TOMOYO: Allow using executable's realpath and symlink's target as conditions.
[linux-2.6.git] / security / tomoyo / gc.c
1 /*
2  * security/tomoyo/gc.c
3  *
4  * Implementation of the Domain-Based Mandatory Access Control.
5  *
6  * Copyright (C) 2005-2010  NTT DATA CORPORATION
7  *
8  */
9
10 #include "common.h"
11 #include <linux/kthread.h>
12 #include <linux/slab.h>
13
14 /* The list for "struct tomoyo_io_buffer". */
15 static LIST_HEAD(tomoyo_io_buffer_list);
16 /* Lock for protecting tomoyo_io_buffer_list. */
17 static DEFINE_SPINLOCK(tomoyo_io_buffer_list_lock);
18
19 /* Size of an element. */
20 static const u8 tomoyo_element_size[TOMOYO_MAX_POLICY] = {
21         [TOMOYO_ID_GROUP] = sizeof(struct tomoyo_group),
22         [TOMOYO_ID_PATH_GROUP] = sizeof(struct tomoyo_path_group),
23         [TOMOYO_ID_NUMBER_GROUP] = sizeof(struct tomoyo_number_group),
24         [TOMOYO_ID_AGGREGATOR] = sizeof(struct tomoyo_aggregator),
25         [TOMOYO_ID_TRANSITION_CONTROL] =
26         sizeof(struct tomoyo_transition_control),
27         [TOMOYO_ID_MANAGER] = sizeof(struct tomoyo_manager),
28         /* [TOMOYO_ID_CONDITION] = "struct tomoyo_condition"->size, */
29         /* [TOMOYO_ID_NAME] = "struct tomoyo_name"->size, */
30         /* [TOMOYO_ID_ACL] =
31            tomoyo_acl_size["struct tomoyo_acl_info"->type], */
32         [TOMOYO_ID_DOMAIN] = sizeof(struct tomoyo_domain_info),
33 };
34
35 /* Size of a domain ACL element. */
36 static const u8 tomoyo_acl_size[] = {
37         [TOMOYO_TYPE_PATH_ACL] = sizeof(struct tomoyo_path_acl),
38         [TOMOYO_TYPE_PATH2_ACL] = sizeof(struct tomoyo_path2_acl),
39         [TOMOYO_TYPE_PATH_NUMBER_ACL] = sizeof(struct tomoyo_path_number_acl),
40         [TOMOYO_TYPE_MKDEV_ACL] = sizeof(struct tomoyo_mkdev_acl),
41         [TOMOYO_TYPE_MOUNT_ACL] = sizeof(struct tomoyo_mount_acl),
42 };
43
44 /**
45  * tomoyo_struct_used_by_io_buffer - Check whether the list element is used by /sys/kernel/security/tomoyo/ users or not.
46  *
47  * @element: Pointer to "struct list_head".
48  *
49  * Returns true if @element is used by /sys/kernel/security/tomoyo/ users,
50  * false otherwise.
51  */
52 static bool tomoyo_struct_used_by_io_buffer(const struct list_head *element)
53 {
54         struct tomoyo_io_buffer *head;
55         bool in_use = false;
56
57         spin_lock(&tomoyo_io_buffer_list_lock);
58         list_for_each_entry(head, &tomoyo_io_buffer_list, list) {
59                 head->users++;
60                 spin_unlock(&tomoyo_io_buffer_list_lock);
61                 if (mutex_lock_interruptible(&head->io_sem)) {
62                         in_use = true;
63                         goto out;
64                 }
65                 if (head->r.domain == element || head->r.group == element ||
66                     head->r.acl == element || &head->w.domain->list == element)
67                         in_use = true;
68                 mutex_unlock(&head->io_sem);
69 out:
70                 spin_lock(&tomoyo_io_buffer_list_lock);
71                 head->users--;
72                 if (in_use)
73                         break;
74         }
75         spin_unlock(&tomoyo_io_buffer_list_lock);
76         return in_use;
77 }
78
79 /**
80  * tomoyo_name_used_by_io_buffer - Check whether the string is used by /sys/kernel/security/tomoyo/ users or not.
