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

[Elena Reshetova <elena.reshetova@intel.com>]

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>
---
 fs/notify/inotify/inotify_user.c |  4 ++--
 fs/notify/mark.c                 | 14 +++++++-------
 include/linux/fsnotify_backend.h |  2 +-
 kernel/audit_tree.c              |  2 +-
 4 files changed, 11 insertions(+), 11 deletions(-)

diff --git a/fs/notify/inotify/inotify_user.c b/fs/notify/inotify/inotify_user.c
index 7cc7d3f..d3c20e0 100644
--- a/fs/notify/inotify/inotify_user.c
+++ b/fs/notify/inotify/inotify_user.c
@@ -376,7 +376,7 @@ static struct inotify_inode_mark *inotify_idr_find_locked(struct fsnotify_group
 
 		fsnotify_get_mark(fsn_mark);
 		/* One ref for being in the idr, one ref we just took */
-		BUG_ON(atomic_read(&fsn_mark->refcnt) < 2);
+		BUG_ON(refcount_read(&fsn_mark->refcnt) < 2);
 	}
 
 	return i_mark;
@@ -446,7 +446,7 @@ static void inotify_remove_from_idr(struct fsnotify_group *group,
 	 * One ref for being in the idr
 	 * one ref grabbed by inotify_idr_find
 	 */
-	if (unlikely(atomic_read(&i_mark->fsn_mark.refcnt) < 2)) {
+	if (unlikely(refcount_read(&i_mark->fsn_mark.refcnt) < 2)) {
 		printk(KERN_ERR "%s: i_mark=%p i_mark->wd=%d i_mark->group=%p\n",
 			 __func__, i_mark, i_mark->wd, i_mark->fsn_mark.group);
 		/* we can't really recover with bad ref cnting.. */
diff --git a/fs/notify/mark.c b/fs/notify/mark.c
index 9991f88..45f1141 100644
--- a/fs/notify/mark.c
+++ b/fs/notify/mark.c
@@ -105,8 +105,8 @@ static DECLARE_WORK(connector_reaper_work, fsnotify_connector_destroy_workfn);
 
 void fsnotify_get_mark(struct fsnotify_mark *mark)
 {
-	WARN_ON_ONCE(!atomic_read(&mark->refcnt));
-	atomic_inc(&mark->refcnt);
+	WARN_ON_ONCE(!refcount_read(&mark->refcnt));
+	refcount_inc(&mark->refcnt);
 }
 
 /*
@@ -116,7 +116,7 @@ void fsnotify_get_mark(struct fsnotify_mark *mark)
  */
 static bool fsnotify_get_mark_safe(struct fsnotify_mark *mark)
 {
-	return atomic_inc_not_zero(&mark->refcnt);
+	return refcount_inc_not_zero(&mark->refcnt);
 }
 
 static void __fsnotify_recalc_mask(struct fsnotify_mark_connector *conn)
@@ -211,7 +211,7 @@ void fsnotify_put_mark(struct fsnotify_mark *mark)
 
 	/* Catch marks that were actually never attached to object */
 	if (!mark->connector) {
-		if (atomic_dec_and_test(&mark->refcnt))
+		if (refcount_dec_and_test(&mark->refcnt))
 			fsnotify_final_mark_destroy(mark);
 		return;
 	}
@@ -220,7 +220,7 @@ void fsnotify_put_mark(struct fsnotify_mark *mark)
 	 * We have to be careful so that traversals of obj_list under lock can
 	 * safely grab mark reference.
 	 */
-	if (!atomic_dec_and_lock(&mark->refcnt, &mark->connector->lock))
+	if (!refcount_dec_and_lock(&mark->refcnt, &mark->connector->lock))
 		return;
 
 	conn = mark->connector;
@@ -338,7 +338,7 @@ void fsnotify_detach_mark(struct fsnotify_mark *mark)
 
 	WARN_ON_ONCE(!mutex_is_locked(&group->mark_mutex));
 	WARN_ON_ONCE(!srcu_read_lock_held(&fsnotify_mark_srcu) &&
-		     atomic_read(&mark->refcnt) < 1 +
+		     refcount_read(&mark->refcnt) < 1 +
 			!!(mark->flags & FSNOTIFY_MARK_FLAG_ATTACHED));
 
 	spin_lock(&mark->lock);
@@ -738,7 +738,7 @@ void fsnotify_init_mark(struct fsnotify_mark *mark,
 {
 	memset(mark, 0, sizeof(*mark));
 	spin_lock_init(&mark->lock);
-	atomic_set(&mark->refcnt, 1);
+	refcount_set(&mark->refcnt, 1);
 	fsnotify_get_group(group);
 	mark->group = group;
 }
diff --git a/include/linux/fsnotify_backend.h b/include/linux/fsnotify_backend.h
index 20a57ba..26485b1 100644
--- a/include/linux/fsnotify_backend.h
+++ b/include/linux/fsnotify_backend.h
@@ -244,7 +244,7 @@ struct fsnotify_mark {
 	__u32 mask;
 	/* We hold one for presence in g_list. Also one ref for each 'thing'
 	 * in kernel that found and may be using this mark. */
-	atomic_t refcnt;
+	refcount_t refcnt;
 	/* Group this mark is for. Set on mark creation, stable until last ref
 	 * is dropped */
 	struct fsnotify_group *group;
diff --git a/kernel/audit_tree.c b/kernel/audit_tree.c
index 011d46e..45ec960 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 = {
-- 
2.7.4