Apply custom Pod-level security policies using Gatekeeper

Autopilot Standard

This page shows you how to use the Gatekeeper admission controller to apply Pod-level security controls to your Google Kubernetes Engine (GKE) clusters.

Overview

Gatekeeper is an admission controller that validates requests to create and update Pods on Kubernetes clusters, using the Open Policy Agent (OPA).

Using Gatekeeper allows administrators to define policies with a constraint, which is a set of conditions that permit or deny deployment behaviors in Kubernetes. You can then enforce these policies on a cluster using a ConstraintTemplate. This document provides examples for restricting the security capabilities of workloads to ensure enforce, test, and audit security policies using Gatekeeper.

In addition to enforcing the same capabilities as Kubernetes PodSecurityPolicies, Gatekeeper can also:

Roll out policies: Enforce policy in a gradual, scoped manner to limit the risk of disrupting workloads.
Dry-run policy changes: Provide mechanisms for testing policy impact and range prior to enforcement.
Audit existing policies: Ensure the application of security controls to new and existing workloads (audit controls).

Concepts

Gatekeeper introduces two concepts in order to provide administrators with a powerful and flexible means of controlling their cluster: constraints, and constraint templates, both of which are concepts inherited from the Open Policy Agent Constraint Framework.

Constraints are the representation of your security policy—they define the requirements and range of enforcement. Constraint templates are reusable statements (written in Rego) that apply logic to evaluate specific fields in Kubernetes objects, based on requirements defined in constraints.

For example, you might have a constraint that declares allowable seccomp profiles that can be applied to Pods in a specific namespace, and a comparable constraint template that provides the logic for extracting these values and handling enforcement.

The following constraint template, from the Gatekeeper repository, checks for the existence of securityContext.privileged in a Pod specification:

apiVersion: templates.gatekeeper.sh/v1beta1
kind: ConstraintTemplate
metadata:
  name: k8spspprivilegedcontainer
spec:
  crd:
    spec:
      names:
        kind: K8sPSPPrivilegedContainer
  targets:
    - target: admission.k8s.gatekeeper.sh
      rego: |
        package k8spspprivileged

        violation[{"msg": msg, "details": {}}] {
            c := input_containers[_]
            c.securityContext.privileged
            msg := sprintf("Privileged container is not allowed: %v, securityContext: %v", [c.name, c.securityContext])
        }
        input_containers[c] {
            c := input.review.object.spec.containers[_]
        }
        input_containers[c] {
            c := input.review.object.spec.initContainers[_]
        }

To extend the above constraint template, the following constraint defines the scope (kinds) for the specific enforcement of this constraint template in a dryrun mode:

apiVersion: constraints.gatekeeper.sh/v1beta1
kind: K8sPSPPrivilegedContainer
metadata:
  name: psp-privileged-container
spec:
  enforcementAction: dryrun
  match:
    kinds:
      - apiGroups: [""]
        kinds: ["Pod"]

With Gatekeeper, you can create your own constraints and constraint templates to meet your specific needs. You can also use a standard set of constraints and constraint templates in the Gatekeeper repository that have been defined to enable quick adoption and security enforcement. Each constraint is also accompanied with example Pod configurations.

Before you begin

Before you start, make sure you have performed the following tasks:

Enable the Google Kubernetes Engine API.

Enable Google Kubernetes Engine API

If you want to use the Google Cloud CLI for this task, install and then initialize the gcloud CLI.
Note: For existing gcloud CLI installations, make sure to set the compute/region and compute/zone properties. By setting default locations, you can avoid errors in gcloud CLI like the following: One of [--zone, --region] must be supplied: Please specify location.

Enable Gatekeeper on a cluster with Anthos Config Management

Anthos Config Management offers Policy Controller, which is a policy engine built on the Gatekeeper open source project. Google recommends the use of Anthos Config Management because it solves common problems associated with enforcing policy at scale, including policy-as-code, multi-cluster support, integration with Cloud Logging, and ability to synchronize configuration.

