;
        g_free(vs->sasl.username);
        g_free(vs->sasl.mechlist);
        vs->sasl.username = vs->sasl.mechlist = NULL;
        sasl_dispose(&vs->sasl.conn);
        vs->sasl.conn = NULL;
    }
}


long vnc_client_write_sasl(VncState *vs)
{
    long ret;

    VNC_DEBUG("Write SASL: Pending output %p size %zd offset %zd "
              "Encoded: %p size %d offset %d\n",
              vs->output.buffer, vs->output.capacity, vs->output.offset,
              vs->sasl.encoded, vs->sasl.encodedLength, vs->sasl.encodedOffset);

    if (!vs->sasl.encoded) {
        int err;
        err = sasl_encode(vs->sasl.conn,
                          (char *)vs->output.buffer,
                          vs->output.offset,
                          (const char **)&vs->sasl.encoded,
                          &vs->sasl.encodedLength);
        if (err != SASL_OK)
            return vnc_client_io_error(vs, -1, NULL);

        vs->sasl.encodedOffset = 0;
    }

    ret = vnc_client_write_buf(vs,
                               vs->sasl.encoded + vs->sasl.encodedOffset,
                               vs->sasl.encodedLength - vs->sasl.encodedOffset);
    if (!ret)
        return 0;

    vs->sasl.encodedOffset += ret;
    if (vs->sasl.encodedOffset == vs->sasl.encodedLength) {
        vs->output.offset = 0;
        vs->sasl.encoded = NULL;
        vs->sasl.encodedOffset = vs->sasl.encodedLength = 0;
    }

    /* Can't merge this block with one above, because
     * someone might have written more unencrypted
     * data in vs->output while we were processing
     * SASL encoded output
     */
    if (vs->output.offset == 0) {
        if (vs->ioc_tag) {
            g_source_remove(vs->ioc_tag);
        }
        vs->ioc_tag = qio_channel_add_watch(
            vs->ioc, G_IO_IN, vnc_client_io, vs, NULL);
    }

    return ret;
}


long vnc_client_read_sasl(VncState *vs)
{
    long ret;
    uint8_t encoded[4096];
    const char *decoded;
    unsigned int decodedLen;
    int err;

    ret = vnc_client_read_buf(vs, encoded, sizeof(encoded));
    if (!ret)
        return 0;

    err = sasl_decode(vs->sasl.conn,
                      (char *)encoded, ret,
                      &decoded, &decodedLen);

    if (err != SASL_OK)
        return vnc_client_io_error(vs, -1, NULL);
    VNC_DEBUG("Read SASL Encoded %p size %ld Decoded %p size %d\n",
              encoded, ret, decoded, decodedLen);
    buffer_reserve(&vs->input, decodedLen);
    buffer_append(&vs->input, decoded, decodedLen);
    return decodedLen;
}


static int vnc_auth_sasl_check_access(VncState *vs)
{
    const void *val;
    int err;
    int allow;

    err = sasl_getprop(vs->sasl.conn, SASL_USERNAME, &val);
    if (err != SASL_OK) {
        VNC_DEBUG("cannot query SASL username on connection %d (%s), denying access\n",
                  err, sasl_errstring(err, NULL, NULL));
        return -1;
    }
    if (val == NULL) {
        VNC_DEBUG("no client username was found, denying access\n");
        return -1;
    }
    VNC_DEBUG("SASL client username %s\n", (const char *)val);

    vs->sasl.username = g_strdup((const char*)val);

    if (vs->vd->sasl.acl == NULL) {
        VNC_DEBUG("no ACL activated, allowing access\n");
        return 0;
    }

    allow = qemu_acl_party_is_allowed(vs->vd->sasl.acl, vs->sasl.username);

    VNC_DEBUG("SASL client %s %s by ACL\n", vs->sasl.username,
              allow ? "allowed" : "denied");
    return allow ? 0 : -1;
}

static int vnc_auth_sasl_check_ssf(VncState *vs)
{
    const void *val;
    int err, ssf;

    if (!vs->sasl.wantSSF)
        return 1;

    err = sasl_getprop(vs->sasl.conn, SASL_SSF, &val);
    if (err != SASL_OK)
        return 0;

    ssf = *(const int *)val;
    VNC_DEBUG("negotiated an SSF of %d\n", ssf);
    if (ssf < 56)
        return 0; /* 56 is good for Kerberos */