81  *
82  * @string: String to check.
83  * @size:   Memory allocated for @string .
84  *
85  * Returns true if @string is used by /sys/kernel/security/tomoyo/ users,
86  * false otherwise.
87  */
88 static bool tomoyo_name_used_by_io_buffer(const char *string,
89                                           const size_t size)
90 {
91         struct tomoyo_io_buffer *head;
92         bool in_use = false;
93
94         spin_lock(&tomoyo_io_buffer_list_lock);
95         list_for_each_entry(head, &tomoyo_io_buffer_list, list) {
96                 int i;
97                 head->users++;
98                 spin_unlock(&tomoyo_io_buffer_list_lock);
99                 if (mutex_lock_interruptible(&head->io_sem)) {
100                         in_use = true;
101                         goto out;
102                 }
103                 for (i = 0; i < TOMOYO_MAX_IO_READ_QUEUE; i++) {
104                         const char *w = head->r.w[i];
105                         if (w < string || w > string + size)
106                                 continue;
107                         in_use = true;
108                         break;
109                 }
110                 mutex_unlock(&head->io_sem);
111 out:
112                 spin_lock(&tomoyo_io_buffer_list_lock);
113                 head->users--;
114                 if (in_use)
115                         break;
116         }
117         spin_unlock(&tomoyo_io_buffer_list_lock);
118         return in_use;
119 }
120
121 /* Structure for garbage collection. */
122 struct tomoyo_gc {
123         struct list_head list;
124         enum tomoyo_policy_id type;
125         size_t size;
126         struct list_head *element;
127 };
128 /* List of entries to be deleted. */
129 static LIST_HEAD(tomoyo_gc_list);
130 /* Length of tomoyo_gc_list. */
131 static int tomoyo_gc_list_len;
132
133 /**
134  * tomoyo_add_to_gc - Add an entry to to be deleted list.
135  *
136  * @type:    One of values in "enum tomoyo_policy_id".
137  * @element: Pointer to "struct list_head".
138  *
139  * Returns true on success, false otherwise.
140  *
141  * Caller holds tomoyo_policy_lock mutex.
142  *
143  * Adding an entry needs kmalloc(). Thus, if we try to add thousands of
144  * entries at once, it will take too long time. Thus, do not add more than 128
145  * entries per a scan. But to be able to handle worst case where all entries
146  * are in-use, we accept one more entry per a scan.
147  *
148  * If we use singly linked list using "struct list_head"->prev (which is
149  * LIST_POISON2), we can avoid kmalloc().
150  */
151 static bool tomoyo_add_to_gc(const int type, struct list_head *element)
152 {
153         struct tomoyo_gc *entry = kzalloc(sizeof(*entry), GFP_ATOMIC);
154         if (!entry)
155                 return false;
156         entry->type = type;
157         if (type == TOMOYO_ID_ACL)
158                 entry->size = tomoyo_acl_size[
159                               container_of(element,
160                                            typeof(struct tomoyo_acl_info),
161                                            list)->type];
162         else if (type == TOMOYO_ID_NAME)
163                 entry->size = strlen(container_of(element,
164                                                   typeof(struct tomoyo_name),
165                                                   head.list)->entry.name) + 1;
166         else if (type == TOMOYO_ID_CONDITION)
167                 entry->size =
168                         container_of(element, typeof(struct tomoyo_condition),
169                                      head.list)->size;
170         else
171                 entry->size = tomoyo_element_size[type];
172         entry->element = element;
173         list_add(&entry->list, &tomoyo_gc_list);
174         list_del_rcu(element);
175         return tomoyo_gc_list_len++ < 128;
176 }
177
178 /**
179  * tomoyo_element_linked_by_gc - Validate next element of an entry.
180  *
181  * @element: Pointer to an element.
