在Kubernetes中，授权(authorization)是在认证(authentication)之后的一个步骤。授权就是决定一个用户(普通用户或ServiceAccount)是否有权请求Kubernetes API做某些事情。

之前，Kubernetes中的授权策略主要是ABAC(Attribute-Based Access Control)。对于ABAC，Kubernetes在实现上是比较难用的，而且需要Master Node的SSH和根文件系统访问权限，授权策略发生变化后还需要重启API Server。

Kubernetes 1.6中，RBAC(Role-Based Access Control)基于角色的访问控制进入Beta阶段。RBAC访问控制策略可以使用kubectl或Kubernetes API进行配置。使用RBAC可以直接授权给用户，让用户拥有授权管理的权限，这样就不再需要直接触碰Master Node。在Kubernetes中RBAC被映射成API资源和操作。

RBAC API的资源对象

在Kubernetes 1.6中通过启动参数--authorization-mode=RBAC.API Overview为API Server启用RBAC。

使用kubeadm初始化的1.6版本的Kubernetes集群，已经默认为API Server开启了RBAC，可以查看Master Node上API Server的静态Pod定义文件：

cat /etc/kubernetes/manifests/kube-apiserver.yaml | grep RBAC
    - --authorization-mode=RBAC

RBAC API定义了四个资源对象用于描述RBAC中用户和资源之间的连接权限：

Role
ClusterRole
RoleBinding
ClusterRoleBinding

Role和ClusterRole

Role是一系列权限的集合。Role是定义在某个Namespace下的资源，在这个具体的Namespace下使用。ClusterRole与Role相似，只是ClusterRole是整个集群范围内使用的。

下面我们使用kubectl打印一下Kubernetes集群中的Role和ClusterRole：

kubectl get roles --all-namespaces
NAMESPACE     NAME                                        AGE
kube-public   system:bootstrap-signer-clusterinfo         6d
kube-public   system:controller:bootstrap-signer          6d
kube-system   extension-apiserver-authentication-reader   6d
kube-system   system:controller:bootstrap-signer          6d
kube-system   system:controller:token-cleaner             6d

kubectl get ClusterRoles
NAME                                           AGE
admin                                          6d
cluster-admin                                  6d
edit                                           6d
flannel                                        5d
system:auth-delegator                          6d
system:basic-user                              6d
system:controller:attachdetach-controller      6d
......
system:kube-aggregator                         6d
system:kube-controller-manager                 6d
system:kube-dns                                6d
system:kube-scheduler                          6d
system:node                                    6d
system:node-bootstrapper                       6d
system:node-problem-detector                   6d
system:node-proxier                            6d
system:persistent-volume-provisioner           6d
view                                           6d

可以看到之前创建的这个Kubernetes集群中已经内置或创建很多的Role和ClusterRole。

下面在default命名空间内创建一个名称为pod-reader的Role，role-pord-reader.yaml文件如下：

kind: Role
apiVersion: rbac.authorization.k8s.io/v1beta1
metadata:
  namespace: default
  name: pod-reader
rules:
- apiGroups: [""] # "" indicates the core API group
  resources: ["pods"]
  verbs: ["get", "watch", "list"]

kubectl create -f role-pord-reader.yaml
role "pod-reader" created
kubectl get roles
NAME         AGE
pod-reader   1m

注意RBAC在Kubernetes 1.6还处于Beta阶段，所以API归属在rbac.authorization.k8s.io，上面的apiVersion为rbac.authorization.k8s.io/v1beta1。

下面再给一个ClusterRole的定义文件：

kind: ClusterRole
apiVersion: rbac.authorization.k8s.io/v1beta1
metadata:
  # "namespace" omitted since ClusterRoles are not namespaced
  name: secret-reader
rules:
- apiGroups: [""]
  resources: ["secrets"]
  verbs: ["get", "watch", "list"]

