summaryrefslogtreecommitdiffstats
path: root/net/sunrpc/auth_gss/gss_rpc_xdr.c
blob: d79f12c2550ac36b0c85aad2cb919e1d0cf2f4bb (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
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
// SPDX-License-Identifier: GPL-2.0+
/*
 * GSS Proxy upcall module
 *
 *  Copyright (C) 2012 Simo Sorce <simo@redhat.com>
 */

#include <linux/sunrpc/svcauth.h>
#include "gss_rpc_xdr.h"

static int gssx_enc_bool(struct xdr_stream *xdr, int v)
{
	__be32 *p;

	p = xdr_reserve_space(xdr, 4);
	if (unlikely(p == NULL))
		return -ENOSPC;
	*p = v ? xdr_one : xdr_zero;
	return 0;
}

static int gssx_dec_bool(struct xdr_stream *xdr, u32 *v)
{
	__be32 *p;

	p = xdr_inline_decode(xdr, 4);
	if (unlikely(p == NULL))
		return -ENOSPC;
	*v = be32_to_cpu(*p);
	return 0;
}

static int gssx_enc_buffer(struct xdr_stream *xdr,
			   const gssx_buffer *buf)
{
	__be32 *p;

	p = xdr_reserve_space(xdr, sizeof(u32) + buf->len);
	if (!p)
		return -ENOSPC;
	xdr_encode_opaque(p, buf->data, buf->len);
	return 0;
}

static int gssx_enc_in_token(struct xdr_stream *xdr,
			     const struct gssp_in_token *in)
{
	__be32 *p;

	p = xdr_reserve_space(xdr, 4);
	if (!p)
		return -ENOSPC;
	*p = cpu_to_be32(in->page_len);

	/* all we need to do is to write pages */
	xdr_write_pages(xdr, in->pages, in->page_base, in->page_len);

	return 0;
}


static int gssx_dec_buffer(struct xdr_stream *xdr,
			   gssx_buffer *buf)
{
	u32 length;
	__be32 *p;

	p = xdr_inline_decode(xdr, 4);
	if (unlikely(p == NULL))
		return -ENOSPC;

	length = be32_to_cpup(p);
	p = xdr_inline_decode(xdr, length);
	if (unlikely(p == NULL))
		return -ENOSPC;

	if (buf->len == 0) {
		/* we intentionally are not interested in this buffer */
		return 0;
	}
	if (length > buf->len)
		return -ENOSPC;

	if (!buf->data) {
		buf->data = kmemdup(p, length, GFP_KERNEL);
		if (!buf->data)
			return -ENOMEM;
	} else {
		memcpy(buf->data, p, length);
	}
	buf->len = length;
	return 0;
}

static int gssx_enc_option(struct xdr_stream *xdr,
			   struct gssx_option *opt)
{
	int err;

	err = gssx_enc_buffer(xdr, &opt->option);
	if (err)
		return err;
	err = gssx_enc_buffer(xdr, &opt->value);
	return err;
}

static int gssx_dec_option(struct xdr_stream *xdr,
			   struct gssx_option *opt)
{
	int err;

	err = gssx_dec_buffer(xdr, &opt->option);
	if (err)
		return err;
	err = gssx_dec_buffer(xdr, &opt->value);
	return err;
}

static int dummy_enc_opt_array(struct xdr_stream *xdr,
				const struct gssx_option_array *oa)
{
	__be32 *p;

	if (oa->count != 0)
		return -EINVAL;

	p = xdr_reserve_space(xdr, 4);
	if (!p)
		return -ENOSPC;
	*p = 0;

	return 0;
}

static int dummy_dec_opt_array(struct xdr_stream *xdr,
				struct gssx_option_array *oa)
{
	struct gssx_option dummy;
	u32 count, i;
	__be32 *p;

	p = xdr_inline_decode(xdr, 4);
	if (unlikely(p == NULL))
		return -ENOSPC;
	count = be32_to_cpup(p++);
	memset(&dummy, 0, sizeof(dummy));
	for (i = 0; i < count; i++) {
		gssx_dec_option(xdr, &dummy);
	}

	oa->count = 0;
	oa->data = NULL;
	return 0;
}

static int get_host_u32(struct xdr_stream *xdr, u32 *res)
{
	__be32 *p;

	p = xdr_inline_decode(xdr, 4);
	if (!p)
		return -EINVAL;
	/* Contents of linux creds are all host-endian: */
	memcpy(res, p, sizeof(u32));
	return 0;
}

static int gssx_dec_linux_creds(struct xdr_stream *xdr,
				struct svc_cred *creds)
{
	u32 length;
	__be32 *p;
	u32 tmp;
	u32 N;
	int i, err;

	p = xdr_inline_decode(xdr, 4);
	if (unlikely(p == NULL))
		return -ENOSPC;

	length = be32_to_cpup(p);

	if (length > (3 + NGROUPS_MAX) * sizeof(u32))
		return -ENOSPC;