    /* Only setup for read initially, because we're about to send an RPC
     * reply which must be in plain text. When the next incoming RPC
     * arrives, we'll switch on writes too
     *
     * cf qemudClientReadSASL  in qemud.c
     */
    vs->sasl.runSSF = 1;

    /* We have a SSF that's good enough */
    return 1;
}

/*
 * Step Msg
 *
 * Input from client:
 *
 * u32 clientin-length
 * u8-array clientin-string
 *
 * Output to client:
 *
 * u32 serverout-length
 * u8-array serverout-strin
 * u8 continue
 */

static int protocol_client_auth_sasl_step_len(VncState *vs, uint8_t *data, size_t len);

static int protocol_client_auth_sasl_step(VncState *vs, uint8_t *data, size_t len)
{
    uint32_t datalen = len;
    const char *serverout;
    unsigned int serveroutlen;
    int err;
    char *clientdata = NULL;

    /* NB, distinction of NULL vs "" is *critical* in SASL */
    if (datalen) {
        clientdata = (char*)data;
        clientdata[datalen-1] = '\0'; /* Wire includes '\0', but make sure */
        datalen--; /* Don't count NULL byte when passing to _start() */
    }

    VNC_DEBUG("Step using SASL Data %p (%d bytes)\n",
              clientdata, datalen);
    err = sasl_server_step(vs->sasl.conn,
                           clientdata,
                           datalen,
                           &serverout,
                           &serveroutlen);
    if (err != SASL_OK &&
        err != SASL_CONTINUE) {
        VNC_DEBUG("sasl step failed %d (%s)\n",
                  err, sasl_errdetail(vs->sasl.conn));
        sasl_dispose(&vs->sasl.conn);
        vs->sasl.conn = NULL;
        goto authabort;
    }

    if (serveroutlen > SASL_DATA_MAX_LEN) {
        VNC_DEBUG("sasl step reply data too long %d\n",
                  serveroutlen);
        sasl_dispose(&vs->sasl.conn);
        vs->sasl.conn = NULL;
        goto authabort;
    }

    VNC_DEBUG("SASL return data %d bytes, nil; %d\n",
              serveroutlen, serverout ? 0 : 1);

    if (serveroutlen) {
        vnc_write_u32(vs, serveroutlen + 1);
        vnc_write(vs, serverout, serveroutlen + 1);
    } else {
        vnc_write_u32(vs, 0);
    }

    /* Whether auth is complete */
    vnc_write_u8(vs, err == SASL_CONTINUE ? 0 : 1);

    if (err == SASL_CONTINUE) {
        VNC_DEBUG("%s", "Authentication must continue\n");
        /* Wait for step length */
        vnc_read_when(vs, protocol_client_auth_sasl_step_len, 4);
    } else {
        if (!vnc_auth_sasl_check_ssf(vs)) {
            VNC_DEBUG("Authentication rejected for weak SSF %p\n", vs->ioc);
            goto authreject;
        }

        /* Check username whitelist ACL */
        if (vnc_auth_sasl_check_access(vs) < 0) {
            VNC_DEBUG("Authentication rejected for ACL %p\n", vs->ioc);
            goto authreject;
        }

        VNC_DEBUG("Authentication successful %p\n", vs->ioc);
        vnc_write_u32(vs, 0); /* Accept auth */
        /*
         * Delay writing in SSF encoded mode until pending output
         * buffer is written
         */
        if (vs->sasl.runSSF)
            vs->sasl.waitWriteSSF = vs->output.offset;
        start_client_init(vs);
    }

    return 0;

 authreject:
    vnc_write_u32(vs, 1); /* Reject auth */
    vnc_write_u32(vs, sizeof("Authentication failed"));
    vnc_write(vs, "Authentication failed", sizeof("Authentication failed"));
    vnc_flush(vs);
    vnc_client_error(vs);
    return -1;

 authabort:
    vnc_client_error(vs);
    return -1;
}

static int protocol_client_auth_sasl_step_len(VncState *vs, uint8_t *data, size_t len)
{
    uint32_t steplen = read_u32(data, 0);
    VNC_DEBUG("Got client step len %d\n", steplen);
    if (steplen > SASL_DATA_MAX_LEN) {
        VNC_DEBUG("Too much SASL data %d\n", steplen);
        vnc_client_error(vs);
        return -1;
    }

    if (steplen == 0)
        return protocol_client_auth_sasl_step(vs, NULL, 0);
    else
        vnc_read_when(vs, protocol_client_auth_sasl_step, steplen);
    return 0;
}

