aboutsummaryrefslogtreecommitdiffstats
path: root/security/keys/keyring.c
blob: a06ffab38568809f5187970261888d6a9e673f18 (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
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
/* Keyring handling
 *
 * Copyright (C) 2004-2005, 2008 Red Hat, Inc. All Rights Reserved.
 * Written by David Howells (dhowells@redhat.com)
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version
 * 2 of the License, or (at your option) any later version.
 */

#include <linux/module.h>
#include <linux/init.h>
#include <linux/sched.h>
#include <linux/slab.h>
#include <linux/security.h>
#include <linux/seq_file.h>
#include <linux/err.h>
#include <keys/keyring-type.h>
#include <linux/uaccess.h>
#include "internal.h"

#define rcu_dereference_locked_keyring(keyring)				\
	(rcu_dereference_protected(					\
		(keyring)->payload.subscriptions,			\
		rwsem_is_locked((struct rw_semaphore *)&(keyring)->sem)))

#define KEY_LINK_FIXQUOTA 1UL

/*
 * When plumbing the depths of the key tree, this sets a hard limit
 * set on how deep we're willing to go.
 */
#define KEYRING_SEARCH_MAX_DEPTH 6

/*
 * We keep all named keyrings in a hash to speed looking them up.
 */
#define KEYRING_NAME_HASH_SIZE	(1 << 5)

static struct list_head	keyring_name_hash[KEYRING_NAME_HASH_SIZE];
static DEFINE_RWLOCK(keyring_name_lock);

static inline unsigned keyring_hash(const char *desc)
{
	unsigned bucket = 0;

	for (; *desc; desc++)
		bucket += (unsigned char)*desc;

	return bucket & (KEYRING_NAME_HASH_SIZE - 1);
}

/*
 * The keyring key type definition.  Keyrings are simply keys of this type and
 * can be treated as ordinary keys in addition to having their own special
 * operations.
 */
static int keyring_instantiate(struct key *keyring,
			       const void *data, size_t datalen);
static int keyring_match(const struct key *keyring, const void *criterion);
static void keyring_revoke(struct key *keyring);
static void keyring_destroy(struct key *keyring);
static void keyring_describe(const struct key *keyring, struct seq_file *m);
static long keyring_read(const struct key *keyring,
			 char __user *buffer, size_t buflen);

struct key_type key_type_keyring = {
	.name		= "keyring",
	.def_datalen	= sizeof(struct keyring_list),
	.instantiate	= keyring_instantiate,
	.match		= keyring_match,
	.revoke		= keyring_revoke,
	.destroy	= keyring_destroy,
	.describe	= keyring_describe,
	.read		= keyring_read,
};
EXPORT_SYMBOL(key_type_keyring);

/*
 * Semaphore to serialise link/link calls to prevent two link calls in parallel
 * introducing a cycle.
 */
static DECLARE_RWSEM(keyring_serialise_link_sem);

/*
 * Publish the name of a keyring so that it can be found by name (if it has
 * one).
 */
static void keyring_publish_name(struct key *keyring)
{
	int bucket;

	if (keyring->description) {
		bucket = keyring_hash(keyring->description);

		write_lock(&keyring_name_lock);

		if (!keyring_name_hash[bucket].next)
			INIT_LIST_HEAD(&keyring_name_hash[bucket]);

		list_add_tail(&keyring->type_data.link,
			      &keyring_name_hash[bucket]);

		write_unlock(&keyring_name_lock);
	}
}

/*
 * Initialise a keyring.
 *
 * Returns 0 on success, -EINVAL if given any data.
 */
static int keyring_instantiate(struct key *keyring,
			       const void *data, size_t datalen)
{
	int ret;

	ret = -EINVAL;
	if (datalen == 0) {
		/* make the keyring available by name if it has one */
		keyring_publish_name(keyring);
		ret = 0;
	}

	return ret;
}

/*
 * Match keyrings on their name
 */
static int keyring_match(const struct key *keyring, const void *description)
{
	return keyring->description &&
		strcmp(keyring->description, description) == 0;
}

/*
 * Clean up a keyring when it is destroyed.  Unpublish its name if it had one
 * and dispose of its data.
 */
static void keyring_destroy(struct key *keyring)
{
	struct keyring_list *klist;
	int loop;

	if (keyring->description) {
		write_lock(&keyring_name_lock);

		if (keyring->type_data.link.next != NULL &&
		    !list_empty(&keyring->type_data.link))
			list_del(&keyring->type_data.link);

		write_unlock(&keyring_name_lock);
	}

	klist = rcu_dereference_check(keyring->payload.subscriptions,
				      rcu_read_lock_held() ||
				      atomic_read(&keyring->usage) == 0);
	if (klist) {
		for (loop = klist->nkeys - 1; loop >= 0; loop--)
			key_put(klist->keys[loop]);
		kfree(klist);
	}
}