182  * @size:    Size of @element in byte.
183  *
184  * Returns true if @element is linked by other elements in the garbage
185  * collector's queue, false otherwise.
186  */
187 static bool tomoyo_element_linked_by_gc(const u8 *element, const size_t size)
188 {
189         struct tomoyo_gc *p;
190         list_for_each_entry(p, &tomoyo_gc_list, list) {
191                 const u8 *ptr = (const u8 *) p->element->next;
192                 if (ptr < element || element + size < ptr)
193                         continue;
194                 return true;
195         }
196         return false;
197 }
198
199 /**
200  * tomoyo_del_transition_control - Delete members in "struct tomoyo_transition_control".
201  *
202  * @element: Pointer to "struct list_head".
203  *
204  * Returns nothing.
205  */
206 static void tomoyo_del_transition_control(struct list_head *element)
207 {
208         struct tomoyo_transition_control *ptr =
209                 container_of(element, typeof(*ptr), head.list);
210         tomoyo_put_name(ptr->domainname);
211         tomoyo_put_name(ptr->program);
212 }
213
214 /**
215  * tomoyo_del_aggregator - Delete members in "struct tomoyo_aggregator".
216  *
217  * @element: Pointer to "struct list_head".
218  *
219  * Returns nothing.
220  */
221 static void tomoyo_del_aggregator(struct list_head *element)
222 {
223         struct tomoyo_aggregator *ptr =
224                 container_of(element, typeof(*ptr), head.list);
225         tomoyo_put_name(ptr->original_name);
226         tomoyo_put_name(ptr->aggregated_name);
227 }
228
229 /**
230  * tomoyo_del_manager - Delete members in "struct tomoyo_manager".
231  *
232  * @element: Pointer to "struct list_head".
233  *
234  * Returns nothing.
235  */
236 static void tomoyo_del_manager(struct list_head *element)
237 {
238         struct tomoyo_manager *ptr =
239                 container_of(element, typeof(*ptr), head.list);
240         tomoyo_put_name(ptr->manager);
241 }
242
243 /**
244  * tomoyo_del_acl - Delete members in "struct tomoyo_acl_info".
245  *
246  * @element: Pointer to "struct list_head".
247  *
248  * Returns nothing.
249  */
250 static void tomoyo_del_acl(struct list_head *element)
251 {
252         struct tomoyo_acl_info *acl =
253                 container_of(element, typeof(*acl), list);
254         tomoyo_put_condition(acl->cond);
255         switch (acl->type) {
256         case TOMOYO_TYPE_PATH_ACL:
257                 {
258                         struct tomoyo_path_acl *entry
259                                 = container_of(acl, typeof(*entry), head);
260                         tomoyo_put_name_union(&entry->name);
261                 }
262                 break;
263         case TOMOYO_TYPE_PATH2_ACL:
264                 {
265                         struct tomoyo_path2_acl *entry
266                                 = container_of(acl, typeof(*entry), head);
267                         tomoyo_put_name_union(&entry->name1);
268                         tomoyo_put_name_union(&entry->name2);
269                 }
270                 break;
271         case TOMOYO_TYPE_PATH_NUMBER_ACL:
272                 {
273                         struct tomoyo_path_number_acl *entry
274                                 = container_of(acl, typeof(*entry), head);
275                         tomoyo_put_name_union(&entry->name);
276                         tomoyo_put_number_union(&entry->number);
277                 }
278                 break;
279         case TOMOYO_TYPE_MKDEV_ACL:
280                 {
281                         struct tomoyo_mkdev_acl *entry
282                                 = container_of(acl, typeof(*entry), head);
283                         tomoyo_put_name_union(&entry->name);
284                         tomoyo_put_number_union(&entry->mode);
285                         tomoyo_put_number_union(&entry->major);
286                         tomoyo_put_number_union(&entry->minor);
287                 }
288                 break;
289         case TOMOYO_TYPE_MOUNT_ACL:
290                 {
291                         struct tomoyo_mount_acl *entry
292                                 = container_of(acl, typeof(*entry), head);
293                         tomoyo_put_name_union(&entry->dev_name);
294                         tomoyo_put_name_union(&entry->dir_name);
295                         tomoyo_put_name_union(&entry->fs_type);
296                         tomoyo_put_number_union(&entry->flags);
297                 }
298                 break;
299         }
300 }
301
302 /**
303  * tomoyo_del_domain - Delete members in "struct tomoyo_domain_info".
