Gatekeeper

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How is Gatekeeper different from OPA?

Compared to using OPA with its sidecar kube-mgmt (aka Gatekeeper v1.0), Gatekeeper introduces the following functionality:

An extensible, parameterized policy library
Native Kubernetes CRDs for instantiating the policy library (aka "constraints")
Native Kubernetes CRDs for extending the policy library (aka "constraint templates")
Audit functionality

Goals

Every organization has policies. Some are essential to meet governance and legal requirements. Others help ensure adherance to best practices and institutional conventions. Attempting to ensure compliance manually would be error-prone and frustrating. Automating policy enforcement ensures consistency, lowers development latency through immediate feedback, and helps with agility by allowing developers to operate independently without sacrificing compliance.

Kubernetes allows decoupling policy decisions from the inner workings of the API Server by means of admission controller webhooks, which are executed whenever a resource is created, updated or deleted. Gatekeeper is a validating (mutating TBA) webhook that enforces CRD-based policies executed by Open Policy Agent, a policy engine for Cloud Native environments hosted by CNCF as an incubation-level project.

In addition to the admission scenario, Gatekeeper's audit functionality allows administrators to see what resources are currently violating any given policy.

Finally, Gatekeeper's engine is designed to be portable, allowing administrators to detect and reject non-compliant commits to an infrastructure-as-code system's source-of-truth, further strengthening compliance efforts and preventing bad state from slowing down the organization.

Installation Instructions

Installation

Prerequisites

Minimum Kubernetes Version

To use Gatekeeper, you should have a minimum Kubernetes version of 1.14, which adds webhook timeouts.

You can install Gatekeeper in earlier versions of Kubernetes either by removing incompatible fields from the manifest or by setting --validate=false when applying the manifest. Be warned that, without timeouts on the webhook, your API Server could timeout when Gatekeeper is down. Kubernetes 1.14 fixes this issue.

RBAC Permissions

For either installation method, make sure you have cluster admin permissions:

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

Deploying a Release using Prebuilt Image

If you want to deploy a released version of Gatekeeper in your cluster with a prebuilt image, then you can run the following command:

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

Deploying HEAD Using make

Currently the most reliable way of installing Gatekeeper is to build and install from HEAD:

Make sure that:
- Kubebuilder and Kustomize are installed.
- Your kubectl context is set to the desired installation cluster.
- You have a container registry you can write to that is readable by the target cluster.

Clone the Gatekeeper repository to your local system:

git clone https://github.com/open-policy-agent/gatekeeper.git

cd to the repository directory.

Define your destination Docker image location:

export DESTINATION_GATEKEEPER_DOCKER_IMAGE=<YOUR DESIRED DESTINATION DOCKER IMAGE>

Build and push your Docker image:

make docker-build REPOSITORY="$DESTINATION_GATEKEEPER_DOCKER_IMAGE"
make docker-push-release REPOSITORY="$DESTINATION_GATEKEEPER_DOCKER_IMAGE"

Finally, deploy:

make deploy REPOSITORY="$DESTINATION_GATEKEEPER_DOCKER_IMAGE"

Deploying via Helm

A basic Helm v2 template exists in chart/gatekeeper-operator. If you have Helm installed and Tiller initialized on your cluster you can deploy via

helm install chart/gatekeeper-operator/

You can alter the variables in chart/gatekeeper-operator/values.yaml to customize your deployment. To regenerate the base template, run make manifests.

Uninstallation

Before uninstalling Gatekeeper, be sure to clean up old Constraints, ConstraintTemplates, and the Config resource in the gatekeeper-system namespace. This will make sure all finalizers are removed by Gatekeeper. Otherwise the finalizers will need to be removed manually.

Before Uninstall, Clean Up Old Constraints

Currently the uninstall mechanism only removes the Gatekeeper system, it does not remove any ConstraintTemplate, Constraint, and Config resources that have been created by the user, nor does it remove their accompanying CRDs.

