[PATCH 1/2] fsnotify: convert fsnotify_group.refcnt from atomic_t to refcount_t

From: Elena Reshetova
Date: Fri Oct 20 2017 - 06:14:17 EST


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@xxxxxxxxxxxx>
Reviewed-by: David Windsor <dwindsor@xxxxxxxxx>
Reviewed-by: Hans Liljestrand <ishkamiel@xxxxxxxxx>
Signed-off-by: Elena Reshetova <elena.reshetova@xxxxxxxxx>
---
fs/notify/group.c | 6 +++---
include/linux/fsnotify_backend.h | 3 ++-
2 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/fs/notify/group.c b/fs/notify/group.c
index 3235753..b7a4b6a 100644
--- a/fs/notify/group.c
+++ b/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);

diff --git a/include/linux/fsnotify_backend.h b/include/linux/fsnotify_backend.h
index c6c6931..20a57ba 100644
--- a/include/linux/fsnotify_backend.h
+++ b/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 */

--
2.7.4