Add Flux Multitenant Policies to docs

Signed-off-by: Stefan Prodan <stefan.prodan@gmail.com>

docs/guides/flux-policies.md

@@ -0,0 +1,251 @@
+# Flux Multitenant Policies
+
+In this guide we will cover how to use the Kubernetes built-in validating admission engine
+to enforce policies on multi-tenant clusters managed by Flux.
+
+## Kubernetes Validating Admission Policy
+
+Starting from Kubernetes 1.30, the Kubernetes API server comes with a built-in validating
+admission controller that allows defining custom policies using the
+[Common Expression Language](https://kubernetes.io/docs/reference/using-api/cel/) (CEL).
+
+In a GitOps setup, the Kubernetes `ValidatingAdmissionPolicy` and `ValidatingAdmissionPolicyBinding`
+resources are defined by cluster admins in the fleet repository. Using Flux `dependsOn` feature,
+the cluster admins can ensure that the policies are applied before any other resources are reconciled,
+thus enforcing the policies on all tenant's namespaces and workloads.
+
+## Restricting Access to Flux Sources
+
+One common policy is to restrict the access to external Flux sources such as Git repositories,
+OCI registries and Helm repositories which are controlled by tenants.
+
+### Define the Allow List
+
+Assuming that cluster admins want to restrict the access to a specific GitHub organization,
+first, they need to define `ConfigMap` in the `flux-system` namespace
+with the list of allowed Flux sources:
+
+```yaml
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: allowlist
+  namespace: flux-system
+  labels:
+    fluxcd.controlplane.io/role: "policy"
+data:
+  sources: >-
+    oci://ghcr.io/controlplaneio-fluxcd/charts/
+    https://github.com/controlplaneio-fluxcd/
+    ssh://git@github.com/controlplaneio-fluxcd/
+```
+
+The `sources` list contains the allowed URL prefixes separated by a new line for
+tenant-owned Flux sources e.g. `GitRepository`, `OCIRepository` and `HelmRepository`. 
+
+### Define the Admission Policy
+
+Next, the cluster admins need to define a `ValidatingAdmissionPolicy` resource that
+verifies the source URL against the allow list:
+
+```yaml
+apiVersion: admissionregistration.k8s.io/v1
+kind: ValidatingAdmissionPolicy
+metadata:
+  name: "source.policy.fluxcd.controlplane.io"
+spec:
+  failurePolicy: Fail
+  matchConstraints:
+    resourceRules:
+      - apiGroups: [ "source.toolkit.fluxcd.io" ]
+        apiVersions: [ "*" ]
+        operations: [ "CREATE", "UPDATE" ]
+        resources: [ "gitrepositories", "ocirepositories", "helmrepositories" ]
+  paramKind:
+    apiVersion: v1
+    kind: ConfigMap
+  variables:
+    - name: url
+      expression: object.spec.url
+    - name: sources
+      expression: params.data.sources.split(' ')
+  validations:
+    - expression: >
+        variables.sources.exists_one(prefix, variables.url.startsWith(prefix))
+      messageExpression: >
+        "Source " + variables.url + " is not allowed, must be one of " + variables.sources.join(", ")
+      reason: Invalid
+```
+
+### Define the Admission Policy Binding
+
+Finally, the cluster admins need to define a `ValidatingAdmissionPolicyBinding` resource
+that binds the policy to all tenant namespaces and references the `ConfigMap` with the allow list:
+
+```yaml
+apiVersion: admissionregistration.k8s.io/v1
+kind: ValidatingAdmissionPolicyBinding
+metadata:
+  name: tenant-sources
+spec:
+  policyName: "source.policy.fluxcd.controlplane.io"
+  validationActions: [ "Deny" ]
+  paramRef:
+    name: allowlist
+    namespace: flux-system
+    parameterNotFoundAction: "Deny"
+  matchResources:
+    namespaceSelector:
+      matchExpressions:
+        - key: kubernetes.io/metadata.name
+          operator: NotIn
+          values:
+            - flux-system
+            - kube-system
+```
+
+With the above policy in place, any tenant trying to create or update a Flux source that is not
+listed in the allow list will receive a validation error and the operation will be denied.
+
+### Testing the Policy
+
+If a tenant adds an `OCIRepository` manifest to their repository that tries to
+pull Helm charts from a registry that is not in the allow list, for example:
+
+```yaml
+apiVersion: source.toolkit.fluxcd.io/v1beta2
+kind: OCIRepository
+metadata:
+  name: podinfo
+  namespace: apps
+spec:
+  interval: 10m
+  url: oci://ghcr.io/stefanprodan/charts/podinfo
+  ref:
+    tag: 6.x
+```
+
+The admission controller will deny the creation of the `OCIRepository` and the tenant will receive
+an alert from Flux about the policy violation:
+
+```
+The ocirepositories "podinfo" is invalid:
+ValidatingAdmissionPolicy 'source.policy.fluxcd.controlplane.io' with binding 'tenant-sources' denied request:
+Source oci://ghcr.io/stefanprodan/charts/podinfo is not allowed, must be one of oci://ghcr.io/controlplaneio-fluxcd/charts/, https://github.com/controlplaneio-fluxcd/, ssh://git@github.com/controlplaneio-fluxcd/
