fsnotify: convert fsnotify_mark.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_mark.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>

1 files changed, 1 insertions, 1 deletions

diff --git a/kernel/audit_tree.c b/kernel/audit_tree.c
index 011d46e5f73f..45ec960ad536 100644
--- a/kernel/audit_tree.c
+++ b/kernel/audit_tree.c
@@ -1007,7 +1007,7 @@ static void audit_tree_freeing_mark(struct fsnotify_mark *entry, struct fsnotify
          * We are guaranteed to have at least one reference to the mark from
          * either the inode or the caller of fsnotify_destroy_mark().
          */
-        BUG_ON(atomic_read(&entry->refcnt) < 1);
+        BUG_ON(refcount_read(&entry->refcnt) < 1);
 }
 
 static const struct fsnotify_ops audit_tree_ops = {