btrfs: convert btrfs_root.refs 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: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.h

@@ -1222,7 +1222,7 @@ struct btrfs_root {
 	dev_t anon_dev;
 
 	spinlock_t root_item_lock;
-	atomic_t refs;
+	refcount_t refs;
 
 	struct mutex delalloc_mutex;
 	spinlock_t delalloc_lock;

fs/btrfs/disk-io.c

@@ -1340,7 +1340,7 @@ static void __setup_root(struct btrfs_root *root, struct btrfs_fs_info *fs_info,
 	atomic_set(&root->log_writers, 0);
 	atomic_set(&root->log_batch, 0);
 	atomic_set(&root->orphan_inodes, 0);
-	atomic_set(&root->refs, 1);
+	refcount_set(&root->refs, 1);
 	atomic_set(&root->will_be_snapshoted, 0);
 	atomic64_set(&root->qgroup_meta_rsv, 0);
 	root->log_transid = 0;

fs/btrfs/disk-io.h

@@ -101,14 +101,14 @@ struct btrfs_root *btrfs_alloc_dummy_root(struct btrfs_fs_info *fs_info);
  */
 static inline struct btrfs_root *btrfs_grab_fs_root(struct btrfs_root *root)
 {
-	if (atomic_inc_not_zero(&root->refs))
+	if (refcount_inc_not_zero(&root->refs))
 		return root;
 	return NULL;
 }
 
 static inline void btrfs_put_fs_root(struct btrfs_root *root)
 {
-	if (atomic_dec_and_test(&root->refs))
+	if (refcount_dec_and_test(&root->refs))
 		kfree(root);
 }