kernfs: kernfs_find_and_get_node_by_ino() should only look up activated nodes

kernfs node can be created in two separate steps - allocation and
activation.  This is used to make kernfs nodes visible only after the
internal states attached to the node are fully initialized.
kernfs_find_and_get_node_by_id() currently allows lookups of nodes
which aren't activated yet and thus can expose nodes are which are
still being prepped by kernfs users.

Fix it by disallowing lookups of nodes which aren't activated yet.

kernfs_find_and_get_node_by_ino()
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Namhyung Kim <namhyung@kernel.org>

fs/kernfs/dir.c

@@ -714,7 +714,13 @@ struct kernfs_node *kernfs_find_and_get_node_by_ino(struct kernfs_root *root,
 	if (!kn)
 		goto err_unlock;
 
-	if (unlikely(!atomic_inc_not_zero(&kn->count)))
+	/*
+	 * ACTIVATED is protected with kernfs_mutex but it was clear when
+	 * @kn was added to idr and we just wanna see it set.  No need to
+	 * grab kernfs_mutex.
+	 */
+	if (unlikely(!(kn->flags & KERNFS_ACTIVATED) ||
+		     !atomic_inc_not_zero(&kn->count)))
 		goto err_unlock;
 
 	spin_unlock(&kernfs_idr_lock);