[RFC-0003] Flux Multi-Tenancy Mode

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

rfcs/0003-multi-tenancy-mode/README.md

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+# RFC-0003 Flux Multi-Tenancy Mode
+
+## Summary
+
+For multi-tenant environments, we want to offer an easy way of configuring Flux to enforce tenant isolation
+(as defined by the Soft Multi-Tenancy model from RFC-0001).
+
+When running in the multi-tenant mode, Flux will lock down access to sources (as defined by RFC-0002),
+and will use the tenant service account instead of defaulting to `cluster-admin`.
+
+From an end-user perspective, the multi-tenancy mode means that:
+
+- Platform admins have to create a Kubernetes service account and RBAC in each namespace where
+  Flux performs source-to-cluster reconciliation on behalf of tenants.
+  By default, Flux will have no permissions to reconcile the tenants sources onto clusters.
+- Source owners have to specify with which tenants they wish to share their sources.
+  By default, nothing is shared between tenants.
+
+## Motivation
+
+Currently, configuring Flux for soft multi-tenancy requires additional tooling such as Kyverno or OPA Gatekeeper
+to overcome caveats such as:
+- Flux does not require for a service account name to be specified on Flux custom resources that perform
+  source-to-cluster reconciliation. When a service account is not specified, Flux defaults to cluster-admin.
+- Flux does not prevent tenants from accessing known sources outside of their namespaces.
+
+Flux users have been asking for a way to enforce multi-tenancy
+without having to use 3rd party validation webhooks e.g.
+[fluxcd/kustomize-controller#422](https://github.com/fluxcd/kustomize-controller/issues/422).
+
+### Goals
+
+- Enforce service account impersonation for source-to-cluster reconciliation.
+- Enforce ACLs for cross-namespace access to sources.
+
+### Non-Goals
+
+- Enforce tenant's workload isolation with network policies and pod security standards as described
+  [here](https://kubernetes.io/blog/2021/04/15/three-tenancy-models-for-kubernetes/#security-considerations).
+
+## Proposal
+
+### User Stories
+
+#### Story 1
+
+> As a platform admin, I want to install Flux with lowest privilege/permission level possible.
+
+#### Story 2
+
+> As a platform admin, I want to give tenants full control over their assigned namespaces.
+> So that tenants could use their own repositories and manager the app delivery with Flux.
+
+#### Story 3
+
+> As a platform admin, I want to prevent tenants from changing the cluster-wide configuration.
+> If a tenant adds to their repository a cluster-scoped resource such as a namespace or cluster role,
+> Flux should reject the change and notify the tenant that this operation is not allowed.
+
+### Multi-tenant Bootstrap
+
+When bootstrapping Flux, platform admins should have the option to lock down Flux for multi-tenant environments e.g.:
+
+```shell
+flux bootstrap --security-profile=multi-tenant
+```
+
+The security profile flag accepts two values: `single-tenant` and `multi-tenant`.
+Platform admins may switch between the two modes at any time, either by rerunning bootstrap
+or by patching the Flux manifests in Git.
+
+The `multi-tenant` profile is just a shortcut to setting the following container args in the Flux deployment manifests:
+
+```yaml
+      containers:
+      - name: manager
+        args:
+          - --default-service-account=flux
+          - --enable-source-acl=true
+```
+
+When running in the `multi-tenant` mode, Flux behaves differently:
+
+- The source-to-cluster reconciliation no longer runs under the service account of
+  the Flux controllers. The controller service account, is only used to impersonate
+  the service account specified in the Flux custom resources (`Kustomizations`, `HelmReleases`).
+- When no service account name is specified in a Flux custom resource,
+  a default will be used e.g. `system:serviceaccount:<tenant-namespace>:flux`.
+- When a Flux custom resource (`Kustomizations`, `HelmReleases`, `ImagePolicies`, `ImageUpdateAutomations`)
+  refers to a source in a different namespace, access is granted based the source access control list.
+  If no ACL is defined for a source, cross-namespace access is denied.
+
+### Tenants Onboarding
+
+When onboarding tenants, platform admins should have the option to assign namespaces, set
+permissions and register the tenants repositories onto clusters in a declarative manner. 
+
+The Flux CLI offers an easy way of generating all the Kubernetes manifests needed to onboard tenants:
+
+- `flux create tenant` command generates namespaces, service accounts and Kubernetes RBAC
+  with restricted access to the cluster resources, given tenants access only to their namespaces.
+- `flux create secret git` command generates SSH keys used by Flux to clone the tenants repositories.
+- `flux create source git` command generates the configuration that tells Flux which repositories belong to tenants.
+- `flux create kustomization` command generates the configuration that tells Flux how to reconcile the manifests found in the tenants repositories.
+
+All the above commands have an `--export` flag for generating the Kubernetes resources in YAML format.
+The platform admins should place the generated manifests in the repository that defines the cluster(s) desired state.
+
+Here is an example of the generated manifests:
+
+```yaml
+---
+apiVersion: v1
+kind: Namespace
+metadata:
+  name: tenant1
+---
+apiVersion: v1
+kind: ServiceAccount
+metadata:
+  name: flux
+  namespace: tenant1
+---
+apiVersion: rbac.authorization.k8s.io/v1
+kind: RoleBinding
+metadata:
+  name: flux
+  namespace: tenant1
+roleRef:
+  apiGroup: rbac.authorization.k8s.io
+  kind: ClusterRole
+  name: cluster-admin
+subjects:
+  - kind: ServiceAccount
+    name: flux
+    namespace: tenant1
+---
+apiVersion: source.toolkit.fluxcd.io/v1beta1
+kind: GitRepository
+metadata:
+  name: tenant1
+  namespace: tenant1
+spec:
+  interval: 5m0s
+  ref:
+    branch: main
+  secretRef:
+    name: tenant1-git-auth
+  url: ssh://git@github.com/org/tenant1
+---
+apiVersion: kustomize.toolkit.fluxcd.io/v1beta2
+kind: Kustomization
+metadata:
+  name: tenant1
+  namespace: tenant1
+spec:
+  interval: 10m0s
+  path: ./
+  prune: true
+  serviceAccountName: flux
+  sourceRef:
+    kind: GitRepository
+    name: tenant1
+```
+
+Note that the [cluster-admin](https://kubernetes.io/docs/reference/access-authn-authz/rbac/#user-facing-roles)
+role is used in a `RoleBinding`, this only gives full control over every resource in the role binding's namespace.
+
+Once the tenants repositories are registered on the cluster(s), the tenants can configure their app delivery 
+in Git using Kubernetes namespace-scoped resources such as `Deployments`, `Services`, Flagger `Canaries`,
+Flux `Kustomizations`, `HelmReleases`, `ImageUpdateAutomations`, `Alerts`, `Receivers`, etc.
+
+## Alternatives
+
+Instead of introducing the security profile flag to `flux bootstrap`,
+we could document how to patch each controller deployment with Kustomize.
+
+Having an easy way of locking down Flux with a single flag, make users aware of the security implications
+and improves the user experience.