/*
 * Start Msg
 *
 * Input from client:
 *
 * u32 clientin-length
 * u8-array clientin-string
 *
 * Output to client:
 *
 * u32 serverout-length
 * u8-array serverout-strin
 * u8 continue
 */

#define SASL_DATA_MAX_LEN (1024 * 1024)

static int protocol_client_auth_sasl_start(VncState *vs, uint8_t *data, size_t len)
{
    uint32_t datalen = len;
    const char *serverout;
    unsigned int serveroutlen;
    int err;
    char *clientdata = NULL;

    /* NB, distinction of NULL vs "" is *critical* in SASL */
    if (datalen) {
        clientdata = (char*)data;
        clientdata[datalen-1] = '\0'; /* Should be on wire, but make sure */
        datalen--; /* Don't count NULL byte when passing to _start() */
    }

    VNC_DEBUG("Start SASL auth with mechanism %s. Data %p (%d bytes)\n",
              vs->sasl.mechlist, clientdata, datalen);
    err = sasl_server_start(vs->sasl.conn,
                            vs->sasl.mechlist,
                            clientdata,
                            datalen,
                            &serverout,
                            &serveroutlen);
    if (err != SASL_OK &&
        err != SASL_CONTINUE) {
        VNC_DEBUG("sasl start failed %d (%s)\n",
                  err, sasl_errdetail(vs->sasl.conn));
        sasl_dispose(&vs->sasl.conn);
        vs->sasl.conn = NULL;
        goto authabort;
    }
    if (serveroutlen > SASL_DATA_MAX_LEN) {
        VNC_DEBUG("sasl start reply data too long %d\n",
                  serveroutlen);
        sasl_dispose(&vs->sasl.conn);
        vs->sasl.conn = NULL;
        goto authabort;
    }

    VNC_DEBUG("SASL return data %d bytes, nil; %d\n",
              serveroutlen, serverout ? 0 : 1);

    if (serveroutlen) {
        vnc_write_u32(vs, serveroutlen + 1);
        vnc_write(vs, serverout, serveroutlen + 1);
    } else {
        vnc_write_u32(vs, 0);
    }

    /* Whether auth is complete */
    vnc_write_u8(vs, err == SASL_CONTINUE ? 0 : 1);

    if (err == SASL_CONTINUE) {
        VNC_DEBUG("%s", "Authentication must continue\n");
        /* Wait for step length */
        vnc_read_when(vs, protocol_client_auth_sasl_step_len, 4);
    } else {
        if (!vnc_auth_sasl_check_ssf(vs)) {
            VNC_DEBUG("Authentication rejected for weak SSF %p\n", vs->ioc);
            goto authreject;
        }

        /* Check username whitelist ACL */
        if (vnc_auth_sasl_check_access(vs) < 0) {
            VNC_DEBUG("Authentication rejected for ACL %p\n", vs->ioc);
            goto authreject;
        }

        VNC_DEBUG("Authentication successful %p\n", vs->ioc);
        vnc_write_u32(vs, 0); /* Accept auth */
        start_client_init(vs);
    }

    return 0;

 authreject:
    vnc_write_u32(vs, 1); /* Reject auth */
    vnc_write_u32(vs, sizeof("Authentication failed"));
    vnc_write(vs, "Authentication failed", sizeof("Authentication failed"));
    vnc_flush(vs);
    vnc_client_error(vs);
    return -1;

