Add an working example for rbac.md.

Signed-off-by: Xun Jiang <jxun@vmware.com>

site/content/docs/main/rbac.md

@@ -9,9 +9,9 @@ By default Velero runs with an RBAC policy of ClusterRole `cluster-admin`. This
 
 For more information about RBAC and access control generally in Kubernetes, see the Kubernetes documentation about [access control][1], [managing service accounts][2], and [RBAC authorization][3].
 
-## Set up Roles and RoleBindings
+## Set up with restricted RBAC permissions
 
-Here's a sample Role and RoleBinding pair.
+Here's a sample of restricted permission setting.
 
 ```yaml
 apiVersion: rbac.authorization.k8s.io/v1
@@ -28,23 +28,68 @@ rules:
       - "*"
     resources:
       - "*"
-```
-
-```yaml
+---
 apiVersion: rbac.authorization.k8s.io/v1
 kind: RoleBinding
 metadata:
   name: ROLEBINDING_NAME_HERE
+  namespace: YOUR_NAMESPACE_HERE
 subjects:
   - kind: ServiceAccount
     name: YOUR_SERVICEACCOUNT_HERE
 roleRef:
   kind: Role
   name: ROLE_NAME_HERE
   apiGroup: rbac.authorization.k8s.io
+---
+apiVersion: rbac.authorization.k8s.io/v1
+kind: ClusterRole
+metadata:
+  name: velero-clusterrole
+rules:
+- apiGroups: 
+  - ""
+  resources:
+  - persistentvolumes
+  - namespaces
+  verbs:
+  - '*'
+- apiGroups: 
+  - '*'
+  resources:
+  - '*'
+  verbs:
+  - list
+- apiGroups:
+  - 'apiextensions.k8s.io'
+  resources:
+  - 'customresourcedefinitions'
+  verbs:
+  - get
+---
+apiVersion: rbac.authorization.k8s.io/v1
+kind: ClusterRoleBinding
+metadata:
+  name: velero-clusterrolebinding
+roleRef:
+  apiGroup: rbac.authorization.k8s.io
+  kind: ClusterRole
+  name: velero-clusterrole
+subjects:
+  - kind: ServiceAccount
+    name: YOUR_SERVICEACCOUNT_HERE
+    namespace: YOUR_NAMESPACE_HERE
 ```
 
+You can add more permissions into the `Role` setting according to the need.
+`velero-clusterrole` ClusterRole is verified to work in most cases.
+`Namespaces` resource permission is needed to create namespace during restore. If you don't need that, the `create` permission can be removed, but `list` and `get` permissions of `Namespaces` resource is still needed, because Velero needs to know whether the namespace it's assigned exists in the cluster.
+`PersistentVolumes` resource permission is needed for back up and restore volumes. If that is not needed, it can be removed too.
+`CustomResourceDefinitions` resource permission is needed to backup CR instances' CRD. It's better to keep them.
+It's better to have the `list` permission for all resources, because Velero needs to read some resources during backup, for example, `ClusterRoles` is listed for backing `ServiceAccount` up, and `VolumeSnapshotContent` for CSI `PersistentVolumeClaim`. If you just enable `list` permissions for the resources you want to back up and restore, it's possible that backup or restore end with failure.
+
 [1]: https://kubernetes.io/docs/reference/access-authn-authz/controlling-access/
 [2]: https://kubernetes.io/docs/reference/access-authn-authz/service-accounts-admin/
 [3]: https://kubernetes.io/docs/reference/access-authn-authz/rbac/
 [4]: namespace.md
+