/*
 * Describe a keyring for /proc.
 */
static void keyring_describe(const struct key *keyring, struct seq_file *m)
{
	struct keyring_list *klist;

	if (keyring->description)
		seq_puts(m, keyring->description);
	else
		seq_puts(m, "[anon]");

	if (key_is_instantiated(keyring)) {
		rcu_read_lock();
		klist = rcu_dereference(keyring->payload.subscriptions);
		if (klist)
			seq_printf(m, ": %u/%u", klist->nkeys, klist->maxkeys);
		else
			seq_puts(m, ": empty");
		rcu_read_unlock();
	}
}

/*
 * Read a list of key IDs from the keyring's contents in binary form
 *
 * The keyring's semaphore is read-locked by the caller.
 */
static long keyring_read(const struct key *keyring,
			 char __user *buffer, size_t buflen)
{
	struct keyring_list *klist;
	struct key *key;
	size_t qty, tmp;
	int loop, ret;

	ret = 0;
	klist = rcu_dereference_locked_keyring(keyring);
	if (klist) {
		/* calculate how much data we could return */
		qty = klist->nkeys * sizeof(key_serial_t);

		if (buffer && buflen > 0) {
			if (buflen > qty)
				buflen = qty;

			/* copy the IDs of the subscribed keys into the
			 * buffer */
			ret = -EFAULT;

			for (loop = 0; loop < klist->nkeys; loop++) {
				key = klist->keys[loop];

				tmp = sizeof(key_serial_t);
				if (tmp > buflen)
					tmp = buflen;

				if (copy_to_user(buffer,
						 &key->serial,
						 tmp) != 0)
					goto error;

				buflen -= tmp;
				if (buflen == 0)
					break;
				buffer += tmp;
			}
		}

		ret = qty;
	}

error:
	return ret;
}

/*
 * Allocate a keyring and link into the destination keyring.
 */
struct key *keyring_alloc(const char *description, uid_t uid, gid_t gid,
			  const struct cred *cred, unsigned long flags,
			  struct key *dest)
{
	struct key *keyring;
	int ret;

	keyring = key_alloc(&key_type_keyring, description,
			    uid, gid, cred,
			    (KEY_POS_ALL & ~KEY_POS_SETATTR) | KEY_USR_ALL,
			    flags);

	if (!IS_ERR(keyring)) {
		ret = key_instantiate_and_link(keyring, NULL, 0, dest, NULL);
		if (ret < 0) {
			key_put(keyring);
			keyring = ERR_PTR(ret);
		}
	}

	return keyring;
}

/**
 * keyring_search_aux - Search a keyring tree for a key matching some criteria
 * @keyring_ref: A pointer to the keyring with possession indicator.
 * @cred: The credentials to use for permissions checks.
 * @type: The type of key to search for.
 * @description: Parameter for @match.
 * @match: Function to rule on whether or not a key is the one required.
 * @no_state_check: Don't check if a matching key is bad
 *
 * Search the supplied keyring tree for a key that matches the criteria given.
 * The root keyring and any linked keyrings must grant Search permission to the
 * caller to be searchable and keys can only be found if they too grant Search
 * to the caller. The possession flag on the root keyring pointer controls use
 * of the possessor bits in permissions checking of the entire tree.  In
 * addition, the LSM gets to forbid keyring searches and key matches.
 *
 * The search is performed as a breadth-then-depth search up to the prescribed
 * limit (KEYRING_SEARCH_MAX_DEPTH).
 *
 * Keys are matched to the type provided and are then filtered by the match
 * function, which is given the description to use in any way it sees fit.  The
 * match function may use any attributes of a key that it wishes to to
 * determine the match.  Normally the match function from the key type would be
 * used.
 *
 * RCU is used to prevent the keyring key lists from disappearing without the
 * need to take lots of locks.
 *
 * Returns a pointer to the found key and increments the key usage count if
 * successful; -EAGAIN if no matching keys were found, or if expired or revoked
 * keys were found; -ENOKEY if only negative keys were found; -ENOTDIR if the
 * specified keyring wasn't a keyring.
 *
 * In the case of a successful return, the possession attribute from
 * @keyring_ref is propagated to the returned key reference.
 */
key_ref_t keyring_search_aux(key_ref_t keyring_ref,
			     const struct cred *cred,
			     struct key_type *type,
			     const void *description,
			     key_match_func_t match,
			     bool no_state_check)
{
	struct {
		struct keyring_list *keylist;
		int kix;
	} stack[KEYRING_SEARCH_MAX_DEPTH];

