RBAC: Pods, Permissions and Prohibitions

You can also read this over at towardsk8s.com.

If you’ve ever used Linux, you’ve probably used, or at least heard of, RBAC. RBAC stands for Role Based Access Control, and it provides exactly that.

It’s intuitive to think of RBAC as allowing or restricting users to do stuff. But, what if you wanted a Kubernetes pod to be able to do something on the Kubernetes cluster. What if you wanted it to be able to not do some things? Kubernetes RBAC also provides features for it.

First, you’d need to create a role that defines what the pod should be able to do. Whatever you don’t explicitly specify will be restricted.

Let’s create a role named scholar-role in the existing namespace library that allows the pods to see information about the available pods. Let’s also give the pod ability to see other pod’s logs in the same namespace.

# scholar-role.yaml
apiVersion: rbac.authorization.k8s.io/v1
kind: Role
metadata:
  name: scholar-role
  namespace: library
rules:
- apiGroups:
  - ""
  resources:
  - pods
  - pods/log
  verbs:
  - list
  - get

Let’s apply the configuration to create the role:

kubectl apply -f ./scholar-role.yaml

Now, for that role to be used, you have to create a rolebinding that binds the role to the relevant account. But wait, we’re doing this for a pod. Unlike users, pods don’t have their own accounts that they use to login to the system. Now that’s where ServiceAccounts come in.

Let’s create a ServiceAccount named scholar-account in the same namespace library

# scholar-account.yaml
apiVersion: v1
kind: ServiceAccount
metadata:
  name: scholar-account
  namespace: library

Let’s apply the configuration to create the ServiceAccount:

kubectl apply -f ./scholar-account.yaml

Now, we can assign this ServiceAccount to any pod we want. Let’s create a couple of pods — scholar-pod and dumb-pod in the same namespace library . We will assign scholar-account ServiceAccount to the scholar-pod pod, and no ServiceAccount to the dumb-pod. They’ll both use the nginx image, but that’s irrelevant here, the image used doesn’t make any difference.

# pods.yaml
apiVersion: v1
kind: Pod 
metadata:
  name: scholar-pod
  namespace: library
spec:
  serviceAccountName: scholar-account
  containers:
  - image: nginx
    name: scholar-pod

---
apiVersion: v1
kind: Pod 
metadata:
  name: dumb-pod
  namespace: library
spec:
  containers:
  - image: nginx
    name: dumb-pod

Let’s apply the configuration to create the pods:

kubectl apply -f ./pods.yaml

At this point, we are ready to bind our ServiceAccount to the role we created. Let’s create a RoleBinding that binds the role scholar-role to ServiceAccount scholar-account:

# scholar-binding.yaml
apiVersion: rbac.authorization.k8s.io/v1
kind: RoleBinding
metadata:
  name: scholar-rolebinding
  namespace: library
roleRef:
  apiGroup: rbac.authorization.k8s.io
  kind: Role
  name: scholar-role
subjects:
- kind: ServiceAccount
  name: scholar-account
  namespace: library

Let’s apply the configuration to create the RoleBinding:

kubectl apply -f ./scholar-binding.yaml

We are done! But how do we make sure things work as they should. Precisely, how do we make sure that the scholar-pod can read the pods and pods’ logs, and that dumb-pod cannot do that.

One way we could verify the permissions is by calling the Kubernetes API to perform the read operations from each of the pods. But that’s too much work just to verify that our ServiceAccounts work as expected. Instead, we can use the can-i subcommand provided by kubectl auth to test the permissions.

Let’s see if our scholar-account ServiceAccount can read the pod logs in the library namespsce.

kubectl auth can-i get pods --subresource=log --as=system:serviceaccount:library:scholar-account -n library
yes

Well, yes, as expected.

While we’re at it, let’s also see if the ServiceAccount can list the pods themselves:

kubectl auth can-i list pods --as=system:serviceaccount:library:scholar-account -n library
yes

As expected. But, can the ServiceAccount do what it’s not supposed to, say deleting pods. Let’s check that:

kubectl auth can-i delete pods --as=system:serviceaccount:library:scholar-account -n library
no

no is the exact result we were looking for. Perfect!

🏡

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RBAC: Pods, Permissions and Prohibitions