+```
+
+## Restricting Access to Container Registries
+
+Another common policy is to restrict the access to container registries for workloads running in tenant namespaces.
+
+Assuming that cluster admins want to restrict the access to specific container registries,
+they can add the allowed registry prefixes to the `ConfigMap` in the `flux-system` namespace:
+
+```yaml
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: allowlist
+  namespace: flux-system
+  labels:
+    fluxcd.controlplane.io/role: "policy"
+data:
+  registries: >-
+    ghcr.io/controlplaneio-fluxcd/
+    709825985650.dkr.ecr.us-east-1.amazonaws.com/controlplane/
+  sources: "omitted for brevity"
+```
+
+Next, the cluster admins need to define a `ValidatingAdmissionPolicy` resource that
+verifies the containers image URL against the allow list:
+
+```yaml
+apiVersion: admissionregistration.k8s.io/v1
+kind: ValidatingAdmissionPolicy
+metadata:
+  name: "registry.policy.fluxcd.controlplane.io"
+spec:
+  failurePolicy: Fail
+  matchConstraints:
+    resourceRules:
+      - apiGroups: [ "apps" ]
+        apiVersions: [ "v1" ]
+        operations: [ "CREATE", "UPDATE" ]
+        resources: [ "deployments", "statefulsets", "daemonsets" ]
+  paramKind:
+    apiVersion: v1
+    kind: ConfigMap
+  variables:
+    - name: registries
+      expression: params.data.registries.split(' ')
+  validations:
+    - expression: >
+        object.spec.template.spec.containers.all(
+          container,
+          variables.registries.exists(
+              prefix, container.image.startsWith(prefix)
+          )
+        )
+      messageExpression: >
+        "Container image is not allowed, must be one of " + variables.registries.join(", ")
+      reason: Invalid
+    - expression: >
+        !has(object.spec.template.spec.initContainers) ||
+        object.spec.template.spec.initContainers.all(
+          container,
+          variables.registries.exists(
+              prefix, container.image.startsWith(prefix)
+          )
+        )
+      messageExpression: >
+        "Init container image is not allowed, must be one of " + variables.registries.join(", ")
+      reason: Invalid
+```
+
+Finally, the cluster admins need to define a `ValidatingAdmissionPolicyBinding` resource
+that binds the policy to all tenant namespaces and references the `ConfigMap` with the allow list:
+
+```yaml
+apiVersion: admissionregistration.k8s.io/v1
+kind: ValidatingAdmissionPolicyBinding
+metadata:
+  name: tenant-registries
+spec:
+  policyName: "registry.policy.fluxcd.controlplane.io"
+  validationActions: [ "Deny" ]
+  paramRef:
+    name: allowlist
+    namespace: flux-system
+    parameterNotFoundAction: "Deny"
+  matchResources:
+    namespaceSelector:
+      matchExpressions:
+        - key: kubernetes.io/metadata.name
+          operator: NotIn
+          values:
+            - flux-system
+            - kube-system
+```
+
+With the above policy in place, any tenant trying to create or update a workload that uses a container image
+from a registry that is not listed in the allow list will receive a validation error and the operation will be denied.
+
+```text
+The deployments "test-deployment" is invalid:
+ValidatingAdmissionPolicy 'registry.policy.fluxcd.controlplane.io' with binding 'tenant-registries' denied request:
+Init container image is not allowed, must be one of ghcr.io/controlplaneio-fluxcd/, 709825985650.dkr.ecr.us-east-1.amazonaws.com/controlplane/
+```
+
+## Conclusion
+
+In this guide we've shown how to use the Kubernetes native validating admission policies
+to enforce policies on multi-tenant clusters managed by Flux. Extending the examples above,
+cluster admins can define policies to enforce best practices, security and compliance requirements
+across all tenant namespaces and workloads.
+
+The new Kubernetes validating admission policies offer an alternative to admission webhooks such as
+[OPA Gatekeeper](https://open-policy-agent.github.io/gatekeeper/website/) or [Kyverno](https://kyverno.io/).
+Admission webhooks are single points of failure and can introduce latency in the API server response time,
+blocking or slowing down the reconciliation of the cluster state performed by Flux.
+Using the built-in policy engine, eliminates the maintenance overhead of running and managing
+custom admission webhooks and makes policy enforcement more reliable and efficient.

mkdocs.yml

@@ -92,6 +92,7 @@ nav:
     - Guides:
         - Flux Architecture Overview: guides/flux-architecture.md
         - D1 Reference Architecture: guides/d1-architecture-reference.md
+        - Flux Multitenant Policies: guides/flux-policies.md
     - Marketplaces:
         - AWS Marketplace: marketplace/aws.md
     - Versions: