aboutsummaryrefslogtreecommitdiff
path: root/security/tomoyo/gc.c
blob: ae135fbbbe955b922c0fe3dbe6d8d158dc6fa9d1 (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
/*
 * security/tomoyo/gc.c
 *
 * Copyright (C) 2005-2011  NTT DATA CORPORATION
 */

#include "common.h"
#include <linux/kthread.h>
#include <linux/slab.h>

/* The list for "struct tomoyo_io_buffer". */
static LIST_HEAD(tomoyo_io_buffer_list);
/* Lock for protecting tomoyo_io_buffer_list. */
static DEFINE_SPINLOCK(tomoyo_io_buffer_list_lock);

/* Size of an element. */
static const u8 tomoyo_element_size[TOMOYO_MAX_POLICY] = {
	[TOMOYO_ID_GROUP] = sizeof(struct tomoyo_group),
	[TOMOYO_ID_PATH_GROUP] = sizeof(struct tomoyo_path_group),
	[TOMOYO_ID_NUMBER_GROUP] = sizeof(struct tomoyo_number_group),
	[TOMOYO_ID_AGGREGATOR] = sizeof(struct tomoyo_aggregator),
	[TOMOYO_ID_TRANSITION_CONTROL] =
	sizeof(struct tomoyo_transition_control),
	[TOMOYO_ID_MANAGER] = sizeof(struct tomoyo_manager),
	/* [TOMOYO_ID_CONDITION] = "struct tomoyo_condition"->size, */
	/* [TOMOYO_ID_NAME] = "struct tomoyo_name"->size, */
	/* [TOMOYO_ID_ACL] =
	   tomoyo_acl_size["struct tomoyo_acl_info"->type], */
	[TOMOYO_ID_DOMAIN] = sizeof(struct tomoyo_domain_info),
};

/* Size of a domain ACL element. */
static const u8 tomoyo_acl_size[] = {
	[TOMOYO_TYPE_PATH_ACL] = sizeof(struct tomoyo_path_acl),
	[TOMOYO_TYPE_PATH2_ACL] = sizeof(struct tomoyo_path2_acl),
	[TOMOYO_TYPE_PATH_NUMBER_ACL] = sizeof(struct tomoyo_path_number_acl),
	[TOMOYO_TYPE_MKDEV_ACL] = sizeof(struct tomoyo_mkdev_acl),
	[TOMOYO_TYPE_MOUNT_ACL] = sizeof(struct tomoyo_mount_acl),
};

/**
 * tomoyo_struct_used_by_io_buffer - Check whether the list element is used by /sys/kernel/security/tomoyo/ users or not.
 *
 * @element: Pointer to "struct list_head".
 *
 * Returns true if @element is used by /sys/kernel/security/tomoyo/ users,
 * false otherwise.
 */
static bool tomoyo_struct_used_by_io_buffer(const struct list_head *element)
{
	struct tomoyo_io_buffer *head;
	bool in_use = false;

	spin_lock(&tomoyo_io_buffer_list_lock);
	list_for_each_entry(head, &tomoyo_io_buffer_list, list) {
		head->users++;
		spin_unlock(&tomoyo_io_buffer_list_lock);
		if (mutex_lock_interruptible(&head->io_sem)) {
			in_use = true;
			goto out;
		}
		if (head->r.domain == element || head->r.group == element ||
		    head->r.acl == element || &head->w.domain->list == element)
			in_use = true;
		mutex_unlock(&head->io_sem);
out:
		spin_lock(&tomoyo_io_buffer_list_lock);
		head->users--;
		if (in_use)
			break;
	}
	spin_unlock(&tomoyo_io_buffer_list_lock);
	return in_use;
}

