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[RFC-0001] Define Flux tenancy models
Signed-off-by: Stefan Prodan <stefan.prodan@gmail.com>
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| # RFC-0001 Flux Multi-Tenancy | ||
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| ## Summary | ||
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| The main goal of this RFC is to define the Kubernetes tenancy models supported by Flux. | ||
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| ## Motivation | ||
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| The Flux documentation contains references to account impersonation, reconciliation isolation and other multi-tenancy | ||
| related features without clearly defining which tenancy models are supported by Flux and how they relate to source | ||
| access control and Kubernetes role-based access control. | ||
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| ### Goals | ||
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| - Define multi-tenancy from a Flux user perspective. | ||
| - List the types of users that interact with Flux in regard to multi-tenancy. | ||
| - List the tenancy models supported by Flux. | ||
| - Explain the differences between tenancy models. | ||
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| ### Non-Goals | ||
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| - Explain in detail the Kubernetes architecture in regard to multi-tenancy. | ||
| - Provide an end-to-end workflow of how to setup multi-tenancy with Flux. | ||
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| ## Proposal | ||
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| The Flux documentation should have a section dedicated to multi-tenancy. What follows is the proposed content | ||
| of the multi-tenancy overview documentation that sets the stage for future enhancements. | ||
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| ### Introduction | ||
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| Flux allows different organizations and/or teams to share the same Kubernetes control plane to deliver applications | ||
| in a GitOps manner. Flux enables segmentation and isolation of resources across tenants by leveraging | ||
| Kubernetes Cluster API, namespaces and role-based access control. | ||
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| ### User Roles | ||
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| There are two types of users that interact with Flux: platform admins and tenants. | ||
| Besides installing Flux, all the other operations (deploy applications, configure ingress, policies, etc) | ||
| do not require users to have direct access to the Kubernetes API. Flux acts as a proxy between users and | ||
| the Kubernetes API, using Git as source of truth for the cluster desired state. Changes to the clusters | ||
| and workloads configuration can be made in a collaborative manner, where the various teams responsible for | ||
| the delivery process propose, review and approve changes via pull request workflows. | ||
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| #### Platform Admins | ||
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| The platform admins have unrestricted access to Kubernetes API. | ||
| They are responsible for installing Flux and granting Flux | ||
| access to the sources (Git, Helm, OCI repositories) that make up the cluster(s) control plane desired state. | ||
| The repository(s) owned by the platform admins are reconciled on the cluster(s) by Flux, under | ||
| the [cluster-admin](https://kubernetes.io/docs/reference/access-authn-authz/rbac/#user-facing-roles) | ||
| Kubernetes cluster role. | ||
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| Example of operations performed by cluster admins: | ||
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| - Bootstrap Flux onto cluster(s). | ||
| - Extend the Kubernetes API with custom resource definitions and validation webhooks. | ||
| - Configure various controllers for ingress, storage, logging, monitoring, progressive delivery, etc. | ||
| - Set up namespaces for tenants and define their level of access with Kubernetes RBAC. | ||
| - Onboard tenants by registering their Git repositories with Flux. | ||
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| #### Tenants | ||
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| The tenants have restricted access to the cluster(s) according to the Kubernetes RBAC configured | ||
| by the platform admins. The repositories owned by tenants are reconciled on the cluster(s) by Flux, | ||
| under the Kubernetes account(s) assigned by platform admins. | ||
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| Example of operations performed by tenants: | ||
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| - Register their sources with Flux (`GitRepositories`, `HelmRepositories` and `Buckets`). | ||
| - Deploy workload(s) into their namespace(s) using Flux custom resources (`Kustomizations` and `HelmReleases`). | ||
| - Automate application updates using Flux custom resources (`ImageRepositories`, `ImagePolicies` and `ImageUpdateAutomations`). | ||
| - Configure the release pipeline(s) using Flagger custom resources (`Canaries` and `MetricsTemplates`). | ||
| - Setup webhooks and alerting for their release pipeline(s) using Flux custom resources (`Receivers` and `Alerts`). | ||
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| ### Tenancy Models | ||
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| The Kubernetes tenancy models supported by Flux are: soft multi-tenancy and hard multi-tenancy. | ||
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| For an overview of the Kubernetes multi-tenant architecture please consult the following documentation: | ||
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| - [Three Tenancy Models For Kubernetes](https://kubernetes.io/blog/2021/04/15/three-tenancy-models-for-kubernetes/) | ||
| - [GKE multi-tenancy overview](https://cloud.google.com/kubernetes-engine/docs/concepts/multitenancy-overview) | ||
| - [EKS multi-tenancy best practices](https://aws.github.io/aws-eks-best-practices/security/docs/multitenancy/) | ||
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| #### Soft Multi-Tenancy | ||
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| With soft multi-tenancy, the platform admins use Kubernetes constructs such as namespaces, accounts, | ||
| roles and role bindings to create a logical separation between tenants. | ||