304  *
305  * @element: Pointer to "struct list_head".
306  *
307  * Returns true if deleted, false otherwise.
308  */
309 static bool tomoyo_del_domain(struct list_head *element)
310 {
311         struct tomoyo_domain_info *domain =
312                 container_of(element, typeof(*domain), list);
313         struct tomoyo_acl_info *acl;
314         struct tomoyo_acl_info *tmp;
315         /*
316          * Since we don't protect whole execve() operation using SRCU,
317          * we need to recheck domain->users at this point.
318          *
319          * (1) Reader starts SRCU section upon execve().
320          * (2) Reader traverses tomoyo_domain_list and finds this domain.
321          * (3) Writer marks this domain as deleted.
322          * (4) Garbage collector removes this domain from tomoyo_domain_list
323          *     because this domain is marked as deleted and used by nobody.
324          * (5) Reader saves reference to this domain into
325          *     "struct linux_binprm"->cred->security .
326          * (6) Reader finishes SRCU section, although execve() operation has
327          *     not finished yet.
328          * (7) Garbage collector waits for SRCU synchronization.
329          * (8) Garbage collector kfree() this domain because this domain is
330          *     used by nobody.
331          * (9) Reader finishes execve() operation and restores this domain from
332          *     "struct linux_binprm"->cred->security.
333          *
334          * By updating domain->users at (5), we can solve this race problem
335          * by rechecking domain->users at (8).
336          */
337         if (atomic_read(&domain->users))
338                 return false;
339         list_for_each_entry_safe(acl, tmp, &domain->acl_info_list, list) {
340                 tomoyo_del_acl(&acl->list);
341                 tomoyo_memory_free(acl);
342         }
343         tomoyo_put_name(domain->domainname);
344         return true;
345 }
346
347 /**
348  * tomoyo_del_condition - Delete members in "struct tomoyo_condition".
349  *
350  * @element: Pointer to "struct list_head".
351  *
352  * Returns nothing.
353  */
354 void tomoyo_del_condition(struct list_head *element)
355 {
356         struct tomoyo_condition *cond = container_of(element, typeof(*cond),
357                                                      head.list);
358         const u16 condc = cond->condc;
359         const u16 numbers_count = cond->numbers_count;
360         const u16 names_count = cond->names_count;
361         unsigned int i;
362         const struct tomoyo_condition_element *condp
363                 = (const struct tomoyo_condition_element *) (cond + 1);
364         struct tomoyo_number_union *numbers_p
365                 = (struct tomoyo_number_union *) (condp + condc);
366         struct tomoyo_name_union *names_p
367                 = (struct tomoyo_name_union *) (numbers_p + numbers_count);
368         for (i = 0; i < numbers_count; i++)
369                 tomoyo_put_number_union(numbers_p++);
370         for (i = 0; i < names_count; i++)
371                 tomoyo_put_name_union(names_p++);
372 }
373
374 /**
375  * tomoyo_del_name - Delete members in "struct tomoyo_name".
376  *
377  * @element: Pointer to "struct list_head".
378  *
379  * Returns nothing.
380  */
381 static void tomoyo_del_name(struct list_head *element)
382 {
383         const struct tomoyo_name *ptr =
384                 container_of(element, typeof(*ptr), head.list);
385 }
386
387 /**
388  * tomoyo_del_path_group - Delete members in "struct tomoyo_path_group".
389  *
390  * @element: Pointer to "struct list_head".
391  *
392  * Returns nothing.