/**
 * tomoyo_name_used_by_io_buffer - Check whether the string is used by /sys/kernel/security/tomoyo/ users or not.
 *
 * @string: String to check.
 * @size:   Memory allocated for @string .
 *
 * Returns true if @string is used by /sys/kernel/security/tomoyo/ users,
 * false otherwise.
 */
static bool tomoyo_name_used_by_io_buffer(const char *string,
					  const size_t size)
{
	struct tomoyo_io_buffer *head;
	bool in_use = false;

	spin_lock(&tomoyo_io_buffer_list_lock);
	list_for_each_entry(head, &tomoyo_io_buffer_list, list) {
		int i;
		head->users++;
		spin_unlock(&tomoyo_io_buffer_list_lock);
		if (mutex_lock_interruptible(&head->io_sem)) {
			in_use = true;
			goto out;
		}
		for (i = 0; i < TOMOYO_MAX_IO_READ_QUEUE; i++) {
			const char *w = head->r.w[i];
			if (w < string || w > string + size)
				continue;
			in_use = true;
			break;
		}
		mutex_unlock(&head->io_sem);
out:
		spin_lock(&tomoyo_io_buffer_list_lock);
		head->users--;
		if (in_use)
			break;
	}
	spin_unlock(&tomoyo_io_buffer_list_lock);
	return in_use;
}

/* Structure for garbage collection. */
struct tomoyo_gc {
	struct list_head list;
	enum tomoyo_policy_id type;
	size_t size;
	struct list_head *element;
};
/* List of entries to be deleted. */
static LIST_HEAD(tomoyo_gc_list);
/* Length of tomoyo_gc_list. */
static int tomoyo_gc_list_len;

/**
 * tomoyo_add_to_gc - Add an entry to to be deleted list.
 *
 * @type:    One of values in "enum tomoyo_policy_id".
 * @element: Pointer to "struct list_head".
 *
 * Returns true on success, false otherwise.
 *
 * Caller holds tomoyo_policy_lock mutex.
 *
 * Adding an entry needs kmalloc(). Thus, if we try to add thousands of
 * entries at once, it will take too long time. Thus, do not add more than 128
 * entries per a scan. But to be able to handle worst case where all entries
 * are in-use, we accept one more entry per a scan.
 *
 * If we use singly linked list using "struct list_head"->prev (which is
 * LIST_POISON2), we can avoid kmalloc().
 */
static bool tomoyo_add_to_gc(const int type, struct list_head *element)
{
	struct tomoyo_gc *entry = kzalloc(sizeof(*entry), GFP_ATOMIC);
	if (!entry)
		return false;
	entry->type = type;
	if (type == TOMOYO_ID_ACL)
		entry->size = tomoyo_acl_size[
			      container_of(element,
					   typeof(struct tomoyo_acl_info),
					   list)->type];
	else if (type == TOMOYO_ID_NAME)
		entry->size = strlen(container_of(element,
						  typeof(struct tomoyo_name),
						  head.list)->entry.name) + 1;
	else if (type == TOMOYO_ID_CONDITION)
		entry->size =
			container_of(element, typeof(struct tomoyo_condition),
				     head.list)->size;
	else
		entry->size = tomoyo_element_size[type];
	entry->element = element;
	list_add(&entry->list, &tomoyo_gc_list);
	list_del_rcu(element);
	return tomoyo_gc_list_len++ < 128;
}

/**
 * tomoyo_element_linked_by_gc - Validate next element of an entry.
 *
 * @element: Pointer to an element.
 * @size:    Size of @element in byte.
 *
 * Returns true if @element is linked by other elements in the garbage
 * collector's queue, false otherwise.
 */
static bool tomoyo_element_linked_by_gc(const u8 *element, const size_t size)
{
	struct tomoyo_gc *p;
	list_for_each_entry(p, &tomoyo_gc_list, list) {
		const u8 *ptr = (const u8 *) p->element->next;
		if (ptr < element || element + size < ptr)
			continue;
		return true;
	}
	return false;
}

/**
 * tomoyo_del_transition_control - Delete members in "struct tomoyo_transition_control".
 *
 * @element: Pointer to "struct list_head".
 *
 * Returns nothing.
 */
static void tomoyo_del_transition_control(struct list_head *element)
{
	struct tomoyo_transition_control *ptr =
		container_of(element, typeof(*ptr), head.list);
	tomoyo_put_name(ptr->domainname);
	tomoyo_put_name(ptr->program);
}

/**
 * tomoyo_del_aggregator - Delete members in "struct tomoyo_aggregator".
 *
 * @element: Pointer to "struct list_head".
 *
 * Returns nothing.
 */
static void tomoyo_del_aggregator(struct list_head *element)
{
	struct tomoyo_aggregator *ptr =
		container_of(element, typeof(*ptr), head.list);
	tomoyo_put_name(ptr->original_name);
	tomoyo_put_name(ptr->aggregated_name);
}