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| When Flux deploys workloads from a repository belonging to a tenant, it uses the Kubernetes account assigned to that | ||
| tenant to perform the git-to-cluster reconciliation. By leveraging Kubernetes RBAC, Flux can ensure | ||
| that the operations performed by tenants are restricted to their namespaces. | ||
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| Note that with this model, tenants share cluster-wide resources such as | ||
| `ClusterRoles`, `CustomResourceDefinitions`, `IngressClasses`, `StorageClasses`, | ||
| and they cannot create or alter these resources. | ||
| If a tenant adds o cluster-scoped resource definition to their repository, | ||
| Flux will fail the git-to-cluster reconciliation due to Kubernetes RBAC restrictions. | ||
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| To restrict the reconciliation of tenant's sources, a Kubernetes service account name can be specified | ||
| in Flux `Kustomizations` and `HelmReleases` under `.spec.serviceAccountName`. Please consult the Flux | ||
| documentation for more details: | ||
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| - [Kustomization API: Role-based access control](https://fluxcd.io/docs/components/kustomize/kustomization/#role-based-access-control) | ||
| - [HelmRelease API: Role-based access control](https://fluxcd.io/docs/components/helm/helmreleases/#role-based-access-control) | ||
| - [Flux multi-tenancy example repository](https://github.com/fluxcd/flux2-multi-tenancy) | ||
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| Note that with soft multi-tenancy, true tenant isolation requires security measures beyond Kubernetes RBAC. | ||
| Please refer to the Kubernetes [security considerations documentation](https://kubernetes.io/blog/2021/04/15/three-tenancy-models-for-kubernetes/#security-considerations) | ||
| for more details on how to harden shared clusters. | ||
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| ##### Caveats | ||
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| Currently, Flux does not enforce a service account to be specified on Flux `Kustomizations` and `HelmReleases`. | ||
| When a service account is not specified, Flux defaults to cluster-admin. | ||
| In order to enforce the tenant isolation, an admission controller such as Kyverno or OPA Gatekeeper must be used | ||
| to make the `.spec.serviceAccountName` a required field for the Flux custom resources created by tenants. | ||
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| We provide an [example](https://github.com/fluxcd/flux2-multi-tenancy/blob/main/infrastructure/kyverno-policies/flux-multi-tenancy.yaml) | ||
| for enforcing service accounts using a Kyverno cluster policy. | ||
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| Currently, Flux allows for `Kustomizations` and `HelmReleases` to reference sources | ||
| (`GitRepositories`, `HelmRepositories` and `Buckets`) across namespaces. | ||
| In order to prevent tenants from accessing each other sources, an admission controller such as Kyverno or OPA Gatekeeper | ||
| must be used to block cross-namespace references. | ||
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| We provide an [example](https://github.com/fluxcd/flux2-multi-tenancy/blob/main/infrastructure/kyverno-policies/flux-multi-tenancy.yaml) | ||
| for blocking source cross-namespace references using a Kyverno cluster policy. | ||
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| #### Hard Multi-Tenancy | ||
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| With hard multi-tenancy, the platform admins use Kubernetes Cluster API to create dedicated clusters for each tenant. | ||
| The Flux instance installed on the management cluster is responsible | ||
| for reconciling the cluster definitions belonging to tenants. | ||
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| To enable GitOps for the tenant's clusters, the platform admins can configure the Flux instance running on the | ||
| management cluster to connect to the tenant's cluster using the `kubeConfig` generated by the Cluster API provider. | ||
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| To configure Flux reconciliation of remote clusters, a Kubernetes secret containing a `kubeConfig` can be specified | ||
| in Flux `Kustomizations` and `HelmReleases` under `.spec.kubeConfig.secretRef`. Please consult the Flux API | ||
| documentation for more details: | ||
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| - [Kustomization API: Remote Clusters](https://fluxcd.io/docs/components/kustomize/kustomization/#remote-clusters--cluster-api) | ||
| - [HelmRelease API: Remote Clusters](https://fluxcd.io/docs/components/helm/helmreleases/#remote-clusters--cluster-api) | ||
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| Note that with hard multi-tenancy, tenants have full access to cluster-wide resources, so they have the option | ||
| to manage Flux independently of platform admins, by deploying a Flux instances on each cluster. | ||
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| ##### Caveats | ||
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| When using a Kubernetes Cluster API provider, the `kubeConfig` secret is automatically generated and Flux can | ||
| make use of it without any manual actions. For clusters created by other means than Cluster API, the | ||
| platform team has to create the `kubeConfig` secrets to allow Flux access to the remote clusters. | ||
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| Currently, we don't provide any guidance for cluster admins on how to generate the `kubeConfig` secrets. | ||
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| ## Implementation History | ||
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| - Soft multi-tenancy based on service account impersonation was first released in flux2 **v0.0.1**. | ||
| - Generating namespaces and RBAC for defining tenants with `flux create tenant` was first released in flux2 **v0.1.0**. | ||
| - Hard multi-tenancy based on remote cluster reconciliation was first released in flux2 **v0.2.0**. | ||
| - Soft multi-tenancy end-to-end workflow example was first published on 27 Nov 2020 at | ||
| [fluxcd/flux2-multi-tenancy](https://github.com/fluxcd/flux2-multi-tenancy). | ||
| - Soft multi-tenancy [CVE-2021-41254](https://github.com/fluxcd/kustomize-controller/security/advisories/GHSA-35rf-v2jv-gfg7) | ||
| "Privilege escalation to cluster admin on multi-tenant environments" was fixed in flux2 **v0.15.0**. |