Enforce policies using Gatekeeper

Gatekeeper is the policy controller for Kubernetes, allowing organizations to enforce configurable policies using the Open Policy Agent. Open Policy Agend is a policy engine for Cloud Native environments hosted by CNCF as an incubation-level project.

This topic describes how to use Gatekeeper to enforce policies by rejecting non-compliant resources. Specifically, this topic describes two constraints as a way to show how to use Gatekeeper as an alternative to Pod Security Policies:

• Prevent running privileged pods
• Prevent mounting host path volumes

Deploy Gatekeeper

See the Gatekeeper installation instructions for details.

Check RBAC Permission

Ensure you have cluster admin permissions:

kubectl create clusterrolebinding cluster-admin-binding \
    --clusterrole cluster-admin \
    --user <YOUR USER NAME>

Deploy a Release Using a Prebuilt Image

To deploy a released version of Gatekeeper in your cluster with a prebuilt image, run the following command:

kubectl apply -f https://raw.githubusercontent.com/open-policy-agent/gatekeeper/master/deploy/gatekeeper.yaml

Use Gatekeeper

Gatekeeper uses the OPA Constraint Framework to describe and enforce a policy. Before you can define a constraint, you must first define a ConstraintTemplate. A ConstraintTemplate describes both the Rego (a powerful query language) that enforces the constraint and the schema of the constraint. The constraint schema allows an admin to fine-tune the behavior of a constraint, much like arguments to a function.

The Gatekeeper repository includes a library of policies to replace Pod Security Policies which we will use in the following steps.

Prevent Privileged Pods

Define the ConstraintTemplate

Create the privileged pod policy constraint template k8spspprivilegedcontainer by running the following command:

kubectl apply -f https://raw.githubusercontent.com/open-policy-agent/gatekeeper/master/library/pod-security-policy/privileged-containers/template.yaml

Define the Constraint

Constraints are then used to inform Gatekeeper that the admin wants a ConstraintTemplate to be enforced, and how.

Create the privileged pod policy constraint psp-privileged-container by running the following command:

kubectl apply -f https://raw.githubusercontent.com/open-policy-agent/gatekeeper/master/library/pod-security-policy/privileged-containers/constraint.yaml

Test That the Constraint is Enforced

Create a privileged pod by running the following command:

kubectl apply -f https://raw.githubusercontent.com/open-policy-agent/gatekeeper/master/library/pod-security-policy/privileged-containers/example.yaml

You should see the following output:

Error from server ([denied by psp-privileged-container] Privileged container is not allowed: nginx, securityContext: {"privileged": true}): error when creating "https://raw.githubusercontent.com/open-policy-agent/gatekeeper/master/library/pod-security-policy/privileged-containers/example.yaml": admission webhook "validation.gatekeeper.sh" denied the request: [denied by psp-privileged-container] Privileged container is not allowed: nginx, securityContext: {"privileged": true}

Prevent Host Path Volumes

Define the ConstraintTemplate

Create the host path volume policy constraint template k8spsphostfilesystem by running the following command:

kubectl apply -f https://raw.githubusercontent.com/open-policy-agent/gatekeeper/master/library/pod-security-policy/host-filesystem/template.yaml

Define the Constraint

Constraints are then used to inform Gatekeeper that the admin wants a ConstraintTemplate to be enforced, and how.

Create the host path volume policy constraint psp-host-filesystem by running the following command to only allow /foo to be mounted as a host path volume:

cat <<EOF | kubectl apply -f -
apiVersion: constraints.gatekeeper.sh/v1beta1
kind: K8sPSPHostFilesystem
metadata:
  name: psp-host-filesystem
spec:
  match:
    kinds:
      - apiGroups: [""]
        kinds: ["Pod"]
  parameters:
    allowedHostPaths:
    - readOnly: true
      pathPrefix: "/foo"
EOF

Test That the Constraint is Enforced

Create a pod that mounts a disallowed host path by running the following command:

cat <<EOF | kubectl apply -f -
apiVersion: v1
kind: Pod
metadata:
  name: nginx-host-filesystem
  labels:
    app: nginx-host-filesystem
spec:
  containers:
  - name: nginx
    image: nginx
    volumeMounts:
    - mountPath: /cache
      name: cache-volume
      readOnly: true
  volumes:
  - name: cache-volume
    hostPath:
      path: /tmp # directory location on host
EOF

You should see the following output:

Error from server ([denied by psp-host-filesystem] HostPath volume {"hostPath": {"path": "/tmp", "type": ""}, "name": "cache-volume"} is not allowed, pod: nginx-host-filesystem. Allowed path: [{"readOnly": true, "pathPrefix": "/foo"}]): error when creating "STDIN": admission webhook "validation.gatekeeper.sh" denied the request: [denied by psp-host-filesystem] HostPath volume {"hostPath": {"path": "/tmp", "type": ""}, "name": "cache-volume"} is not allowed, pod: nginx-host-filesystem. Allowed path: [{"readOnly": true, "pathPrefix": "/foo"}]

Test That the Constraint Allows ‘Allowed Host Paths’

Create a pod that mounts an allowed host path by running the following command:

cat <<EOF | kubectl apply -f -
apiVersion: v1
kind: Pod
metadata:
  name: nginx-host-filesystem
  labels:
    app: nginx-host-filesystem
spec:
  containers:
  - name: nginx
    image: nginx
    volumeMounts:
    - mountPath: /cache
      name: cache-volume
      readOnly: true
  volumes:
  - name: cache-volume
    hostPath:
      path: /foo # directory location on host
EOF

You should see the following output:

pod/nginx-host-filesystem created