	/* uid */
	err = get_host_u32(xdr, &tmp);
	if (err)
		return err;
	creds->cr_uid = make_kuid(&init_user_ns, tmp);

	/* gid */
	err = get_host_u32(xdr, &tmp);
	if (err)
		return err;
	creds->cr_gid = make_kgid(&init_user_ns, tmp);

	/* number of additional gid's */
	err = get_host_u32(xdr, &tmp);
	if (err)
		return err;
	N = tmp;
	if ((3 + N) * sizeof(u32) != length)
		return -EINVAL;
	creds->cr_group_info = groups_alloc(N);
	if (creds->cr_group_info == NULL)
		return -ENOMEM;

	/* gid's */
	for (i = 0; i < N; i++) {
		kgid_t kgid;
		err = get_host_u32(xdr, &tmp);
		if (err)
			goto out_free_groups;
		err = -EINVAL;
		kgid = make_kgid(&init_user_ns, tmp);
		if (!gid_valid(kgid))
			goto out_free_groups;
		creds->cr_group_info->gid[i] = kgid;
	}
	groups_sort(creds->cr_group_info);

	return 0;
out_free_groups:
	groups_free(creds->cr_group_info);
	return err;
}

static int gssx_dec_option_array(struct xdr_stream *xdr,
				 struct gssx_option_array *oa)
{
	struct svc_cred *creds;
	u32 count, i;
	__be32 *p;
	int err;

	p = xdr_inline_decode(xdr, 4);
	if (unlikely(p == NULL))
		return -ENOSPC;
	count = be32_to_cpup(p++);
	if (!count)
		return 0;

	/* we recognize only 1 currently: CREDS_VALUE */
	oa->count = 1;

	oa->data = kmalloc(sizeof(struct gssx_option), GFP_KERNEL);
	if (!oa->data)
		return -ENOMEM;

	creds = kzalloc(sizeof(struct svc_cred), GFP_KERNEL);
	if (!creds) {
		kfree(oa->data);
		return -ENOMEM;
	}

	oa->data[0].option.data = CREDS_VALUE;
	oa->data[0].option.len = sizeof(CREDS_VALUE);
	oa->data[0].value.data = (void *)creds;
	oa->data[0].value.len = 0;

	for (i = 0; i < count; i++) {
		gssx_buffer dummy = { 0, NULL };
		u32 length;

		/* option buffer */
		p = xdr_inline_decode(xdr, 4);
		if (unlikely(p == NULL))
			return -ENOSPC;

		length = be32_to_cpup(p);
		p = xdr_inline_decode(xdr, length);
		if (unlikely(p == NULL))
			return -ENOSPC;

		if (length == sizeof(CREDS_VALUE) &&
		    memcmp(p, CREDS_VALUE, sizeof(CREDS_VALUE)) == 0) {
			/* We have creds here. parse them */
			err = gssx_dec_linux_creds(xdr, creds);
			if (err)
				return err;
			oa->data[0].value.len = 1; /* presence */
		} else {
			/* consume uninteresting buffer */
			err = gssx_dec_buffer(xdr, &dummy);
			if (err)
				return err;
		}
	}
	return 0;
}

static int gssx_dec_status(struct xdr_stream *xdr,
			   struct gssx_status *status)
{
	__be32 *p;
	int err;

	/* status->major_status */
	p = xdr_inline_decode(xdr, 8);
	if (unlikely(p == NULL))
		return -ENOSPC;
	p = xdr_decode_hyper(p, &status->major_status);

	/* status->mech */
	err = gssx_dec_buffer(xdr, &status->mech);
	if (err)
		return err;

	/* status->minor_status */
	p = xdr_inline_decode(xdr, 8);
	if (unlikely(p == NULL))
		return -ENOSPC;
	p = xdr_decode_hyper(p, &status->minor_status);

	/* status->major_status_string */
	err = gssx_dec_buffer(xdr, &status->major_status_string);
	if (err)
		return err;

	/* status->minor_status_string */
	err = gssx_dec_buffer(xdr, &status->minor_status_string);
	if (err)
		return err;

	/* status->server_ctx */
	err = gssx_dec_buffer(xdr, &status->server_ctx);
	if (err)
		return err;