When Gatekeeper is running it is possible to remove unwanted constraints by:

Deleting all instances of the constraint resource
Deleting the ConstraintTemplate resource, which should automatically clean up the CRD
Deleting the Config resource removes finalizers on synced resources

Uninstall Gatekeeper

Using Prebuilt Image

If you used a prebuilt image to deploy Gatekeeper, then you can delete all the Gatekeeper components with the following command:

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

Using make

If you used make to deploy, then run the following to uninstall Gatekeeper:

cd to the repository directory
run make uninstall

Using Helm

If you used helm to deploy, then run the following to uninstall Gatekeeper:

helm delete <release name> --purge

Manually Removing Constraints

If Gatekeeper is no longer running and there are extra constraints in the cluster, then the finalizers, CRDs and other artifacts must be removed manually:

Delete all instances of the constraint resource
Executing kubectl patch crd constrainttemplates.templates.gatekeeper.sh -p '{"metadata":{"finalizers":[]}}' --type=merge. Note that this will remove all finalizers on every CRD. If this is not something you want to do, the finalizers must be removed individually.
Delete the CRD and ConstraintTemplate resources associated with the unwanted constraint.

How to Use Gatekeeper

Gatekeeper uses the OPA Constraint Framework to describe and enforce policy. Look there for more detailed information on their semantics and advanced usage.

Constraint Templates

Before you can define a constraint, you must first define a ConstraintTemplate, which describes both the Rego that enforces the constraint and the schema of the constraint. The schema of the constraint allows an admin to fine-tune the behavior of a constraint, much like arguments to a function.

Here is an example constraint template that requires all labels described by the constraint to be present:

apiVersion: templates.gatekeeper.sh/v1beta1
kind: ConstraintTemplate
metadata:
  name: k8srequiredlabels
spec:
  crd:
    spec:
      names:
        kind: K8sRequiredLabels
        listKind: K8sRequiredLabelsList
        plural: k8srequiredlabels
        singular: k8srequiredlabels
      validation:
        # Schema for the `parameters` field
        openAPIV3Schema:
          properties:
            labels:
              type: array
              items: string
  targets:
    - target: admission.k8s.gatekeeper.sh
      rego: |
        package k8srequiredlabels

        violation[{"msg": msg, "details": {"missing_labels": missing}}] {
          provided := {label | input.review.object.metadata.labels[label]}
          required := {label | label := input.parameters.labels[_]}
          missing := required - provided
          count(missing) > 0
          msg := sprintf("you must provide labels: %v", [missing])
        }

You can install this ConstraintTemplate with the following command:

kubectl apply -f https://raw.githubusercontent.com/open-policy-agent/gatekeeper/master/demo/basic/templates/k8srequiredlabels_template.yaml

Constraints

Constraints are then used to inform Gatekeeper that the admin wants a ConstraintTemplate to be enforced, and how. This constraint uses the K8sRequiredLabels constraint template above to make sure the gatekeeper label is defined on all namespaces:

apiVersion: constraints.gatekeeper.sh/v1beta1
kind: K8sRequiredLabels
metadata:
  name: ns-must-have-gk
spec:
  match:
    kinds:
      - apiGroups: [""]
        kinds: ["Namespace"]
  parameters:
    labels: ["gatekeeper"]

You can install this Constraint with the following command:

kubectl apply -f https://raw.githubusercontent.com/open-policy-agent/gatekeeper/master/demo/basic/constraints/all_ns_must_have_gatekeeper.yaml

Note the match field, which defines the scope of objects to which a given constraint will be applied. It supports the following matchers:

kinds accepts a list of objects with apiGroups and kinds fields that list the groups/kinds of objects to which the constraint will apply. If multiple groups/kinds objects are specified, only one match is needed for the resource to be in scope.
namespaces is a list of namespace names. If defined, a constraint will only apply to resources in a listed namespace.
excludedNamespaces is a list of namespace names. If defined, a constraint will only apply to resources not in a listed namespace.
labelSelector is a standard Kubernetes label selector.
namespaceSelector is a standard Kubernetes namespace selector. If defined, make sure to add Namespaces to your configs.config.gatekeeper.sh object to ensure namespaces are synced into OPA. Refer to the Replicating Data section for more details.

