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Commit 7761daa6 authored by Elena Reshetova's avatar Elena Reshetova Committed by Jan Kara
Browse files

fsnotify: convert fsnotify_group.refcnt from atomic_t to refcount_t 



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: default avatarKees Cook <keescook@chromium.org>
Reviewed-by: default avatarDavid Windsor <dwindsor@gmail.com>
Reviewed-by: default avatarHans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: default avatarElena Reshetova <elena.reshetova@intel.com>
Signed-off-by: default avatarJan Kara <jack@suse.cz>
parent 9cf90cef

fs/notify/group.c

+3 −3
Original line number Diff line number Diff line
@@ -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);

include/linux/fsnotify_backend.h

+2 −1
Original line number Diff line number Diff line
@@ -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 */