	/* we assume we have no options for now, so simply consume them */
	/* status->options */
	err = dummy_dec_opt_array(xdr, &status->options);

	return err;
}

static int gssx_enc_call_ctx(struct xdr_stream *xdr,
			     const struct gssx_call_ctx *ctx)
{
	struct gssx_option opt;
	__be32 *p;
	int err;

	/* ctx->locale */
	err = gssx_enc_buffer(xdr, &ctx->locale);
	if (err)
		return err;

	/* ctx->server_ctx */
	err = gssx_enc_buffer(xdr, &ctx->server_ctx);
	if (err)
		return err;

	/* we always want to ask for lucid contexts */
	/* ctx->options */
	p = xdr_reserve_space(xdr, 4);
	*p = cpu_to_be32(2);

	/* we want a lucid_v1 context */
	opt.option.data = LUCID_OPTION;
	opt.option.len = sizeof(LUCID_OPTION);
	opt.value.data = LUCID_VALUE;
	opt.value.len = sizeof(LUCID_VALUE);
	err = gssx_enc_option(xdr, &opt);

	/* ..and user creds */
	opt.option.data = CREDS_OPTION;
	opt.option.len = sizeof(CREDS_OPTION);
	opt.value.data = CREDS_VALUE;
	opt.value.len = sizeof(CREDS_VALUE);
	err = gssx_enc_option(xdr, &opt);

	return err;
}

static int gssx_dec_name_attr(struct xdr_stream *xdr,
			     struct gssx_name_attr *attr)
{
	int err;

	/* attr->attr */
	err = gssx_dec_buffer(xdr, &attr->attr);
	if (err)
		return err;

	/* attr->value */
	err = gssx_dec_buffer(xdr, &attr->value);
	if (err)
		return err;

	/* attr->extensions */
	err = dummy_dec_opt_array(xdr, &attr->extensions);

	return err;
}

static int dummy_enc_nameattr_array(struct xdr_stream *xdr,
				    struct gssx_name_attr_array *naa)
{
	__be32 *p;

	if (naa->count != 0)
		return -EINVAL;

	p = xdr_reserve_space(xdr, 4);
	if (!p)
		return -ENOSPC;
	*p = 0;

	return 0;
}

static int dummy_dec_nameattr_array(struct xdr_stream *xdr,
				    struct gssx_name_attr_array *naa)
{
	struct gssx_name_attr dummy = { .attr = {.len = 0} };
	u32 count, i;
	__be32 *p;

	p = xdr_inline_decode(xdr, 4);
	if (unlikely(p == NULL))
		return -ENOSPC;
	count = be32_to_cpup(p++);
	for (i = 0; i < count; i++) {
		gssx_dec_name_attr(xdr, &dummy);
	}

	naa->count = 0;
	naa->data = NULL;
	return 0;
}

static struct xdr_netobj zero_netobj = {};

static struct gssx_name_attr_array zero_name_attr_array = {};

static struct gssx_option_array zero_option_array = {};

static int gssx_enc_name(struct xdr_stream *xdr,
			 struct gssx_name *name)
{
	int err;

	/* name->display_name */
	err = gssx_enc_buffer(xdr, &name->display_name);
	if (err)
		return err;

	/* name->name_type */
	err = gssx_enc_buffer(xdr, &zero_netobj);
	if (err)
		return err;

	/* name->exported_name */
	err = gssx_enc_buffer(xdr, &zero_netobj);
	if (err)
		return err;

	/* name->exported_composite_name */
	err = gssx_enc_buffer(xdr, &zero_netobj);
	if (err)
		return err;

	/* leave name_attributes empty for now, will add once we have any
	 * to pass up at all */
	/* name->name_attributes */
	err = dummy_enc_nameattr_array(xdr, &zero_name_attr_array);
	if (err)
		return err;

	/* leave options empty for now, will add once we have any options
	 * to pass up at all */
	/* name->extensions */
	err = dummy_enc_opt_array(xdr, &zero_option_array);

	return err;
}


static int gssx_dec_name(struct xdr_stream *xdr,
			 struct gssx_name *name)
{
	struct xdr_netobj dummy_netobj = { .len = 0 };
	struct gssx_name_attr_array dummy_name_attr_array = { .count = 0 };
	struct gssx_option_array dummy_option_array = { .count = 0 };
	int err;

	/* name->display_name */
	err = gssx_dec_buffer(xdr, &name->display_name);
	if (err)
		return err;

	/* name->name_type */
	err = gssx_dec_buffer(xdr, &dummy_netobj);
	if (err)
		return err;

