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: 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>

fs/notify/group.c

@@ -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

@@ -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 */