使用UploadFile，出错 [ 2.0 版本 ]

move_uploaded_file(/home/127.0.0.1:8080/yii2/backend/web/uploads/14515443368124.jpg): failed to open stream: Permission denied
这是我的代码，有人知道原因吗

$rootPath = Yii::getAlias('@backend') . '/web/uploads/';
$image = UploadedFile::getInstance($model, 'img');
$ext = $image->getExtension();
$randName = time() . rand(1000, 9999) . "." . $ext;
$image->saveAs($rootPath . $randName);
$params['img'] = '/uploads/' . $randName;