	struct keyring_list *keylist;
	struct timespec now;
	unsigned long possessed, kflags;
	struct key *keyring, *key;
	key_ref_t key_ref;
	long err;
	int sp, kix;

	keyring = key_ref_to_ptr(keyring_ref);
	possessed = is_key_possessed(keyring_ref);
	key_check(keyring);

	/* top keyring must have search permission to begin the search */
	err = key_task_permission(keyring_ref, cred, KEY_SEARCH);
	if (err < 0) {
		key_ref = ERR_PTR(err);
		goto error;
	}

	key_ref = ERR_PTR(-ENOTDIR);
	if (keyring->type != &key_type_keyring)
		goto error;

	rcu_read_lock();

	now = current_kernel_time();
	err = -EAGAIN;
	sp = 0;

	/* firstly we should check to see if this top-level keyring is what we
	 * are looking for */
	key_ref = ERR_PTR(-EAGAIN);
	kflags = keyring->flags;
	if (keyring->type == type && match(keyring, description)) {
		key = keyring;
		if (no_state_check)
			goto found;

		/* check it isn't negative and hasn't expired or been
		 * revoked */
		if (kflags & (1 << KEY_FLAG_REVOKED))
			goto error_2;
		if (key->expiry && now.tv_sec >= key->expiry)
			goto error_2;
		key_ref = ERR_PTR(key->type_data.reject_error);
		if (kflags & (1 << KEY_FLAG_NEGATIVE))
			goto error_2;
		goto found;
	}

	/* otherwise, the top keyring must not be revoked, expired, or
	 * negatively instantiated if we are to search it */
	key_ref = ERR_PTR(-EAGAIN);
	if (kflags & ((1 << KEY_FLAG_REVOKED) | (1 << KEY_FLAG_NEGATIVE)) ||
	    (keyring->expiry && now.tv_sec >= keyring->expiry))
		goto error_2;

	/* start processing a new keyring */
descend:
	if (test_bit(KEY_FLAG_REVOKED, &keyring->flags))
		goto not_this_keyring;

	keylist = rcu_dereference(keyring->payload.subscriptions);
	if (!keylist)
		goto not_this_keyring;

	/* iterate through the keys in this keyring first */
	for (kix = 0; kix < keylist->nkeys; kix++) {
		key = keylist->keys[kix];
		kflags = key->flags;

		/* ignore keys not of this type */
		if (key->type != type)
			continue;

		/* skip revoked keys and expired keys */
		if (!no_state_check) {
			if (kflags & (1 << KEY_FLAG_REVOKED))
				continue;

			if (key->expiry && now.tv_sec >= key->expiry)
				continue;
		}

		/* keys that don't match */
		if (!match(key, description))
			continue;

		/* key must have search permissions */
		if (key_task_permission(make_key_ref(key, possessed),
					cred, KEY_SEARCH) < 0)
			continue;

		if (no_state_check)
			goto found;

		/* we set a different error code if we pass a negative key */
		if (kflags & (1 << KEY_FLAG_NEGATIVE)) {
			err = key->type_data.reject_error;
			continue;
		}

		goto found;
	}

	/* search through the keyrings nested in this one */
	kix = 0;
ascend:
	for (; kix < keylist->nkeys; kix++) {
		key = keylist->keys[kix];
		if (key->type != &key_type_keyring)
			continue;

		/* recursively search nested keyrings
		 * - only search keyrings for which we have search permission
		 */
		if (sp >= KEYRING_SEARCH_MAX_DEPTH)
			continue;

		if (key_task_permission(make_key_ref(key, possessed),
					cred, KEY_SEARCH) < 0)
			continue;

		/* stack the current position */
		stack[sp].keylist = keylist;
		stack[sp].kix = kix;
		sp++;

		/* begin again with the new keyring */
		keyring = key;
		goto descend;
	}

	/* the keyring we're looking at was disqualified or didn't contain a
	 * matching key */
not_this_keyring:
	if (sp > 0) {
		/* resume the processing of a keyring higher up in the tree */
		sp--;
		keylist = stack[sp].keylist;
		kix = stack[sp].kix + 1;
		goto ascend;
	}

	key_ref = ERR_PTR(err);
	goto error_2;