/**
 * tomoyo_del_manager - Delete members in "struct tomoyo_manager".
 *
 * @element: Pointer to "struct list_head".
 *
 * Returns nothing.
 */
static void tomoyo_del_manager(struct list_head *element)
{
	struct tomoyo_manager *ptr =
		container_of(element, typeof(*ptr), head.list);
	tomoyo_put_name(ptr->manager);
}

/**
 * tomoyo_del_acl - Delete members in "struct tomoyo_acl_info".
 *
 * @element: Pointer to "struct list_head".
 *
 * Returns nothing.
 */
static void tomoyo_del_acl(struct list_head *element)
{
	struct tomoyo_acl_info *acl =
		container_of(element, typeof(*acl), list);
	tomoyo_put_condition(acl->cond);
	switch (acl->type) {
	case TOMOYO_TYPE_PATH_ACL:
		{
			struct tomoyo_path_acl *entry
				= container_of(acl, typeof(*entry), head);
			tomoyo_put_name_union(&entry->name);
		}
		break;
	case TOMOYO_TYPE_PATH2_ACL:
		{
			struct tomoyo_path2_acl *entry
				= container_of(acl, typeof(*entry), head);
			tomoyo_put_name_union(&entry->name1);
			tomoyo_put_name_union(&entry->name2);
		}
		break;
	case TOMOYO_TYPE_PATH_NUMBER_ACL:
		{
			struct tomoyo_path_number_acl *entry
				= container_of(acl, typeof(*entry), head);
			tomoyo_put_name_union(&entry->name);
			tomoyo_put_number_union(&entry->number);
		}
		break;
	case TOMOYO_TYPE_MKDEV_ACL:
		{
			struct tomoyo_mkdev_acl *entry
				= container_of(acl, typeof(*entry), head);
			tomoyo_put_name_union(&entry->name);
			tomoyo_put_number_union(&entry->mode);
			tomoyo_put_number_union(&entry->major);
			tomoyo_put_number_union(&entry->minor);
		}
		break;
	case TOMOYO_TYPE_MOUNT_ACL:
		{
			struct tomoyo_mount_acl *entry
				= container_of(acl, typeof(*entry), head);
			tomoyo_put_name_union(&entry->dev_name);
			tomoyo_put_name_union(&entry->dir_name);
			tomoyo_put_name_union(&entry->fs_type);
			tomoyo_put_number_union(&entry->flags);
		}
		break;
	}
}

/**
 * tomoyo_del_domain - Delete members in "struct tomoyo_domain_info".
 *
 * @element: Pointer to "struct list_head".
 *
 * Returns true if deleted, false otherwise.
 */
static bool tomoyo_del_domain(struct list_head *element)
{
	struct tomoyo_domain_info *domain =
		container_of(element, typeof(*domain), list);
	struct tomoyo_acl_info *acl;
	struct tomoyo_acl_info *tmp;
	/*
	 * Since we don't protect whole execve() operation using SRCU,
	 * we need to recheck domain->users at this point.
	 *
	 * (1) Reader starts SRCU section upon execve().
	 * (2) Reader traverses tomoyo_domain_list and finds this domain.
	 * (3) Writer marks this domain as deleted.
	 * (4) Garbage collector removes this domain from tomoyo_domain_list
	 *     because this domain is marked as deleted and used by nobody.
	 * (5) Reader saves reference to this domain into
	 *     "struct linux_binprm"->cred->security .
	 * (6) Reader finishes SRCU section, although execve() operation has
	 *     not finished yet.
	 * (7) Garbage collector waits for SRCU synchronization.
	 * (8) Garbage collector kfree() this domain because this domain is
	 *     used by nobody.
	 * (9) Reader finishes execve() operation and restores this domain from
	 *     "struct linux_binprm"->cred->security.
	 *
	 * By updating domain->users at (5), we can solve this race problem
	 * by rechecking domain->users at (8).
	 */
	if (atomic_read(&domain->users))
		return false;
	list_for_each_entry_safe(acl, tmp, &domain->acl_info_list, list) {
		tomoyo_del_acl(&acl->list);
		tomoyo_memory_free(acl);
	}
	tomoyo_put_name(domain->domainname);
	return true;
}