Note that if multiple matchers are specified, a resource must satisfy each top-level matcher (kinds, namespaces, etc.) to be in scope. Each top-level matcher has its own semantics for what qualifies as a match. An empty matcher is deemed to be inclusive (matches everything).

Replicating Data

Some constraints are impossible to write without access to more state than just the object under test. For example, it is impossible to know if an ingress's hostname is unique among all ingresses unless a rule has access to all other ingresses. To make such rules possible, we enable syncing of data into OPA.

The audit feature does not require replication by default. However, when the audit-from-cache flag is set to true, the OPA cache will be used as the source-of-truth for audit queries; thus, an object must first be cached before it can be audited for constraint violations.

Kubernetes data can be replicated into OPA via the sync config resource. Currently resources defined in syncOnly will be synced into OPA. Updating syncOnly should dynamically update what objects are synced. Below is an example:

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"

You can install this config with the following command:

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

Once data is synced into OPA, rules can access the cached data under the data.inventory document.

The data.inventory document has the following format:

For cluster-scoped objects: data.inventory.cluster[<groupVersion>][<kind>][<name>]
- Example referencing the Gatekeeper namespace: data.inventory.cluster["v1"].Namespace["gatekeeper"]
For namespace-scoped objects: data.inventory.namespace[<namespace>][groupVersion][<kind>][<name>]
- Example referencing the Gatekeeper pod: data.inventory.namespace["gatekeeper"]["v1"]["Pod"]["gatekeeper-controller-manager-d4c98b788-j7d92"]

Audit

The audit functionality enables periodic evaluations of replicated resources against the policies enforced in the cluster to detect pre-existing misconfigurations. Audit results are stored as violations listed in the status field of the failed constraint.

apiVersion: constraints.gatekeeper.sh/v1beta1
kind: K8sRequiredLabels
metadata:
  name: ns-must-have-gk
spec:
  match:
    kinds:
      - apiGroups: [""]
        kinds: ["Namespace"]
  parameters:
    labels: ["gatekeeper"]
status:
  auditTimestamp: "2019-05-11T01:46:13Z"
  enforced: true
  violations:
  - enforcementAction: deny
    kind: Namespace
    message: 'you must provide labels: {"gatekeeper"}'
    name: default
  - enforcementAction: deny
    kind: Namespace
    message: 'you must provide labels: {"gatekeeper"}'
    name: gatekeeper-system
  - enforcementAction: deny
    kind: Namespace
    message: 'you must provide labels: {"gatekeeper"}'
    name: kube-public
  - enforcementAction: deny
    kind: Namespace
    message: 'you must provide labels: {"gatekeeper"}'
    name: kube-system

Audit interval: set --audit-interval=123 (defaults to every 60 seconds)
Audit violations per constraint: set --constraint-violations-limit=123 (defaults to 20)
Disable: set --audit-interval=0

By default, the audit will request each resource from the Kubernetes API during each cycle of the audit. To instead rely on the OPA cache, use the flag --audit-from-cache=true. Note that this requires replication of Kubernetes resources into OPA before they can be evaluated against the enforced policies. Refer to the Replicating data section for more information.

Log denies

Set the --log-denies flag to log all denies and dryrun failures. This is useful when trying to see what is being denied/fails dry-run and keeping a log to debug cluster problems without having to enable syncing or looking through the status of all constraints.

Dry Run

When rolling out new constraints to running clusters, the dry run functionality can be helpful as it enables constraints to be deployed in the cluster without making actual changes. This allows constraints to be tested in a running cluster without enforcing them. Cluster resources that are impacted by the dry run constraint are surfaced as violations in the status field of the constraint.

To use the dry run feature, add enforcementAction: dryrun to the constraint spec to ensure no actual changes are made as a result of the constraint. By default, enforcementAction is set to deny as the default behavior is to deny admission requests with any violation.