	/* name->exported_name */
	err = gssx_dec_buffer(xdr, &dummy_netobj);
	if (err)
		return err;

	/* name->exported_composite_name */
	err = gssx_dec_buffer(xdr, &dummy_netobj);
	if (err)
		return err;

	/* we assume we have no attributes for now, so simply consume them */
	/* name->name_attributes */
	err = dummy_dec_nameattr_array(xdr, &dummy_name_attr_array);
	if (err)
		return err;

	/* we assume we have no options for now, so simply consume them */
	/* name->extensions */
	err = dummy_dec_opt_array(xdr, &dummy_option_array);

	return err;
}

static int dummy_enc_credel_array(struct xdr_stream *xdr,
				  struct gssx_cred_element_array *cea)
{
	__be32 *p;

	if (cea->count != 0)
		return -EINVAL;

	p = xdr_reserve_space(xdr, 4);
	if (!p)
		return -ENOSPC;
	*p = 0;

	return 0;
}

static int gssx_enc_cred(struct xdr_stream *xdr,
			 struct gssx_cred *cred)
{
	int err;

	/* cred->desired_name */
	err = gssx_enc_name(xdr, &cred->desired_name);
	if (err)
		return err;

	/* cred->elements */
	err = dummy_enc_credel_array(xdr, &cred->elements);
	if (err)
		return err;

	/* cred->cred_handle_reference */
	err = gssx_enc_buffer(xdr, &cred->cred_handle_reference);
	if (err)
		return err;

	/* cred->needs_release */
	err = gssx_enc_bool(xdr, cred->needs_release);

	return err;
}

static int gssx_enc_ctx(struct xdr_stream *xdr,
			struct gssx_ctx *ctx)
{
	__be32 *p;
	int err;

	/* ctx->exported_context_token */
	err = gssx_enc_buffer(xdr, &ctx->exported_context_token);
	if (err)
		return err;

	/* ctx->state */
	err = gssx_enc_buffer(xdr, &ctx->state);
	if (err)
		return err;

	/* ctx->need_release */
	err = gssx_enc_bool(xdr, ctx->need_release);
	if (err)
		return err;

	/* ctx->mech */
	err = gssx_enc_buffer(xdr, &ctx->mech);
	if (err)
		return err;

	/* ctx->src_name */
	err = gssx_enc_name(xdr, &ctx->src_name);
	if (err)
		return err;

	/* ctx->targ_name */
	err = gssx_enc_name(xdr, &ctx->targ_name);
	if (err)
		return err;

	/* ctx->lifetime */
	p = xdr_reserve_space(xdr, 8+8);
	if (!p)
		return -ENOSPC;
	p = xdr_encode_hyper(p, ctx->lifetime);

	/* ctx->ctx_flags */
	p = xdr_encode_hyper(p, ctx->ctx_flags);

	/* ctx->locally_initiated */
	err = gssx_enc_bool(xdr, ctx->locally_initiated);
	if (err)
		return err;

	/* ctx->open */
	err = gssx_enc_bool(xdr, ctx->open);
	if (err)
		return err;

	/* leave options empty for now, will add once we have any options
	 * to pass up at all */
	/* ctx->options */
	err = dummy_enc_opt_array(xdr, &ctx->options);

	return err;
}

static int gssx_dec_ctx(struct xdr_stream *xdr,
			struct gssx_ctx *ctx)
{
	__be32 *p;
	int err;

	/* ctx->exported_context_token */
	err = gssx_dec_buffer(xdr, &ctx->exported_context_token);
	if (err)
		return err;

	/* ctx->state */
	err = gssx_dec_buffer(xdr, &ctx->state);
	if (err)
		return err;

	/* ctx->need_release */
	err = gssx_dec_bool(xdr, &ctx->need_release);
	if (err)
		return err;

	/* ctx->mech */
	err = gssx_dec_buffer(xdr, &ctx->mech);
	if (err)
		return err;

	/* ctx->src_name */
	err = gssx_dec_name(xdr, &ctx->src_name);
	if (err)
		return err;

	/* ctx->targ_name */
	err = gssx_dec_name(xdr, &ctx->targ_name);
	if (err)
		return err;

	/* ctx->lifetime */
	p = xdr_inline_decode(xdr, 8+8);
	if (unlikely(p == NULL))
		return -ENOSPC;
	p = xdr_decode_hyper(p, &ctx->lifetime);

	/* ctx->ctx_flags */
	p = xdr_decode_hyper(p, &ctx->ctx_flags);