393  */
394 static void tomoyo_del_path_group(struct list_head *element)
395 {
396         struct tomoyo_path_group *member =
397                 container_of(element, typeof(*member), head.list);
398         tomoyo_put_name(member->member_name);
399 }
400
401 /**
402  * tomoyo_del_group - Delete "struct tomoyo_group".
403  *
404  * @element: Pointer to "struct list_head".
405  *
406  * Returns nothing.
407  */
408 static void tomoyo_del_group(struct list_head *element)
409 {
410         struct tomoyo_group *group =
411                 container_of(element, typeof(*group), head.list);
412         tomoyo_put_name(group->group_name);
413 }
414
415 /**
416  * tomoyo_del_number_group - Delete members in "struct tomoyo_number_group".
417  *
418  * @element: Pointer to "struct list_head".
419  *
420  * Returns nothing.
421  */
422 static void tomoyo_del_number_group(struct list_head *element)
423 {
424         struct tomoyo_number_group *member =
425                 container_of(element, typeof(*member), head.list);
426 }
427
428 /**
429  * tomoyo_collect_member - Delete elements with "struct tomoyo_acl_head".
430  *
431  * @id:          One of values in "enum tomoyo_policy_id".
432  * @member_list: Pointer to "struct list_head".
433  *
434  * Returns true if some elements are deleted, false otherwise.
435  */
436 static bool tomoyo_collect_member(const enum tomoyo_policy_id id,
437                                   struct list_head *member_list)
438 {
439         struct tomoyo_acl_head *member;
440         list_for_each_entry(member, member_list, list) {
441                 if (!member->is_deleted)
442                         continue;
443                 if (!tomoyo_add_to_gc(id, &member->list))
444                         return false;
445         }
446         return true;
447 }
448
449 /**
450  * tomoyo_collect_acl - Delete elements in "struct tomoyo_domain_info".
451  *
452  * @list: Pointer to "struct list_head".
453  *
454  * Returns true if some elements are deleted, false otherwise.
455  */
456 static bool tomoyo_collect_acl(struct list_head *list)
457 {
458         struct tomoyo_acl_info *acl;
459         list_for_each_entry(acl, list, list) {
460                 if (!acl->is_deleted)
461                         continue;
462                 if (!tomoyo_add_to_gc(TOMOYO_ID_ACL, &acl->list))
463                         return false;
464         }
465         return true;
466 }
467
468 /**
469  * tomoyo_collect_entry - Scan lists for deleted elements.
470  *
471  * Returns nothing.
472  */
473 static void tomoyo_collect_entry(void)
474 {
475         int i;
476         enum tomoyo_policy_id id;
477         struct tomoyo_policy_namespace *ns;
478         int idx;
479         if (mutex_lock_interruptible(&tomoyo_policy_lock))
480                 return;
481         idx = tomoyo_read_lock();
482         {
483                 struct tomoyo_domain_info *domain;
484                 list_for_each_entry_rcu(domain, &tomoyo_domain_list, list) {
485                         if (!tomoyo_collect_acl(&domain->acl_info_list))
486                                 goto unlock;
487                         if (!domain->is_deleted || atomic_read(&domain->users))
488                                 continue;
489                         /*
490                          * Nobody is referring this domain. But somebody may
491                          * refer this domain after successful execve().
492                          * We recheck domain->users after SRCU synchronization.