	/* we found a viable match */
found:
	atomic_inc(&key->usage);
	key_check(key);
	key_ref = make_key_ref(key, possessed);
error_2:
	rcu_read_unlock();
error:
	return key_ref;
}

/**
 * keyring_search - Search the supplied keyring tree for a matching key
 * @keyring: The root of the keyring tree to be searched.
 * @type: The type of keyring we want to find.
 * @description: The name of the keyring we want to find.
 *
 * As keyring_search_aux() above, but using the current task's credentials and
 * type's default matching function.
 */
key_ref_t keyring_search(key_ref_t keyring,
			 struct key_type *type,
			 const char *description)
{
	if (!type->match)
		return ERR_PTR(-ENOKEY);

	return keyring_search_aux(keyring, current->cred,
				  type, description, type->match, false);
}
EXPORT_SYMBOL(keyring_search);

/*
 * Search the given keyring only (no recursion).
 *
 * The caller must guarantee that the keyring is a keyring and that the
 * permission is granted to search the keyring as no check is made here.
 *
 * RCU is used to make it unnecessary to lock the keyring key list here.
 *
 * Returns a pointer to the found key with usage count incremented if
 * successful and returns -ENOKEY if not found.  Revoked keys and keys not
 * providing the requested permission are skipped over.
 *
 * If successful, the possession indicator is propagated from the keyring ref
 * to the returned key reference.
 */
key_ref_t __keyring_search_one(key_ref_t keyring_ref,
			       const struct key_type *ktype,
			       const char *description,
			       key_perm_t perm)
{
	struct keyring_list *klist;
	unsigned long possessed;
	struct key *keyring, *key;
	int loop;

	keyring = key_ref_to_ptr(keyring_ref);
	possessed = is_key_possessed(keyring_ref);

	rcu_read_lock();

	klist = rcu_dereference(keyring->payload.subscriptions);
	if (klist) {
		for (loop = 0; loop < klist->nkeys; loop++) {
			key = klist->keys[loop];

			if (key->type == ktype &&
			    (!key->type->match ||
			     key->type->match(key, description)) &&
			    key_permission(make_key_ref(key, possessed),
					   perm) == 0 &&
			    !test_bit(KEY_FLAG_REVOKED, &key->flags)
			    )
				goto found;
		}
	}

	rcu_read_unlock();
	return ERR_PTR(-ENOKEY);

found:
	atomic_inc(&key->usage);
	rcu_read_unlock();
	return make_key_ref(key, possessed);
}

/*
 * Find a keyring with the specified name.
 *
 * All named keyrings in the current user namespace are searched, provided they
 * grant Search permission directly to the caller (unless this check is
 * skipped).  Keyrings whose usage points have reached zero or who have been
 * revoked are skipped.
 *
 * Returns a pointer to the keyring with the keyring's refcount having being
 * incremented on success.  -ENOKEY is returned if a key could not be found.
 */
struct key *find_keyring_by_name(const char *name, bool skip_perm_check)
{
	struct key *keyring;
	int bucket;

	if (!name)
		return ERR_PTR(-EINVAL);

	bucket = keyring_hash(name);

	read_lock(&keyring_name_lock);

	if (keyring_name_hash[bucket].next) {
		/* search this hash bucket for a keyring with a matching name
		 * that's readable and that hasn't been revoked */
		list_for_each_entry(keyring,
				    &keyring_name_hash[bucket],
				    type_data.link
				    ) {
			if (keyring->user->user_ns != current_user_ns())
				continue;

			if (test_bit(KEY_FLAG_REVOKED, &keyring->flags))
				continue;

			if (strcmp(keyring->description, name) != 0)
				continue;

			if (!skip_perm_check &&
			    key_permission(make_key_ref(keyring, 0),
					   KEY_SEARCH) < 0)
				continue;

			/* we've got a match but we might end up racing with
			 * key_cleanup() if the keyring is currently 'dead'
			 * (ie. it has a zero usage count) */
			if (!atomic_inc_not_zero(&keyring->usage))
				continue;
			goto out;
		}
	}

	keyring = ERR_PTR(-ENOKEY);
out:
	read_unlock(&keyring_name_lock);
	return keyring;
}

