fuse: convert fuse_conn.count from atomic_t to refcount_t

refcount_t type and corresponding API should be
used instead of atomic_t when the variable is used as
a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free
situations.
Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David Windsor <dwindsor@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/fuse_i.h

@@ -449,7 +449,7 @@ struct fuse_conn {
 	spinlock_t lock;
 
 	/** Refcount */
-	atomic_t count;
+	refcount_t count;
 
 	/** Number of fuse_dev's */
 	atomic_t dev_count;

fs/fuse/inode.c

@@ -608,7 +608,7 @@ void fuse_conn_init(struct fuse_conn *fc)
 	memset(fc, 0, sizeof(*fc));
 	spin_lock_init(&fc->lock);
 	init_rwsem(&fc->killsb);
-	atomic_set(&fc->count, 1);
+	refcount_set(&fc->count, 1);
 	atomic_set(&fc->dev_count, 1);
 	init_waitqueue_head(&fc->blocked_waitq);
 	init_waitqueue_head(&fc->reserved_req_waitq);
@@ -631,7 +631,7 @@ EXPORT_SYMBOL_GPL(fuse_conn_init);
 
 void fuse_conn_put(struct fuse_conn *fc)
 {
-	if (atomic_dec_and_test(&fc->count)) {
+	if (refcount_dec_and_test(&fc->count)) {
 		if (fc->destroy_req)
 			fuse_request_free(fc->destroy_req);
 		fc->release(fc);
@@ -641,7 +641,7 @@ EXPORT_SYMBOL_GPL(fuse_conn_put);
 
 struct fuse_conn *fuse_conn_get(struct fuse_conn *fc)
 {
-	atomic_inc(&fc->count);
+	refcount_inc(&fc->count);
 	return fc;
 }
 EXPORT_SYMBOL_GPL(fuse_conn_get);