RoleBinding和ClusterRoleBinding

RoleBinding把Role绑定到账户主体Subject，让Subject继承Role所在namespace下的权限。ClusterRoleBinding把ClusterRole绑定到Subject，让Subject集成ClusterRole在整个集群中的权限。

账户主体Subject在这里还是叫“用户”吧，包含组group，用户user和ServiceAccount。

kubectl get rolebinding --all-namespaces
NAMESPACE     NAME                                   AGE
kube-public   kubeadm:bootstrap-signer-clusterinfo   6d
kube-public   system:controller:bootstrap-signer     6d
kube-system   system:controller:bootstrap-signer     6d
kube-system   system:controller:token-cleaner        6d

kubectl get clusterrolebinding
NAME                                           AGE
cluster-admin                                  6d
flannel                                        6d
kubeadm:kubelet-bootstrap                      6d
kubeadm:node-proxier                           6d
system:basic-user                              6d
system:controller:attachdetach-controller      6d
system:controller:certificate-controller       6d
......
system:controller:ttl-controller               6d
system:discovery                               6d
system:kube-controller-manager                 6d
system:kube-dns                                6d
system:kube-scheduler                          6d
system:node                                    6d
system:node-proxier                            6d

实际上一个RoleBinding既可以引用相同namespace下的Role；又可以引用一个ClusterRole，RoleBinding引用ClusterRole时用户继承的权限会被限制在RoleBinding所在的namespace下。

kind: RoleBinding
apiVersion: rbac.authorization.k8s.io/v1beta1
metadata:
  name: read-pods
  namespace: default
subjects:
- kind: User
  name: jane
  apiGroup: rbac.authorization.k8s.io
roleRef:
  kind: Role
  name: pod-reader
  apiGroup: rbac.authorization.k8s.io

kind: RoleBinding
apiVersion: rbac.authorization.k8s.io/v1beta1
metadata:
  name: read-secrets
  namespace: development # This only grants permissions within the "development" namespace.
subjects:
- kind: User
  name: dave
  apiGroup: rbac.authorization.k8s.io
roleRef:
  kind: ClusterRole
  name: secret-reader
  apiGroup: rbac.authorization.k8s.io

Kubernetes中默认的Role和RoleBinding

API Server已经创建一系列ClusterRole和ClusterRoleBinding。这些资源对象中名称以system:开头的，表示这个资源对象属于Kubernetes系统基础设施。也就说RBAC默认的集群角色已经完成足够的覆盖，让集群可以完全在 RBAC的管理下运行。修改这些资源对象可能会引起未知的后果，例如对于system:node这个ClusterRole定义了kubelet进程的权限，如果这个角色被修改，可能导致kubelet无法工作。

可以使用kubernetes.io/bootstrapping=rbac-defaults这个label查看默认的ClusterRole和ClusterRoleBinding：

kubectl get clusterrole -l kubernetes.io/bootstrapping=rbac-defaults
NAME                                           AGE
admin                                          6d
cluster-admin                                  6d
edit                                           6d
system:auth-delegator                          6d
system:basic-user                              6d
system:controller:attachdetach-controller      6d
system:controller:certificate-controller       6d
......
system:node-problem-detector                   6d
system:node-proxier                            6d
system:persistent-volume-provisioner           6d
view                                           6d

kubectl get clusterrolebinding -l kubernetes.io/bootstrapping=rbac-defaults
NAME                                           AGE
cluster-admin                                  6d
system:basic-user                              6d
system:controller:attachdetach-controller      6d
system:controller:certificate-controller       6d
system:controller:cronjob-controller           6d
system:controller:daemon-set-controller        6d
system:controller:deployment-controller        6d
......
system:discovery                               6d
system:kube-controller-manager                 6d
system:kube-dns                                6d
system:kube-scheduler                          6d
system:node                                    6d
system:node-proxier                            6d

关于这些角色详细的权限信息可以查看Default Roles and Role Bindings

Kubernetes RBAC

RBAC API的资源对象

Role和ClusterRole

RoleBinding和ClusterRoleBinding

Kubernetes中默认的Role和RoleBinding

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