493                          */
494                         if (!tomoyo_add_to_gc(TOMOYO_ID_DOMAIN, &domain->list))
495                                 goto unlock;
496                 }
497         }
498         list_for_each_entry_rcu(ns, &tomoyo_namespace_list, namespace_list) {
499                 for (id = 0; id < TOMOYO_MAX_POLICY; id++)
500                         if (!tomoyo_collect_member(id, &ns->policy_list[id]))
501                                 goto unlock;
502                 for (i = 0; i < TOMOYO_MAX_ACL_GROUPS; i++)
503                         if (!tomoyo_collect_acl(&ns->acl_group[i]))
504                                 goto unlock;
505                 for (i = 0; i < TOMOYO_MAX_GROUP; i++) {
506                         struct list_head *list = &ns->group_list[i];
507                         struct tomoyo_group *group;
508                         switch (i) {
509                         case 0:
510                                 id = TOMOYO_ID_PATH_GROUP;
511                                 break;
512                         default:
513                                 id = TOMOYO_ID_NUMBER_GROUP;
514                                 break;
515                         }
516                         list_for_each_entry(group, list, head.list) {
517                                 if (!tomoyo_collect_member
518                                     (id, &group->member_list))
519                                         goto unlock;
520                                 if (!list_empty(&group->member_list) ||
521                                     atomic_read(&group->head.users))
522                                         continue;
523                                 if (!tomoyo_add_to_gc(TOMOYO_ID_GROUP,
524                                                       &group->head.list))
525                                         goto unlock;
526                         }
527                 }
528         }
529         id = TOMOYO_ID_CONDITION;
530         for (i = 0; i < TOMOYO_MAX_HASH + 1; i++) {
531                 struct list_head *list = !i ?
532                         &tomoyo_condition_list : &tomoyo_name_list[i - 1];
533                 struct tomoyo_shared_acl_head *ptr;
534                 list_for_each_entry(ptr, list, list) {
535                         if (atomic_read(&ptr->users))
536                                 continue;
537                         if (!tomoyo_add_to_gc(id, &ptr->list))
538                                 goto unlock;
539                 }
540                 id = TOMOYO_ID_NAME;
541         }
542 unlock:
543         tomoyo_read_unlock(idx);
544         mutex_unlock(&tomoyo_policy_lock);
545 }
546
547 /**
548  * tomoyo_kfree_entry - Delete entries in tomoyo_gc_list.
549  *
550  * Returns true if some entries were kfree()d, false otherwise.
551  */
552 static bool tomoyo_kfree_entry(void)
553 {
554         struct tomoyo_gc *p;
555         struct tomoyo_gc *tmp;
556         bool result = false;
557
558         list_for_each_entry_safe(p, tmp, &tomoyo_gc_list, list) {
559                 struct list_head *element = p->element;
560
561                 /*
562                  * list_del_rcu() in tomoyo_add_to_gc() guarantees that the
563                  * list element became no longer reachable from the list which
564                  * the element was originally on (e.g. tomoyo_domain_list).
565                  * Also, synchronize_srcu() in tomoyo_gc_thread() guarantees
566                  * that the list element became no longer referenced by syscall
567                  * users.
568                  *
569                  * However, there are three users which may still be using the
570                  * list element. We need to defer until all of these users
571                  * forget the list element.
572                  *
573                  * Firstly, defer until "struct tomoyo_io_buffer"->r.{domain,
574                  * group,acl} and "struct tomoyo_io_buffer"->w.domain forget
575                  * the list element.
576                  */
577                 if (tomoyo_struct_used_by_io_buffer(element))
578                         continue;
579                 /*
580                  * Secondly, defer until all other elements in the
581                  * tomoyo_gc_list list forget the list element.
582                  */
583                 if (tomoyo_element_linked_by_gc((const u8 *) element, p->size))
584                         continue;
585                 switch (p->type) {
586                 case TOMOYO_ID_TRANSITION_CONTROL:
587                         tomoyo_del_transition_control(element);
588                         break;
589                 case TOMOYO_ID_AGGREGATOR:
590                         tomoyo_del_aggregator(element);
591                         break;
592                 case TOMOYO_ID_MANAGER:
593                         tomoyo_del_manager(element);
594                         break;
595                 case TOMOYO_ID_CONDITION:
596                         tomoyo_del_condition(element);
597                         break;
598                 case TOMOYO_ID_NAME:
599                         /*
600                          * Thirdly, defer until all "struct tomoyo_io_buffer"
601                          * ->r.w[] forget the list element.