/*
 * See if a cycle will will be created by inserting acyclic tree B in acyclic
 * tree A at the topmost level (ie: as a direct child of A).
 *
 * Since we are adding B to A at the top level, checking for cycles should just
 * be a matter of seeing if node A is somewhere in tree B.
 */
static int keyring_detect_cycle(struct key *A, struct key *B)
{
	struct {
		struct keyring_list *keylist;
		int kix;
	} stack[KEYRING_SEARCH_MAX_DEPTH];

	struct keyring_list *keylist;
	struct key *subtree, *key;
	int sp, kix, ret;

	rcu_read_lock();

	ret = -EDEADLK;
	if (A == B)
		goto cycle_detected;

	subtree = B;
	sp = 0;

	/* start processing a new keyring */
descend:
	if (test_bit(KEY_FLAG_REVOKED, &subtree->flags))
		goto not_this_keyring;

	keylist = rcu_dereference(subtree->payload.subscriptions);
	if (!keylist)
		goto not_this_keyring;
	kix = 0;

ascend:
	/* iterate through the remaining keys in this keyring */
	for (; kix < keylist->nkeys; kix++) {
		key = keylist->keys[kix];

		if (key == A)
			goto cycle_detected;

		/* recursively check nested keyrings */
		if (key->type == &key_type_keyring) {
			if (sp >= KEYRING_SEARCH_MAX_DEPTH)
				goto too_deep;

			/* stack the current position */
			stack[sp].keylist = keylist;
			stack[sp].kix = kix;
			sp++;

			/* begin again with the new keyring */
			subtree = key;
			goto descend;
		}
	}

	/* the keyring we're looking at was disqualified or didn't contain a
	 * matching key */
not_this_keyring:
	if (sp > 0) {
		/* resume the checking of a keyring higher up in the tree */
		sp--;
		keylist = stack[sp].keylist;
		kix = stack[sp].kix + 1;
		goto ascend;
	}

	ret = 0; /* no cycles detected */

error:
	rcu_read_unlock();
	return ret;

too_deep:
	ret = -ELOOP;
	goto error;

cycle_detected:
	ret = -EDEADLK;
	goto error;
}

/*
 * Dispose of a keyring list after the RCU grace period, freeing the unlinked
 * key
 */
static void keyring_unlink_rcu_disposal(struct rcu_head *rcu)
{
	struct keyring_list *klist =
		container_of(rcu, struct keyring_list, rcu);

	if (klist->delkey != USHRT_MAX)
		key_put(klist->keys[klist->delkey]);
	kfree(klist);
}

/*
 * Preallocate memory so that a key can be linked into to a keyring.
 */
int __key_link_begin(struct key *keyring, const struct key_type *type,
		     const char *description, unsigned long *_prealloc)
	__acquires(&keyring->sem)
{
	struct keyring_list *klist, *nklist;
	unsigned long prealloc;
	unsigned max;
	size_t size;
	int loop, ret;

	kenter("%d,%s,%s,", key_serial(keyring), type->name, description);

	if (keyring->type != &key_type_keyring)
		return -ENOTDIR;

	down_write(&keyring->sem);

	ret = -EKEYREVOKED;
	if (test_bit(KEY_FLAG_REVOKED, &keyring->flags))
		goto error_krsem;

	/* serialise link/link calls to prevent parallel calls causing a cycle
	 * when linking two keyring in opposite orders */
	if (type == &key_type_keyring)
		down_write(&keyring_serialise_link_sem);

	klist = rcu_dereference_locked_keyring(keyring);

	/* see if there's a matching key we can displace */
	if (klist && klist->nkeys > 0) {
		for (loop = klist->nkeys - 1; loop >= 0; loop--) {
			if (klist->keys[loop]->type == type &&
			    strcmp(klist->keys[loop]->description,
				   description) == 0
			    ) {
				/* found a match - we'll replace this one with
				 * the new key */
				size = sizeof(struct key *) * klist->maxkeys;
				size += sizeof(*klist);
				BUG_ON(size > PAGE_SIZE);

				ret = -ENOMEM;
				nklist = kmemdup(klist, size, GFP_KERNEL);
				if (!nklist)
					goto error_sem;

				/* note replacement slot */
				klist->delkey = nklist->delkey = loop;
				prealloc = (unsigned long)nklist;
				goto done;
			}
		}
	}