For example:

apiVersion: constraints.gatekeeper.sh/v1beta1
kind: K8sRequiredLabels
metadata:
  name: ns-must-have-gk
spec:
  enforcementAction: dryrun
  match:
    kinds:
      - apiGroups: [""]
        kinds: ["Namespace"]
  parameters:
    labels: ["gatekeeper"]
status:
  auditTimestamp: "2019-08-15T01:46:13Z"
  enforced: true
  violations:
  - enforcementAction: dryrun
    kind: Namespace
    message: 'you must provide labels: {"gatekeeper"}'
    name: default
  - enforcementAction: dryrun
    kind: Namespace
    message: 'you must provide labels: {"gatekeeper"}'
    name: gatekeeper-system

NOTE: The supported enforcementActions are [deny, dryrun] for constraints. Update the --disable-enforcementaction-validation=true flag if the desire is to disable enforcementAction validation against the list of supported enforcementActions.

Exempting Namespaces from the Gatekeeper Admission Webhook

Note that the following only exempts resources from the admission webhook. They will still be audited. Editing individual constraints is necessary to exclude them from audit.

If it becomes necessary to exempt a namespace from Gatekeeper entirely (e.g. you want kube-system to bypass admission checks), here's how to do it:

Make sure the validating admission webhook configuration for Gatekeeper has the following namespace selector:
```
  namespaceSelector:
    matchExpressions:
    - key: admission.gatekeeper.sh/ignore
      operator: DoesNotExist
```
the default Gatekeeper manifest should already have added this. The default name for the webhook configuration is gatekeeper-validating-webhook-configuration and the default name for the webhook that needs the namespace selector is validation.gatekeeper.sh
Tell Gatekeeper it's okay for the namespace to be ignored by adding a flag to the pod: --exempt-namespace=<NAMESPACE NAME>. This step is necessary because otherwise the permission to modify a namespace would be equivalent to the permission to exempt everything in that namespace from policy checks. This way a user must explicitly have permissions to configure the Gatekeeper pod before they can add exemptions.
Add the admission.gatekeeper.sh/ignore label to the namespace. The value attached to the label is ignored, so it can be used to annotate the reason for the exemption.

Debugging

NOTE: Verbose logging with DEBUG level can be turned on with --log-level=DEBUG. By default, the --log-level flag is set to minimum log level INFO. Acceptable values for minimum log level are [DEBUG, INFO, WARNING, ERROR]. In production, this flag should not be set to DEBUG.

In debugging decisions and constraints, a few pieces of information can be helpful:

Cached data and existing rules at the time of the request
A trace of the evaluation
The input document being evaluated

Writing out this information for every request would be very expensive, and it would be hard to find the relevant logs for a given request. Instead, Gatekeeper allows users to specify resources and requesting users for which information will be logged. They can do so by configuring the Config resource, which lives in the gatekeeper-system namespace.

Below is an example of a config resource:

apiVersion: config.gatekeeper.sh/v1alpha1
kind: Config
metadata:
  name: config
  namespace: "gatekeeper-system"
spec:
  # Data to be replicated into OPA
  sync:
    syncOnly:
      - group: ""
        version: "v1"
        kind: "Namespace"
  validation:
    # Requests for which we want to run traces
    traces:
        # The requesting user for which traces will be run
      - user: "user_to_trace@company.com"
        kind:
          # The group, version, kind for which we want to run a trace
          group: ""
          version: "v1"
          kind: "Namespace"
          # If dump is defined and set to `All`, also dump the state of OPA
          dump: "All"

Traces will be written to the stdout logs of the Gatekeeper controller.

If there is an error in the Rego in the ConstraintTemplate, there are cases where it is still created via kubectl apply -f [CONSTRAINT_TEMPLATE_FILENAME].yaml.

When applying the constraint using kubectl apply -f constraint.yaml with a ConstraintTemplate that contains incorrect Rego, and error will occur: error: unable to recognize "[CONSTRAINT_FILENAME].yaml": no matches for kind "[NAME_OF_CONSTRAINT]" in version "constraints.gatekeeper.sh/v1beta1".

To find the error, run kubectl get -f [CONSTRAINT_FILENAME].yaml -oyaml. Build errors are shown in the status field.

Customizing Admission Behavior

Gatekeeper is a Kubernetes admission webhook whose default configuration can be found in the gatekeeper.yaml manifest file. By default, it is a ValidatingWebhookConfiguration resource named gatekeeper-validating-webhook-configuration.