/**
 * tomoyo_del_condition - Delete members in "struct tomoyo_condition".
 *
 * @element: Pointer to "struct list_head".
 *
 * Returns nothing.
 */
void tomoyo_del_condition(struct list_head *element)
{
	struct tomoyo_condition *cond = container_of(element, typeof(*cond),
						     head.list);
	const u16 condc = cond->condc;
	const u16 numbers_count = cond->numbers_count;
	const u16 names_count = cond->names_count;
	const u16 argc = cond->argc;
	const u16 envc = cond->envc;
	unsigned int i;
	const struct tomoyo_condition_element *condp
		= (const struct tomoyo_condition_element *) (cond + 1);
	struct tomoyo_number_union *numbers_p
		= (struct tomoyo_number_union *) (condp + condc);
	struct tomoyo_name_union *names_p
		= (struct tomoyo_name_union *) (numbers_p + numbers_count);
	const struct tomoyo_argv *argv
		= (const struct tomoyo_argv *) (names_p + names_count);
	const struct tomoyo_envp *envp
		= (const struct tomoyo_envp *) (argv + argc);
	for (i = 0; i < numbers_count; i++)
		tomoyo_put_number_union(numbers_p++);
	for (i = 0; i < names_count; i++)
		tomoyo_put_name_union(names_p++);
	for (i = 0; i < argc; argv++, i++)
		tomoyo_put_name(argv->value);
	for (i = 0; i < envc; envp++, i++) {
		tomoyo_put_name(envp->name);
		tomoyo_put_name(envp->value);
	}
}

/**
 * tomoyo_del_name - Delete members in "struct tomoyo_name".
 *
 * @element: Pointer to "struct list_head".
 *
 * Returns nothing.
 */
static void tomoyo_del_name(struct list_head *element)
{
	const struct tomoyo_name *ptr =
		container_of(element, typeof(*ptr), head.list);
}

/**
 * tomoyo_del_path_group - Delete members in "struct tomoyo_path_group".
 *
 * @element: Pointer to "struct list_head".
 *
 * Returns nothing.
 */
static void tomoyo_del_path_group(struct list_head *element)
{
	struct tomoyo_path_group *member =
		container_of(element, typeof(*member), head.list);
	tomoyo_put_name(member->member_name);
}

/**
 * tomoyo_del_group - Delete "struct tomoyo_group".
 *
 * @element: Pointer to "struct list_head".
 *
 * Returns nothing.
 */
static void tomoyo_del_group(struct list_head *element)
{
	struct tomoyo_group *group =
		container_of(element, typeof(*group), head.list);
	tomoyo_put_name(group->group_name);
}

/**
 * tomoyo_del_number_group - Delete members in "struct tomoyo_number_group".
 *
 * @element: Pointer to "struct list_head".
 *
 * Returns nothing.
 */
static void tomoyo_del_number_group(struct list_head *element)
{
	struct tomoyo_number_group *member =
		container_of(element, typeof(*member), head.list);
}

/**
 * tomoyo_collect_member - Delete elements with "struct tomoyo_acl_head".
 *
 * @id:          One of values in "enum tomoyo_policy_id".
 * @member_list: Pointer to "struct list_head".
 *
 * Returns true if some elements are deleted, false otherwise.
 */
static bool tomoyo_collect_member(const enum tomoyo_policy_id id,
				  struct list_head *member_list)
{
	struct tomoyo_acl_head *member;
	list_for_each_entry(member, member_list, list) {
		if (!member->is_deleted)
			continue;
		if (!tomoyo_add_to_gc(id, &member->list))
			return false;
	}
	return true;
}

/**
 * tomoyo_collect_acl - Delete elements in "struct tomoyo_domain_info".
 *
 * @list: Pointer to "struct list_head".
 *
 * Returns true if some elements are deleted, false otherwise.
 */
static bool tomoyo_collect_acl(struct list_head *list)
{
	struct tomoyo_acl_info *acl;
	list_for_each_entry(acl, list, list) {
		if (!acl->is_deleted)
			continue;
		if (!tomoyo_add_to_gc(TOMOYO_ID_ACL, &acl->list))
			return false;
	}
	return true;
}