	/* check that we aren't going to overrun the user's quota */
	ret = key_payload_reserve(keyring,
				  keyring->datalen + KEYQUOTA_LINK_BYTES);
	if (ret < 0)
		goto error_sem;

	if (klist && klist->nkeys < klist->maxkeys) {
		/* there's sufficient slack space to append directly */
		nklist = NULL;
		prealloc = KEY_LINK_FIXQUOTA;
	} else {
		/* grow the key list */
		max = 4;
		if (klist)
			max += klist->maxkeys;

		ret = -ENFILE;
		if (max > USHRT_MAX - 1)
			goto error_quota;
		size = sizeof(*klist) + sizeof(struct key *) * max;
		if (size > PAGE_SIZE)
			goto error_quota;

		ret = -ENOMEM;
		nklist = kmalloc(size, GFP_KERNEL);
		if (!nklist)
			goto error_quota;

		nklist->maxkeys = max;
		if (klist) {
			memcpy(nklist->keys, klist->keys,
			       sizeof(struct key *) * klist->nkeys);
			nklist->delkey = klist->nkeys;
			nklist->nkeys = klist->nkeys + 1;
			klist->delkey = USHRT_MAX;
		} else {
			nklist->nkeys = 1;
			nklist->delkey = 0;
		}

		/* add the key into the new space */
		nklist->keys[nklist->delkey] = NULL;
	}

	prealloc = (unsigned long)nklist | KEY_LINK_FIXQUOTA;
done:
	*_prealloc = prealloc;
	kleave(" = 0");
	return 0;

error_quota:
	/* undo the quota changes */
	key_payload_reserve(keyring,
			    keyring->datalen - KEYQUOTA_LINK_BYTES);
error_sem:
	if (type == &key_type_keyring)
		up_write(&keyring_serialise_link_sem);
error_krsem:
	up_write(&keyring->sem);
	kleave(" = %d", ret);
	return ret;
}

/*
 * Check already instantiated keys aren't going to be a problem.
 *
 * The caller must have called __key_link_begin(). Don't need to call this for
 * keys that were created since __key_link_begin() was called.
 */
int __key_link_check_live_key(struct key *keyring, struct key *key)
{
	if (key->type == &key_type_keyring)
		/* check that we aren't going to create a cycle by linking one
		 * keyring to another */
		return keyring_detect_cycle(keyring, key);
	return 0;
}

/*
 * Link a key into to a keyring.
 *
 * Must be called with __key_link_begin() having being called.  Discards any
 * already extant link to matching key if there is one, so that each keyring
 * holds at most one link to any given key of a particular type+description
 * combination.
 */
void __key_link(struct key *keyring, struct key *key,
		unsigned long *_prealloc)
{
	struct keyring_list *klist, *nklist;

	nklist = (struct keyring_list *)(*_prealloc & ~KEY_LINK_FIXQUOTA);
	*_prealloc = 0;

	kenter("%d,%d,%p", keyring->serial, key->serial, nklist);

	klist = rcu_dereference_protected(keyring->payload.subscriptions,
					  rwsem_is_locked(&keyring->sem));

	atomic_inc(&key->usage);

	/* there's a matching key we can displace or an empty slot in a newly
	 * allocated list we can fill */
	if (nklist) {
		kdebug("replace %hu/%hu/%hu",
		       nklist->delkey, nklist->nkeys, nklist->maxkeys);

		nklist->keys[nklist->delkey] = key;

		rcu_assign_pointer(keyring->payload.subscriptions, nklist);

		/* dispose of the old keyring list and, if there was one, the
		 * displaced key */
		if (klist) {
			kdebug("dispose %hu/%hu/%hu",
			       klist->delkey, klist->nkeys, klist->maxkeys);
			call_rcu(&klist->rcu, keyring_unlink_rcu_disposal);
		}
	} else {
		/* there's sufficient slack space to append directly */
		klist->keys[klist->nkeys] = key;
		smp_wmb();
		klist->nkeys++;
	}
}

/*
 * Finish linking a key into to a keyring.
 *
 * Must be called with __key_link_begin() having being called.
 */
void __key_link_end(struct key *keyring, struct key_type *type,
		    unsigned long prealloc)
	__releases(&keyring->sem)
{
	BUG_ON(type == NULL);
	BUG_ON(type->name == NULL);
	kenter("%d,%s,%lx", keyring->serial, type->name, prealloc);

	if (type == &key_type_keyring)
		up_write(&keyring_serialise_link_sem);

	if (prealloc) {
		if (prealloc & KEY_LINK_FIXQUOTA)
			key_payload_reserve(keyring,
					    keyring->datalen -
					    KEYQUOTA_LINK_BYTES);
		kfree((struct keyring_list *)(prealloc & ~KEY_LINK_FIXQUOTA));
	}
	up_write(&keyring->sem);
}