Currently the configuration specifies two webhooks: one for checking a request against the installed constraints and a second webhook for checking labels on namespace requests that would result in bypassing constraints for the namespace. The namespace-label webhook is necessary to prevent a privilege escalation where the permission to add a label to a namespace is equivalent to the ability to bypass all constraints for that namespace. You can read more about the ability to exempt namespaces by label above.

Because Kubernetes adds features with each version, if you want to know how the webhook can be configured it is best to look at the official documentation linked at the top of this section. However, two particularly important configuration options deserve special mention: timeouts and failure policy.

Timeouts allow you to configure how long the API server will wait for a response from the admission webhook before it considers the request to have failed. Note that setting the timeout longer than the overall request timeout means that the main request will time out before the webhook's failure policy is invoked.

Failure policy controls what happens when a webhook fails for whatever reason. Common failure scenarios include timeouts, a 5xx error from the server or the webhook being unavailable. You have the option to ignore errors, allowing the request through, or failing, rejecting the request. This results in a direct tradeoff between availability and enforcement.

Currently Gatekeeper is defaulting to using Ignore for the constraint requests. This is because the webhook server currently only has one instance, which risks downtime during actions like upgrades. As the theoretical availability improves we will likely change the default to Fail.

The namespace label webhook defaults to Fail, this is to help ensure that policies preventing labels that bypass the webhook from being applied are enforced. Because this webhook only gets called for namespace modification requests, the impact of downtime is mitigated, making the theoretical maximum availability less of an issue.

Because the manifest is available for customization, the webhook configuration can be tuned to meet your specific needs if they differ from the defaults.

Emergency Recovery

If a situation arises where Gatekeeper is preventing the cluster from operating correctly, the webhook can be disabled. This will remove all Gatekeeper admission checks. Assuming the default webhook name has been used this can be achieved by running:

kubectl delete validatingwebhookconfigurations.admissionregistration.k8s.io gatekeeper-validating-webhook-configuration

Redeploying the webhook configuration will re-enable Gatekeeper.

Running on private GKE Cluster nodes

By default, firewall rules restrict the cluster master communication to nodes only on ports 443 (HTTPS) and 10250 (kubelet). Although Gatekeeper exposes its service on port 443, GKE by default enables --enable-aggregator-routing option, which makes the master to bypass the service and communicate straight to the POD on port 8443.

Two ways of working around this:

create a new firewall rule from master to private nodes to open port 8443 (or any other custom port)
- https://cloud.google.com/kubernetes-engine/docs/how-to/private-clusters#add_firewall_rules
make the pod to run on privileged port 443 (need to run pod as root)
- update Gatekeeper deployment manifest spec:
  - remove securityContext settings that force the pods not to run as root
  - update port from 8443 to 443
```
containers:
- args:
  - --port=443
  ports:
  - containerPort: 443
    name: webhook-server
    protocol: TCP
```
- update Gatekeeper service manifest spec:
  - update targetPort from 8443 to 443
```
ports:
- port: 443
  targetPort: 443
```

Kick The Tires

The demo/basic directory contains the above examples of simple constraints, templates and configs to play with. The demo/agilebank directory contains more complex examples based on a slightly more realistic scenario. Both folders have a handy demo script to step you through the demos.

FAQ

Finalizers

How can I remove finalizers? Why are they hanging around?

If Gatekeeper is running, it should automatically clean up the finalizer. If it isn't this is a misbehavior that should be investigated. Please file a bug with as much data as you can gather. Including logs, memory usage and utilization, CPU usage and utilization and any other information that may be helpful.

If Gatekeeper is not running:

If it did not have a clean exit, Gatekeeper's garbage collection routine would have been unable to run. Reasons for an unclean exit are:
- The service account was deleted before the Pod exited, blocking the GC process (this can happen if you delete the gatekeeer-system namespace before deleting the deployment or deleting the manifest all at once).
- The container was sent a hard kill signal
- The container had a panic

Finalizers can be removed manually via kubectl edit or kubectl patch

nessex/gatekeeper