602                          */
603                         if (tomoyo_name_used_by_io_buffer(
604                             container_of(element, typeof(struct tomoyo_name),
605                                          head.list)->entry.name, p->size))
606                                 continue;
607                         tomoyo_del_name(element);
608                         break;
609                 case TOMOYO_ID_ACL:
610                         tomoyo_del_acl(element);
611                         break;
612                 case TOMOYO_ID_DOMAIN:
613                         if (!tomoyo_del_domain(element))
614                                 continue;
615                         break;
616                 case TOMOYO_ID_PATH_GROUP:
617                         tomoyo_del_path_group(element);
618                         break;
619                 case TOMOYO_ID_GROUP:
620                         tomoyo_del_group(element);
621                         break;
622                 case TOMOYO_ID_NUMBER_GROUP:
623                         tomoyo_del_number_group(element);
624                         break;
625                 case TOMOYO_MAX_POLICY:
626                         break;
627                 }
628                 tomoyo_memory_free(element);
629                 list_del(&p->list);
630                 kfree(p);
631                 tomoyo_gc_list_len--;
632                 result = true;
633         }
634         return result;
635 }
636
637 /**
638  * tomoyo_gc_thread - Garbage collector thread function.
639  *
640  * @unused: Unused.
641  *
642  * In case OOM-killer choose this thread for termination, we create this thread
643  * as a short live thread whenever /sys/kernel/security/tomoyo/ interface was
644  * close()d.
645  *
646  * Returns 0.
647  */
648 static int tomoyo_gc_thread(void *unused)
649 {
650         /* Garbage collector thread is exclusive. */
651         static DEFINE_MUTEX(tomoyo_gc_mutex);
652         if (!mutex_trylock(&tomoyo_gc_mutex))
653                 goto out;
654         daemonize("GC for TOMOYO");
655         do {
656                 tomoyo_collect_entry();
657                 if (list_empty(&tomoyo_gc_list))
658                         break;
659                 synchronize_srcu(&tomoyo_ss);
660         } while (tomoyo_kfree_entry());
661         {
662                 struct tomoyo_io_buffer *head;
663                 struct tomoyo_io_buffer *tmp;
664
665                 spin_lock(&tomoyo_io_buffer_list_lock);
666                 list_for_each_entry_safe(head, tmp, &tomoyo_io_buffer_list,
667                                          list) {
668                         if (head->users)
669                                 continue;
670                         list_del(&head->list);
671                         kfree(head->read_buf);
672                         kfree(head->write_buf);
673                         kfree(head);
674                 }
675                 spin_unlock(&tomoyo_io_buffer_list_lock);
676         }
677         mutex_unlock(&tomoyo_gc_mutex);
678 out:
679         /* This acts as do_exit(0). */
680         return 0;
681 }
682
683 /**
684  * tomoyo_notify_gc - Register/unregister /sys/kernel/security/tomoyo/ users.
685  *
686  * @head:        Pointer to "struct tomoyo_io_buffer".
687  * @is_register: True if register, false if unregister.
688  *
689  * Returns nothing.
690  */
691 void tomoyo_notify_gc(struct tomoyo_io_buffer *head, const bool is_register)
692 {
693         bool is_write = false;
694
695         spin_lock(&tomoyo_io_buffer_list_lock);
696         if (is_register) {
697                 head->users = 1;
698                 list_add(&head->list, &tomoyo_io_buffer_list);
699         } else {
700                 is_write = head->write_buf != NULL;
701                 if (!--head->users) {
702                         list_del(&head->list);
703                         kfree(head->read_buf);
704                         kfree(head->write_buf);
705                         kfree(head);
706                 }
707         }
708         spin_unlock(&tomoyo_io_buffer_list_lock);
709         if (is_write) {
710                 struct task_struct *task = kthread_create(tomoyo_gc_thread,
711                                                           NULL,
712                                                           "GC for TOMOYO");
713                 if (!IS_ERR(task))
714                         wake_up_process(task);
715         }
716 }