 authabort:
    vnc_client_error(vs);
    return -1;
}

static int protocol_client_auth_sasl_start_len(VncState *vs, uint8_t *data, size_t len)
{
    uint32_t startlen = read_u32(data, 0);
    VNC_DEBUG("Got client start len %d\n", startlen);
    if (startlen > SASL_DATA_MAX_LEN) {
        VNC_DEBUG("Too much SASL data %d\n", startlen);
        vnc_client_error(vs);
        return -1;
    }

    if (startlen == 0)
        return protocol_client_auth_sasl_start(vs, NULL, 0);

    vnc_read_when(vs, protocol_client_auth_sasl_start, startlen);
    return 0;
}

static int protocol_client_auth_sasl_mechname(VncState *vs, uint8_t *data, size_t len)
{
    char *mechname = g_strndup((const char *) data, len);
    VNC_DEBUG("Got client mechname '%s' check against '%s'\n",
              mechname, vs->sasl.mechlist);

    if (strncmp(vs->sasl.mechlist, mechname, len) == 0) {
        if (vs->sasl.mechlist[len] != '\0' &&
            vs->sasl.mechlist[len] != ',') {
            VNC_DEBUG("One %d", vs->sasl.mechlist[len]);
            goto fail;
        }
    } else {
        char *offset = strstr(vs->sasl.mechlist, mechname);
        VNC_DEBUG("Two %p\n", offset);
        if (!offset) {
            goto fail;
        }
        VNC_DEBUG("Two '%s'\n", offset);
        if (offset[-1] != ',' ||
            (offset[len] != '\0'&&
             offset[len] != ',')) {
            goto fail;
        }
    }

    g_free(vs->sasl.mechlist);
    vs->sasl.mechlist = mechname;

    VNC_DEBUG("Validated mechname '%s'\n", mechname);
    vnc_read_when(vs, protocol_client_auth_sasl_start_len, 4);
    return 0;

 fail:
    vnc_client_error(vs);
    g_free(mechname);
    return -1;
}

static int protocol_client_auth_sasl_mechname_len(VncState *vs, uint8_t *data, size_t len)
{
    uint32_t mechlen = read_u32(data, 0);
    VNC_DEBUG("Got client mechname len %d\n", mechlen);
    if (mechlen > 100) {
        VNC_DEBUG("Too long SASL mechname data %d\n", mechlen);
        vnc_client_error(vs);
        return -1;
    }
    if (mechlen < 1) {
        VNC_DEBUG("Too short SASL mechname %d\n", mechlen);
        vnc_client_error(vs);
        return -1;
    }
    vnc_read_when(vs, protocol_client_auth_sasl_mechname,mechlen);
    return 0;
}

static char *
vnc_socket_ip_addr_string(QIOChannelSocket *ioc,
                          bool local,
                          Error **errp)
{
    SocketAddress *addr;
    char *ret;

    if (local) {
        addr = qio_channel_socket_get_local_address(ioc, errp);
    } else {
        addr = qio_channel_socket_get_remote_address(ioc, errp);
    }
    if (!addr) {
        return NULL;
    }

    if (addr->type != SOCKET_ADDRESS_KIND_INET) {
        error_setg(errp, "Not an inet socket type");
        return NULL;
    }
    ret = g_strdup_printf("%s;%s", addr->u.inet->host, addr->u.inet->port);
    qapi_free_SocketAddress(addr);
    return ret;
}

void start_auth_sasl(VncState *vs)
{
    const char *mechlist = NULL;
    sasl_security_properties_t secprops;
    int err;
    char *localAddr, *remoteAddr;
    int mechlistlen;

    VNC_DEBUG("Initialize SASL auth %p\n", vs->ioc);

    /* Get local & remote client addresses in form  IPADDR;PORT */
    localAddr = vnc_socket_ip_addr_string(vs->sioc, true, NULL);
    if (!localAddr) {
        goto authabort;
    }

    remoteAddr = vnc_socket_ip_addr_string(vs->sioc, false, NULL);
    if (!remoteAddr) {
        g_free(localAddr);
        goto authabort;
    }

    err = sasl_server_new("vnc",
                          NULL, /* FQDN - just delegates to gethostname */
                          NULL, /* User realm */
                          localAddr,
                          remoteAddr,
                          NULL, /* Callbacks, not needed */
                          SASL_SUCCESS_DATA,
                          &vs->sasl.conn);
    g_free(localAddr);
    g_free(remoteAddr);
    localAddr = remoteAddr = NULL;

    if (err != SASL_OK) {
        VNC_DEBUG("sasl context setup failed %d (%s)",
                  err, sasl_errstring(err, NULL, NULL));
        vs->sasl.conn = NULL;
        goto authabort;
    }