To enable Policy Controller on a cluster, follow the Anthos Config Management installation guide.

Enable constraints and constraint templates

Unlike PodSecurityPolicy, Gatekeeper and its constraint templates can be installed and enabled without adversely impacting existing or new workloads. For this reason, it's recommended that all applicable Pod security constraint templates be applied to the cluster.

Additionally, Gatekeeper constraints can be implemented to enforce controls for specific objects, such as namespaces and Pods.

Observe the example below that limits the scope to Pods located in the production namespace by defining them in the constraint match statement:

...
spec:
  match:
    kinds:
      - apiGroups: [""]
        kinds: ["Pod"]
    namespaces:
      - "production"

For more information about the available options for Constraint and ConstraintTemplate objects, see How to use Gatekeeper.

Testing policies

Introducing new policies to existing clusters can have adverse behavior, for example by restricting existing workloads. One of the benefits of using Gatekeeper for Pod security is the ability to test the effectiveness and impact a policy will have without making actual changes, using a dry-run mode. This allows for policy configuration to be tested against running clusters without enforcement. Policy violations are logged and identified without interference.

The following steps demonstrate how a developer, operator, or administrator can apply constraint templates and constraints to determine their effectiveness or potential impact:

Apply the Gatekeeper config for replicating data for audit and dry-run functionality:

kubectl create -f- <<EOF
apiVersion: config.gatekeeper.sh/v1alpha1
kind: Config
metadata:
  name: config
  namespace: "gatekeeper-system"
spec:
  sync:
    syncOnly:
      - group: ""
        version: "v1"
        kind: "Namespace"
      - group: ""
        version: "v1"
        kind: "Pod"
EOF

With no constraints applied, let's run a workload with elevated privileges:

kubectl create -f- <<EOF
apiVersion: v1
kind: Pod
metadata:
  name: nginx
  labels:
    app: nginx
spec:
  containers:
  - name: nginx
    image: nginx
    securityContext:
      privileged: true
EOF

Load the k8spspprivilegedcontainer constraint template mentioned above:

kubectl create -f- <<EOF
apiVersion: templates.gatekeeper.sh/v1beta1
kind: ConstraintTemplate
metadata:
  name: k8spspprivilegedcontainer
spec:
  crd:
    spec:
      names:
        kind: K8sPSPPrivilegedContainer
  targets:
    - target: admission.k8s.gatekeeper.sh
      rego: |
        package k8spspprivileged

        violation[{"msg": msg, "details": {}}] {
            c := input_containers[_]
            c.securityContext.privileged
            msg := sprintf("Privileged container is not allowed: %v, securityContext: %v", [c.name, c.securityContext])
        }
        input_containers[c] {
            c := input.review.object.spec.containers[_]
        }
        input_containers[c] {
            c := input.review.object.spec.initContainers[_]
        }
EOF

Now let's create a new constraint to extend this constraint template. This time, we will set the enforcementAction to dryrun:

kubectl create -f- <<EOF
apiVersion: constraints.gatekeeper.sh/v1beta1
kind: K8sPSPPrivilegedContainer
metadata:
  name: psp-privileged-container
spec:
  enforcementAction: dryrun
  match:
    kinds:
      - apiGroups: [""]
        kinds: ["Pod"]
EOF

With Gatekeeper synchronizing running object data, and passively checking for violations, we can confirm if any violations were found by checking the status of the constraint:

kubectl get k8spspprivilegedcontainer.constraints.gatekeeper.sh/psp-privileged-container -o yaml

apiVersion: constraints.gatekeeper.sh/v1beta1
kind: K8sPSPPrivilegedContainer
metadata:
...
 name: psp-privileged-container
...
spec:
 enforcementAction: dryrun
 match:
   kinds:
   - apiGroups:
     - ""
     kinds:
     - Pod
status:
 auditTimestamp: "2019-12-15T22:19:54Z"
 byPod:
 - enforced: true
   id: gatekeeper-controller-manager-0
 violations:
 - enforcementAction: dryrun
   kind: Pod
   message: 'Privileged container is not allowed: nginx, securityContext: {"privileged":
     true}'
   name: nginx
   namespace: default

