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CA Rotation in Extensions

GEP-18 proposes adding support for automated rotation of Shoot cluster certificate authorities (CAs). This document outlines all the requirements that Gardener extensions need to fulfill in order to support the CA rotation feature.

Requirements for Shoot Cluster CA Rotation

Extensions must not rely on static CA Secret names managed by the gardenlet, because their names are changing during CA rotation.
Extensions cannot issue or use client certificates for authenticating against shoot API servers. Instead, they should use short-lived auto-rotated ServiceAccount tokens via gardener-resource-manager’s TokenRequestor. Also see Conventions and TokenRequestor documents.
Extensions need to generate dedicated CAs for signing server certificates (e.g. cloud-controller-manager). There should be one CA per controller and purpose in order to bind the lifecycle to the reconciliation cycle of the respective object for which it is created.
CAs managed by extensions should be rotated in lock-step with the shoot cluster CA. When the user triggers a rotation, the gardenlet writes phase and initiation time to Shoot.status.credentials.rotation.certificateAuthorities.{phase,lastInitiationTime}. See GEP-18 for a detailed description on what needs to happen in each phase. Extensions can retrieve this information from Cluster.shoot.status.

Utilities for Secrets Management

In order to fulfill the requirements listed above, extension controllers can reuse the SecretsManager that the gardenlet uses to manage all shoot cluster CAs, certificates, and other secrets as well. It implements the core logic for managing secrets that need to be rotated, auto-renewed, etc.

Additionally, there are utilities for reusing SecretsManager in extension controllers. They already implement the above requirements based on the Cluster resource and allow focusing on the extension controllers’ business logic.

For example, a simple SecretsManager usage in an extension controller could look like this:

const (
  // identity for SecretsManager instance in ControlPlane controller
  identity = "provider-foo-controlplane"
  // secret config name of the dedicated CA
  caControlPlaneName = "ca-provider-foo-controlplane"
)

func Reconcile() {
  var (
    cluster *extensionscontroller.Cluster
    client  client.Client

    // define wanted secrets with options
    secretConfigs = []extensionssecretsmanager.SecretConfigWithOptions{
      {
        // dedicated CA for ControlPlane controller
        Config: &secretutils.CertificateSecretConfig{
          Name:       caControlPlaneName,
          CommonName: "ca-provider-foo-controlplane",
          CertType:   secretutils.CACert,
        },
        // persist CA so that it gets restored on control plane migration
        Options: []secretsmanager.GenerateOption{secretsmanager.Persist()},
      },
      {
        // server cert for control plane component
        Config: &secretutils.CertificateSecretConfig{
          Name:       "cloud-controller-manager",
          CommonName: "cloud-controller-manager",
          DNSNames:   kutil.DNSNamesForService("cloud-controller-manager", namespace),
          CertType:   secretutils.ServerCert,
        },
        // sign with our dedicated CA
        Options: []secretsmanager.GenerateOption{secretsmanager.SignedByCA(caControlPlaneName)},
      },
    }
  )

  // initialize SecretsManager based on Cluster object
  sm, err := extensionssecretsmanager.SecretsManagerForCluster(ctx, logger.WithName("secretsmanager"), clock.RealClock{}, client, cluster, identity, secretConfigs)
  
  // generate all wanted secrets (first CAs, then the rest)
  secrets, err := extensionssecretsmanager.GenerateAllSecrets(ctx, sm, secretConfigs)

  // cleanup any secrets that are not needed any more (e.g. after rotation)
  err = sm.Cleanup(ctx)
}

Please pay attention to the following points:

There should be one SecretsManager identity per controller (and purpose if applicable) in order to prevent conflicts between different instances. E.g., there should be different identities for Infrastructrue, Worker controller, etc., and the ControlPlane controller should use dedicated SecretsManager identities per purpose (e.g. provider-foo-controlplane and provider-foo-controlplane-exposure).
All other points in Reusing the SecretsManager in Other Components.