    /* Inform SASL that we've got an external SSF layer from TLS/x509 */
    if (vs->auth == VNC_AUTH_VENCRYPT &&
        vs->subauth == VNC_AUTH_VENCRYPT_X509SASL) {
        Error *local_err = NULL;
        int keysize;
        sasl_ssf_t ssf;

        keysize = qcrypto_tls_session_get_key_size(vs->tls,
                                                   &local_err);
        if (keysize < 0) {
            VNC_DEBUG("cannot TLS get cipher size: %s\n",
                      error_get_pretty(local_err));
            error_free(local_err);
            sasl_dispose(&vs->sasl.conn);
            vs->sasl.conn = NULL;
            goto authabort;
        }
        ssf = keysize * CHAR_BIT; /* tls key size is bytes, sasl wants bits */

        err = sasl_setprop(vs->sasl.conn, SASL_SSF_EXTERNAL, &ssf);
        if (err != SASL_OK) {
            VNC_DEBUG("cannot set SASL external SSF %d (%s)\n",
                      err, sasl_errstring(err, NULL, NULL));
            sasl_dispose(&vs->sasl.conn);
            vs->sasl.conn = NULL;
            goto authabort;
        }
    } else {
        vs->sasl.wantSSF = 1;
    }

    memset (&secprops, 0, sizeof secprops);
    /* Inform SASL that we've got an external SSF layer from TLS.
     *
     * Disable SSF, if using TLS+x509+SASL only. TLS without x509
     * is not sufficiently strong
     */
    if (vs->vd->is_unix ||
        (vs->auth == VNC_AUTH_VENCRYPT &&
         vs->subauth == VNC_AUTH_VENCRYPT_X509SASL)) {
        /* If we've got TLS or UNIX domain sock, we don't care about SSF */
        secprops.min_ssf = 0;
        secprops.max_ssf = 0;
        secprops.maxbufsize = 8192;
        secprops.security_flags = 0;
    } else {
        /* Plain TCP, better get an SSF layer */
        secprops.min_ssf = 56; /* Good enough to require kerberos */
        secprops.max_ssf = 100000; /* Arbitrary big number */
        secprops.maxbufsize = 8192;
        /* Forbid any anonymous or trivially crackable auth */
        secprops.security_flags =
            SASL_SEC_NOANONYMOUS | SASL_SEC_NOPLAINTEXT;
    }

    err = sasl_setprop(vs->sasl.conn, SASL_SEC_PROPS, &secprops);
    if (err != SASL_OK) {
        VNC_DEBUG("cannot set SASL security props %d (%s)\n",
                  err, sasl_errstring(err, NULL, NULL));
        sasl_dispose(&vs->sasl.conn);
        vs->sasl.conn = NULL;
        goto authabort;
    }

    err = sasl_listmech(vs->sasl.conn,
                        NULL, /* Don't need to set user */
                        "", /* Prefix */
                        ",", /* Separator */
                        "", /* Suffix */
                        &mechlist,
                        NULL,
                        NULL);
    if (err != SASL_OK) {
        VNC_DEBUG("cannot list SASL mechanisms %d (%s)\n",
                  err, sasl_errdetail(vs->sasl.conn));
        sasl_dispose(&vs->sasl.conn);
        vs->sasl.conn = NULL;
        goto authabort;
    }
    VNC_DEBUG("Available mechanisms for client: '%s'\n", mechlist);

    vs->sasl.mechlist = g_strdup(mechlist);
    mechlistlen = strlen(mechlist);
    vnc_write_u32(vs, mechlistlen);
    vnc_write(vs, mechlist, mechlistlen);
    vnc_flush(vs);

    VNC_DEBUG("Wait for client mechname length\n");
    vnc_read_when(vs, protocol_client_auth_sasl_mechname_len, 4);

    return;

 authabort:
    vnc_client_error(vs);
}