Let's run another privileged Pod, to confirm that the policy does not interfere with deployments:

kubectl create -f- <<EOF
apiVersion: v1
kind: Pod
metadata:
  name: privpod
  labels:
    app: privpod
spec:
  containers:
  - name: nginx
    image: nginx
    securityContext:
      privileged: true
EOF

This new Pod will be successfully deployed.

To clean up the resources created in this section, run the following commands:

kubectl delete k8spspprivilegedcontainer.constraints.gatekeeper.sh/psp-privileged-container
kubectl delete constrainttemplate k8spspprivilegedcontainer
kubectl delete pod/nginx
kubectl delete pod/privpod

Enforcing policies

Now that we can confirm the validity and impact of a policy without impacting existing or new workloads, let's implement a policy with full enforcement.

Building on the examples used to validate the policy above, the following steps demonstrate how a developer, operator, or administrator can apply constraint templates and constraints to enforce a policy:

Load the k8spspprivilegedcontainer constraint template mentioned earlier:

kubectl create -f- <<EOF
apiVersion: templates.gatekeeper.sh/v1beta1
kind: ConstraintTemplate
metadata:
  name: k8spspprivilegedcontainer
spec:
  crd:
    spec:
      names:
        kind: K8sPSPPrivilegedContainer
  targets:
    - target: admission.k8s.gatekeeper.sh
      rego: |
        package k8spspprivileged

        violation[{"msg": msg, "details": {}}] {
            c := input_containers[_]
            c.securityContext.privileged
            msg := sprintf("Privileged container is not allowed: %v, securityContext: %v", [c.name, c.securityContext])
        }
        input_containers[c] {
            c := input.review.object.spec.containers[_]
        }
        input_containers[c] {
            c := input.review.object.spec.initContainers[_]
        }
EOF

Now let's create a new constraint to extend this constraint template. This time, we won't set the enforcementAction key. By default, the enforcementAction key is set to deny:

kubectl create -f- <<EOF
apiVersion: constraints.gatekeeper.sh/v1beta1
kind: K8sPSPPrivilegedContainer
metadata:
  name: psp-privileged-container
spec:
  match:
    kinds:
      - apiGroups: [""]
        kinds: ["Pod"]
EOF

Attempt to deploy a container that declares privileged permissions:

kubectl create -f- <<EOF
apiVersion: v1
kind: Pod
metadata:
  name: nginx
  labels:
    app: nginx
spec:
  containers:
  - name: nginx
    image: nginx
    securityContext:
      privileged: true
EOF

The following error message should be received:

Error from server ([denied by psp-privileged-container] Privileged container is not allowed:
nginx, securityContext: {"privileged": true}): error when creating "STDIN": admission webhook "validation.gatekeeper.sh" denied the request: [denied by psp-privileged-container]
Privileged container is not allowed: nginx, securityContext: {"privileged": true}

To clean up, run the following commands:

kubectl delete k8spspprivilegedcontainer.constraints.gatekeeper.sh/psp-privileged-container
kubectl delete constrainttemplate k8spspprivilegedcontainer

Alternatives to Gatekeeper

Gatekeeper lets you declare and apply custom Pod-level security policies. You can also use Kubernetes' built-in PodSecurity admission controller to apply predefined Pod-level security policies. These predefined policies are aligned with the levels defined by the Pod Security Standards.

What's next

Gatekeeper provides an incredibly powerful means to enforce and validate security on GKE clusters using declarative policies. Gatekeeper's use extends beyond security however, and can be used in other aspects of administration and operations.