Kubernetes Security - ClusterRoles and ClusterRoleBindings

Hello in this post we will explore about ClusterRoles and ClusterRoleBindings on Kubernetes Cluster. The ClusterRoleBindings are mapping a subjects with ClusterRole. Here Subjects are nothing but rules that can be applicable with an action on the Cluster resources. It deals with Users, Groups and service accounts. In this post we will try to focus with 'User' specific rules.

Kubernetes User Access Control with ClusterRoleBindings to ClusterRole

 

Prerequisite: 

1. Kubernetes Cluster up and running 

2. Basic understand on RBAC

These system related resources such as pods, nodes, storage etcs will be administrated using ClusterRole and ClusterRoleBindings by assigning to a user.

 
To list the ClusterRoles in the Kubernetes cluster

kubectl get clusterrole
# Get the Count 
kubectl get clusterrole --no-headers |wc -l

To know about the api-resources that have clusterrole and clusterrolebindings.

k api-resources |grep cluster 

To veiew the clusterrolebindings available in this Kubernetes Cluster

kubectl get clusterrolebindings 
# Get the Count 
kubectl get clusterrolebindings --no-headers |wc -l

Imperative way

You can have single verb to used to create clusterrole. Here is an example, Create a role which should have access to list the deamonsets.

# Initial check 
kubectl get ds --as krish 

kubectl create clusterrole list-ds-role --resource=daemonsets --verb=list
kubectl describe clusterrole list-ds-role

Create clusterrolebinding list-ds-rb for user 'krish' to map that clusterrole list-ds which created above.

kubectl create clusterrolebinding list-ds-rb --clusterrole=list-ds-role --user=krish 

After ClusterRoleBinding assigned to krish

kubectl get ds --as krish 

Create ClusterRole, ClusterRoleBinding imperative way

Cleanup for ClusterRoles

Cleanup activity can be in the reverse order. First delete the ClusterRoleBinding then clusterrole

kubectl delete clusterrolebinding list-ds-rb 

kubectl delete clusterrole list-ds 

Cleanup ClusterRole and ClusterRoleBindings

 
ClusterRole are Kubernetes Cluster wide and they are not part of any namespace. To know about user or groups are associated with cluster-admin role, use ClusterRoleBindings and describe it. Where we can see in the subject section that will reveals you about user/groups.

kubectl describe clusterrolebinding cluster-admin

To inspect the clusterrole 'cluster-admin' privileges describe will show the PolicyRules where what resources can be used? and what you can do? The '*' astriek is to indicate that 'all'. If you want to get all resources access then '*.*' should be given. And same way to indicate all actions such as create, delete, list, watch, get use '*'. A new user mahi joined the Kubernetes Administrtors team. She will be focusing on the nodes in the cluster. Let's create a ClusterRole and ClusterRoleBindings so that she gets access to the nodes .

 
Initially we will check that she is able to access the nodes or not.

kubectl create clusterrole node-admin 
 --verb=get,list,watch --resource=nodes --dry-run=client -o yaml > node-admin.yaml
kubectl apply -f node-admin.yaml
kubectl describe clusterrole node-admin

Let's bind the node-admin clusterrole to mahi user using clusterrolebinding.

kubectl create clusterrolebinding mahi-rb --clusterrole=node-admin --user=mahi --dry-run=client -o yaml > mahi-rb.yaml

kubectl create -f node-admin-rb.yaml 
kubectl describe clusterrolebindings node-admin-rb

# Check michelle have the access to nodes 
kubectl --as mahi get nodes

If a user responsibilities are growing as they are into the organization for atime being. Here Maheshwari(mahi) user got more responsibilities for maintaining storge that used for Kubernetes cluster. Create the required ClusterRole and ClusterRoleBindings to allow her access Storage. Requirements:

ClusterRole: storage-admin
Resource: persistentvolumes
Resource: storageclasses
ClusterRoleBinding: mahi-storage-admin
ClusterRoleBinding Subject: mahi
ClusterRoleBinding Role: storage-admin

Now you know all the steps how to proceed on the clusterrole, clusterrolebindings

 kubectl create clusterrole storage-admin \
  --verb=* --resource=persistentvolumes --resource=storageclasses \
  --dry-run=client -o yaml > storage-admin.yaml
  
kubectl apply -f storage-admin.yaml
kubectl describe clusterrole storage-admin

kubectl create clusterrolebinding mahi-storage-admin-rb \
 --clusterrole=storage-admin --user=mahi --dry-run=client -o yaml > mahi-storage-admin-rb.yaml  
 
 kubectl create -f mahi-storage-admin-rb.yaml
 kubectl describe clusterrolebinding mahi-storage-admin-rb
 
# Validate that authentication given for mahi user to access storage
kubectl get pv --as mahi
kubectl get sc --as mahi

Here the last execution of fetching the storageclasses using 'mahi' is successful.

Reference:

1. Official Documentation