/**
 * tomoyo_collect_entry - Scan lists for deleted elements.
 *
 * Returns nothing.
 */
static void tomoyo_collect_entry(void)
{
	int i;
	enum tomoyo_policy_id id;
	struct tomoyo_policy_namespace *ns;
	int idx;
	if (mutex_lock_interruptible(&tomoyo_policy_lock))
		return;
	idx = tomoyo_read_lock();
	{
		struct tomoyo_domain_info *domain;
		list_for_each_entry_rcu(domain, &tomoyo_domain_list, list) {
			if (!tomoyo_collect_acl(&domain->acl_info_list))
				goto unlock;
			if (!domain->is_deleted || atomic_read(&domain->users))
				continue;
			/*
			 * Nobody is referring this domain. But somebody may
			 * refer this domain after successful execve().
			 * We recheck domain->users after SRCU synchronization.
			 */
			if (!tomoyo_add_to_gc(TOMOYO_ID_DOMAIN, &domain->list))
				goto unlock;
		}
	}
	list_for_each_entry_rcu(ns, &tomoyo_namespace_list, namespace_list) {
		for (id = 0; id < TOMOYO_MAX_POLICY; id++)
			if (!tomoyo_collect_member(id, &ns->policy_list[id]))
				goto unlock;
		for (i = 0; i < TOMOYO_MAX_ACL_GROUPS; i++)
			if (!tomoyo_collect_acl(&ns->acl_group[i]))
				goto unlock;
		for (i = 0; i < TOMOYO_MAX_GROUP; i++) {
			struct list_head *list = &ns->group_list[i];
			struct tomoyo_group *group;
			switch (i) {
			case 0:
				id = TOMOYO_ID_PATH_GROUP;
				break;
			default:
				id = TOMOYO_ID_NUMBER_GROUP;
				break;
			}
			list_for_each_entry(group, list, head.list) {
				if (!tomoyo_collect_member
				    (id, &group->member_list))
					goto unlock;
				if (!list_empty(&group->member_list) ||
				    atomic_read(&group->head.users))
					continue;
				if (!tomoyo_add_to_gc(TOMOYO_ID_GROUP,
						      &group->head.list))
					goto unlock;
			}
		}
	}
	id = TOMOYO_ID_CONDITION;
	for (i = 0; i < TOMOYO_MAX_HASH + 1; i++) {
		struct list_head *list = !i ?
			&tomoyo_condition_list : &tomoyo_name_list[i - 1];
		struct tomoyo_shared_acl_head *ptr;
		list_for_each_entry(ptr, list, list) {
			if (atomic_read(&ptr->users))
				continue;
			if (!tomoyo_add_to_gc(id, &ptr->list))
				goto unlock;
		}
		id = TOMOYO_ID_NAME;
	}
unlock:
	tomoyo_read_unlock(idx);
	mutex_unlock(&tomoyo_policy_lock);
}

/**
 * tomoyo_kfree_entry - Delete entries in tomoyo_gc_list.
 *
 * Returns true if some entries were kfree()d, false otherwise.
 */
static bool tomoyo_kfree_entry(void)
{
	struct tomoyo_gc *p;
	struct tomoyo_gc *tmp;
	bool result = false;

	list_for_each_entry_safe(p, tmp, &tomoyo_gc_list, list) {
		struct list_head *element = p->element;