/**
 * key_link - Link a key to a keyring
 * @keyring: The keyring to make the link in.
 * @key: The key to link to.
 *
 * Make a link in a keyring to a key, such that the keyring holds a reference
 * on that key and the key can potentially be found by searching that keyring.
 *
 * This function will write-lock the keyring's semaphore and will consume some
 * of the user's key data quota to hold the link.
 *
 * Returns 0 if successful, -ENOTDIR if the keyring isn't a keyring,
 * -EKEYREVOKED if the keyring has been revoked, -ENFILE if the keyring is
 * full, -EDQUOT if there is insufficient key data quota remaining to add
 * another link or -ENOMEM if there's insufficient memory.
 *
 * It is assumed that the caller has checked that it is permitted for a link to
 * be made (the keyring should have Write permission and the key Link
 * permission).
 */
int key_link(struct key *keyring, struct key *key)
{
	unsigned long prealloc;
	int ret;

	key_check(keyring);
	key_check(key);

	ret = __key_link_begin(keyring, key->type, key->description, &prealloc);
	if (ret == 0) {
		ret = __key_link_check_live_key(keyring, key);
		if (ret == 0)
			__key_link(keyring, key, &prealloc);
		__key_link_end(keyring, key->type, prealloc);
	}

	return ret;
}
EXPORT_SYMBOL(key_link);

/**
 * key_unlink - Unlink the first link to a key from a keyring.
 * @keyring: The keyring to remove the link from.
 * @key: The key the link is to.
 *
 * Remove a link from a keyring to a key.
 *
 * This function will write-lock the keyring's semaphore.
 *
 * Returns 0 if successful, -ENOTDIR if the keyring isn't a keyring, -ENOENT if
 * the key isn't linked to by the keyring or -ENOMEM if there's insufficient
 * memory.
 *
 * It is assumed that the caller has checked that it is permitted for a link to
 * be removed (the keyring should have Write permission; no permissions are
 * required on the key).
 */
int key_unlink(struct key *keyring, struct key *key)
{
	struct keyring_list *klist, *nklist;
	int loop, ret;

	key_check(keyring);
	key_check(key);

	ret = -ENOTDIR;
	if (keyring->type != &key_type_keyring)
		goto error;

	down_write(&keyring->sem);

	klist = rcu_dereference_locked_keyring(keyring);
	if (klist) {
		/* search the keyring for the key */
		for (loop = 0; loop < klist->nkeys; loop++)
			if (klist->keys[loop] == key)
				goto key_is_present;
	}

	up_write(&keyring->sem);
	ret = -ENOENT;
	goto error;

key_is_present:
	/* we need to copy the key list for RCU purposes */
	nklist = kmalloc(sizeof(*klist) +
			 sizeof(struct key *) * klist->maxkeys,
			 GFP_KERNEL);
	if (!nklist)
		goto nomem;
	nklist->maxkeys = klist->maxkeys;
	nklist->nkeys = klist->nkeys - 1;

	if (loop > 0)
		memcpy(&nklist->keys[0],
		       &klist->keys[0],
		       loop * sizeof(struct key *));

	if (loop < nklist->nkeys)
		memcpy(&nklist->keys[loop],
		       &klist->keys[loop + 1],
		       (nklist->nkeys - loop) * sizeof(struct key *));

	/* adjust the user's quota */
	key_payload_reserve(keyring,
			    keyring->datalen - KEYQUOTA_LINK_BYTES);

	rcu_assign_pointer(keyring->payload.subscriptions, nklist);

	up_write(&keyring->sem);

	/* schedule for later cleanup */
	klist->delkey = loop;
	call_rcu(&klist->rcu, keyring_unlink_rcu_disposal);

	ret = 0;

error:
	return ret;
nomem:
	ret = -ENOMEM;
	up_write(&keyring->sem);
	goto error;
}
EXPORT_SYMBOL(key_unlink);

