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fsnotify: convert fsnotify_group.refcnt from atomic_t to refcount_t

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atomic_t variables are currently used to implement reference
counters with the following properties:
 - counter is initialized to 1 using atomic_set()
 - a resource is freed upon counter reaching zero
 - once counter reaches zero, its further
   increments aren't allowed
 - counter schema uses basic atomic operations
   (set, inc, inc_not_zero, dec_and_test, etc.)

Such atomic variables should be converted to a newly provided
refcount_t type and API that prevents accidental counter overflows
and underflows. This is important since overflows and underflows
can lead to use-after-free situation and be exploitable.

The variable fsnotify_group.refcnt is used as pure reference counter.
Convert it to refcount_t and fix up the operations.

Suggested-by: Kees Cook <keescook@chromium.org>
Reviewed-by: David Windsor <dwindsor@gmail.com>
Reviewed-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Jan Kara <jack@suse.cz>
This commit is contained in:
Elena Reshetova 2017-10-20 13:26:01 +03:00 committed by Jan Kara
parent 9cf90cef36
commit 7761daa6a1
2 changed files with 5 additions and 4 deletions
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6
fs/notify/group.c
View File

@ -107,7 +107,7 @@ void fsnotify_destroy_group(struct fsnotify_group *group)
*/
void fsnotify_get_group(struct fsnotify_group *group)
{
atomic_inc(&group->refcnt);
refcount_inc(&group->refcnt);
}
/*
@ -115,7 +115,7 @@ void fsnotify_get_group(struct fsnotify_group *group)
*/
void fsnotify_put_group(struct fsnotify_group *group)
{
if (atomic_dec_and_test(&group->refcnt))
if (refcount_dec_and_test(&group->refcnt))
fsnotify_final_destroy_group(group);
}
@ -131,7 +131,7 @@ struct fsnotify_group *fsnotify_alloc_group(const struct fsnotify_ops *ops)
return ERR_PTR(-ENOMEM);
/* set to 0 when there a no external references to this group */
atomic_set(&group->refcnt, 1);
refcount_set(&group->refcnt, 1);
atomic_set(&group->num_marks, 0);
atomic_set(&group->user_waits, 0);

3
include/linux/fsnotify_backend.h
View File

@ -17,6 +17,7 @@
#include <linux/types.h>
#include <linux/atomic.h>
#include <linux/user_namespace.h>
#include <linux/refcount.h>
/*
* IN_* from inotfy.h lines up EXACTLY with FS_*, this is so we can easily
@ -135,7 +136,7 @@ struct fsnotify_group {
* inotify_init() and the refcnt will hit 0 only when that fd has been
* closed.
*/
atomic_t refcnt; /* things with interest in this group */
refcount_t refcnt; /* things with interest in this group */
const struct fsnotify_ops *ops; /* how this group handles things */