	/* ctx->locally_initiated */
	err = gssx_dec_bool(xdr, &ctx->locally_initiated);
	if (err)
		return err;

	/* ctx->open */
	err = gssx_dec_bool(xdr, &ctx->open);
	if (err)
		return err;

	/* we assume we have no options for now, so simply consume them */
	/* ctx->options */
	err = dummy_dec_opt_array(xdr, &ctx->options);

	return err;
}

static int gssx_enc_cb(struct xdr_stream *xdr, struct gssx_cb *cb)
{
	__be32 *p;
	int err;

	/* cb->initiator_addrtype */
	p = xdr_reserve_space(xdr, 8);
	if (!p)
		return -ENOSPC;
	p = xdr_encode_hyper(p, cb->initiator_addrtype);

	/* cb->initiator_address */
	err = gssx_enc_buffer(xdr, &cb->initiator_address);
	if (err)
		return err;

	/* cb->acceptor_addrtype */
	p = xdr_reserve_space(xdr, 8);
	if (!p)
		return -ENOSPC;
	p = xdr_encode_hyper(p, cb->acceptor_addrtype);

	/* cb->acceptor_address */
	err = gssx_enc_buffer(xdr, &cb->acceptor_address);
	if (err)
		return err;

	/* cb->application_data */
	err = gssx_enc_buffer(xdr, &cb->application_data);

	return err;
}

void gssx_enc_accept_sec_context(struct rpc_rqst *req,
				 struct xdr_stream *xdr,
				 const void *data)
{
	const struct gssx_arg_accept_sec_context *arg = data;
	int err;

	err = gssx_enc_call_ctx(xdr, &arg->call_ctx);
	if (err)
		goto done;

	/* arg->context_handle */
	if (arg->context_handle)
		err = gssx_enc_ctx(xdr, arg->context_handle);
	else
		err = gssx_enc_bool(xdr, 0);
	if (err)
		goto done;

	/* arg->cred_handle */
	if (arg->cred_handle)
		err = gssx_enc_cred(xdr, arg->cred_handle);
	else
		err = gssx_enc_bool(xdr, 0);
	if (err)
		goto done;

	/* arg->input_token */
	err = gssx_enc_in_token(xdr, &arg->input_token);
	if (err)
		goto done;

	/* arg->input_cb */
	if (arg->input_cb)
		err = gssx_enc_cb(xdr, arg->input_cb);
	else
		err = gssx_enc_bool(xdr, 0);
	if (err)
		goto done;

	err = gssx_enc_bool(xdr, arg->ret_deleg_cred);
	if (err)
		goto done;

	/* leave options empty for now, will add once we have any options
	 * to pass up at all */
	/* arg->options */
	err = dummy_enc_opt_array(xdr, &arg->options);

	xdr_inline_pages(&req->rq_rcv_buf,
		PAGE_SIZE/2 /* pretty arbitrary */,
		arg->pages, 0 /* page base */, arg->npages * PAGE_SIZE);
done:
	if (err)
		dprintk("RPC:       gssx_enc_accept_sec_context: %d\n", err);
}

int gssx_dec_accept_sec_context(struct rpc_rqst *rqstp,
				struct xdr_stream *xdr,
				void *data)
{
	struct gssx_res_accept_sec_context *res = data;
	u32 value_follows;
	int err;
	struct page *scratch;

	scratch = alloc_page(GFP_KERNEL);
	if (!scratch)
		return -ENOMEM;
	xdr_set_scratch_page(xdr, scratch);

	/* res->status */
	err = gssx_dec_status(xdr, &res->status);
	if (err)
		goto out_free;

	/* res->context_handle */
	err = gssx_dec_bool(xdr, &value_follows);
	if (err)
		goto out_free;
	if (value_follows) {
		err = gssx_dec_ctx(xdr, res->context_handle);
		if (err)
			goto out_free;
	} else {
		res->context_handle = NULL;
	}

	/* res->output_token */
	err = gssx_dec_bool(xdr, &value_follows);
	if (err)
		goto out_free;
	if (value_follows) {
		err = gssx_dec_buffer(xdr, res->output_token);
		if (err)
			goto out_free;
	} else {
		res->output_token = NULL;
	}

	/* res->delegated_cred_handle */
	err = gssx_dec_bool(xdr, &value_follows);
	if (err)
		goto out_free;
	if (value_follows) {
		/* we do not support upcall servers sending this data. */
		err = -EINVAL;
		goto out_free;
	}

	/* res->options */
	err = gssx_dec_option_array(xdr, &res->options);

out_free:
	__free_page(scratch);
	return err;
}