		/*
		 * list_del_rcu() in tomoyo_add_to_gc() guarantees that the
		 * list element became no longer reachable from the list which
		 * the element was originally on (e.g. tomoyo_domain_list).
		 * Also, synchronize_srcu() in tomoyo_gc_thread() guarantees
		 * that the list element became no longer referenced by syscall
		 * users.
		 *
		 * However, there are three users which may still be using the
		 * list element. We need to defer until all of these users
		 * forget the list element.
		 *
		 * Firstly, defer until "struct tomoyo_io_buffer"->r.{domain,
		 * group,acl} and "struct tomoyo_io_buffer"->w.domain forget
		 * the list element.
		 */
		if (tomoyo_struct_used_by_io_buffer(element))
			continue;
		/*
		 * Secondly, defer until all other elements in the
		 * tomoyo_gc_list list forget the list element.
		 */
		if (tomoyo_element_linked_by_gc((const u8 *) element, p->size))
			continue;
		switch (p->type) {
		case TOMOYO_ID_TRANSITION_CONTROL:
			tomoyo_del_transition_control(element);
			break;
		case TOMOYO_ID_AGGREGATOR:
			tomoyo_del_aggregator(element);
			break;
		case TOMOYO_ID_MANAGER:
			tomoyo_del_manager(element);
			break;
		case TOMOYO_ID_CONDITION:
			tomoyo_del_condition(element);
			break;
		case TOMOYO_ID_NAME:
			/*
			 * Thirdly, defer until all "struct tomoyo_io_buffer"
			 * ->r.w[] forget the list element.
			 */
			if (tomoyo_name_used_by_io_buffer(
			    container_of(element, typeof(struct tomoyo_name),
					 head.list)->entry.name, p->size))
				continue;
			tomoyo_del_name(element);
			break;
		case TOMOYO_ID_ACL:
			tomoyo_del_acl(element);
			break;
		case TOMOYO_ID_DOMAIN:
			if (!tomoyo_del_domain(element))
				continue;
			break;
		case TOMOYO_ID_PATH_GROUP:
			tomoyo_del_path_group(element);
			break;
		case TOMOYO_ID_GROUP:
			tomoyo_del_group(element);
			break;
		case TOMOYO_ID_NUMBER_GROUP:
			tomoyo_del_number_group(element);
			break;
		case TOMOYO_MAX_POLICY:
			break;
		}
		tomoyo_memory_free(element);
		list_del(&p->list);
		kfree(p);
		tomoyo_gc_list_len--;
		result = true;
	}
	return result;
}

/**
 * tomoyo_gc_thread - Garbage collector thread function.
 *
 * @unused: Unused.
 *
 * In case OOM-killer choose this thread for termination, we create this thread
 * as a short live thread whenever /sys/kernel/security/tomoyo/ interface was
 * close()d.
 *
 * Returns 0.
 */
static int tomoyo_gc_thread(void *unused)
{
	/* Garbage collector thread is exclusive. */
	static DEFINE_MUTEX(tomoyo_gc_mutex);
	if (!mutex_trylock(&tomoyo_gc_mutex))
		goto out;
	daemonize("GC for TOMOYO");
	do {
		tomoyo_collect_entry();
		if (list_empty(&tomoyo_gc_list))
			break;
		synchronize_srcu(&tomoyo_ss);
	} while (tomoyo_kfree_entry());
	{
		struct tomoyo_io_buffer *head;
		struct tomoyo_io_buffer *tmp;

		spin_lock(&tomoyo_io_buffer_list_lock);
		list_for_each_entry_safe(head, tmp, &tomoyo_io_buffer_list,
					 list) {
			if (head->users)
				continue;
			list_del(&head->list);
			kfree(head->read_buf);
			kfree(head->write_buf);
			kfree(head);
		}
		spin_unlock(&tomoyo_io_buffer_list_lock);
	}
	mutex_unlock(&tomoyo_gc_mutex);
out:
	/* This acts as do_exit(0). */
	return 0;
}

/**
 * tomoyo_notify_gc - Register/unregister /sys/kernel/security/tomoyo/ users.
 *
 * @head:        Pointer to "struct tomoyo_io_buffer".
 * @is_register: True if register, false if unregister.
 *
 * Returns nothing.
 */
void tomoyo_notify_gc(struct tomoyo_io_buffer *head, const bool is_register)
{
	bool is_write = false;

	spin_lock(&tomoyo_io_buffer_list_lock);
	if (is_register) {
		head->users = 1;
		list_add(&head->list, &tomoyo_io_buffer_list);
	} else {
		is_write = head->write_buf != NULL;
		if (!--head->users) {
			list_del(&head->list);
			kfree(head->read_buf);
			kfree(head->write_buf);
			kfree(head);
		}
	}
	spin_unlock(&tomoyo_io_buffer_list_lock);
	if (is_write) {
		struct task_struct *task = kthread_create(tomoyo_gc_thread,
							  NULL,
							  "GC for TOMOYO");
		if (!IS_ERR(task))
			wake_up_process(task);
	}
}