/*
 * Dispose of a keyring list after the RCU grace period, releasing the keys it
 * links to.
 */
static void keyring_clear_rcu_disposal(struct rcu_head *rcu)
{
	struct keyring_list *klist;
	int loop;

	klist = container_of(rcu, struct keyring_list, rcu);

	for (loop = klist->nkeys - 1; loop >= 0; loop--)
		key_put(klist->keys[loop]);

	kfree(klist);
}

/**
 * keyring_clear - Clear a keyring
 * @keyring: The keyring to clear.
 *
 * Clear the contents of the specified keyring.
 *
 * Returns 0 if successful or -ENOTDIR if the keyring isn't a keyring.
 */
int keyring_clear(struct key *keyring)
{
	struct keyring_list *klist;
	int ret;

	ret = -ENOTDIR;
	if (keyring->type == &key_type_keyring) {
		/* detach the pointer block with the locks held */
		down_write(&keyring->sem);

		klist = rcu_dereference_locked_keyring(keyring);
		if (klist) {
			/* adjust the quota */
			key_payload_reserve(keyring,
					    sizeof(struct keyring_list));

			rcu_assign_pointer(keyring->payload.subscriptions,
					   NULL);
		}

		up_write(&keyring->sem);

		/* free the keys after the locks have been dropped */
		if (klist)
			call_rcu(&klist->rcu, keyring_clear_rcu_disposal);

		ret = 0;
	}

	return ret;
}
EXPORT_SYMBOL(keyring_clear);

/*
 * Dispose of the links from a revoked keyring.
 *
 * This is called with the key sem write-locked.
 */
static void keyring_revoke(struct key *keyring)
{
	struct keyring_list *klist;

	klist = rcu_dereference_locked_keyring(keyring);

	/* adjust the quota */
	key_payload_reserve(keyring, 0);

	if (klist) {
		rcu_assign_pointer(keyring->payload.subscriptions, NULL);
		call_rcu(&klist->rcu, keyring_clear_rcu_disposal);
	}
}

/*
 * Determine whether a key is dead.
 */
static bool key_is_dead(struct key *key, time_t limit)
{
	return test_bit(KEY_FLAG_DEAD, &key->flags) ||
		(key->expiry > 0 && key->expiry <= limit);
}

/*
 * Collect garbage from the contents of a keyring, replacing the old list with
 * a new one with the pointers all shuffled down.
 *
 * Dead keys are classed as oned that are flagged as being dead or are revoked,
 * expired or negative keys that were revoked or expired before the specified
 * limit.
 */
void keyring_gc(struct key *keyring, time_t limit)
{
	struct keyring_list *klist, *new;
	struct key *key;
	int loop, keep, max;

	kenter("{%x,%s}", key_serial(keyring), keyring->description);

	down_write(&keyring->sem);

	klist = rcu_dereference_locked_keyring(keyring);
	if (!klist)
		goto no_klist;

	/* work out how many subscriptions we're keeping */
	keep = 0;
	for (loop = klist->nkeys - 1; loop >= 0; loop--)
		if (!key_is_dead(klist->keys[loop], limit))
			keep++;

	if (keep == klist->nkeys)
		goto just_return;

	/* allocate a new keyring payload */
	max = roundup(keep, 4);
	new = kmalloc(sizeof(struct keyring_list) + max * sizeof(struct key *),
		      GFP_KERNEL);
	if (!new)
		goto nomem;
	new->maxkeys = max;
	new->nkeys = 0;
	new->delkey = 0;

	/* install the live keys
	 * - must take care as expired keys may be updated back to life
	 */
	keep = 0;
	for (loop = klist->nkeys - 1; loop >= 0; loop--) {
		key = klist->keys[loop];
		if (!key_is_dead(key, limit)) {
			if (keep >= max)
				goto discard_new;
			new->keys[keep++] = key_get(key);
		}
	}
	new->nkeys = keep;

	/* adjust the quota */
	key_payload_reserve(keyring,
			    sizeof(struct keyring_list) +
			    KEYQUOTA_LINK_BYTES * keep);

	if (keep == 0) {
		rcu_assign_pointer(keyring->payload.subscriptions, NULL);
		kfree(new);
	} else {
		rcu_assign_pointer(keyring->payload.subscriptions, new);
	}

	up_write(&keyring->sem);

	call_rcu(&klist->rcu, keyring_clear_rcu_disposal);
	kleave(" [yes]");
	return;

discard_new:
	new->nkeys = keep;
	keyring_clear_rcu_disposal(&new->rcu);
	up_write(&keyring->sem);
	kleave(" [discard]");
	return;

just_return:
	up_write(&keyring->sem);
	kleave(" [no dead]");
	return;

no_klist:
	up_write(&keyring->sem);
	kleave(" [no_klist]");
	return;

nomem:
	up_write(&keyring->sem);
	kleave(" [oom]");
}