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Authorizers

In kcp, a request has four different ways of being admitted:

  • It can be made to one of the preconfigured paths that do not require authorization, like /healthz.
  • It can be performed by a user in one of the configured always-allow groups, by default system:masters.
  • It can pass through the RBAC chain and match configured Roles and ClusterRoles.
  • It can be permitted by an external HTTPS webhook backend.

They are related in the following way:

graph TD
    start(Request):::state --> main_alt[/one of\]:::or
    main_alt --> aapa[Always Allow Paths Auth]
    main_alt --> aaga[Always Allow Groups Auth]
    main_alt --> wa[Webhook Auth]
    main_alt --> rga[Required Groups Auth]

    aapa --> decision(Decision):::state
    aaga --> decision
    wa --> decision

    subgraph "RBAC"
    rga --> wca[Workspace Content Auth]
    wca --> scrda[System CRD Auth]
    scrda --> mppa[Max. Permission Policy Auth]

    mppa --- mppa_alt[/one of\]:::or
    mppa_alt --> lpa[Local Policy Auth]
    mppa_alt --> gpa[Global Policy Auth]
    mppa_alt --> bpa[Bootstrap Policy Auth]
    end

    lpa --> decision
    gpa --> decision
    bpa --> decision

    classDef state color:#F77
    classDef or fill:none,stroke:none

View graph on Kroki

Always Allow Paths Authorizer

Like in vanilla Kubernetes, this authorizer always grants access to the configured URL paths. This is used for the health and liveness checks of kcp.

Always Allow Groups Authorizer

This authorizer always permits access if the user is in one of the configured groups. By default this only includes the system:masters group.

RBAC Chain

The primary authorization flow is handled by a sequence of RBAC-based authorizers that a request must satisfy all in order to be granted access.

The following authorizers work together to implement RBAC in kcp:

Authorizer Description
Workspace content authorizer validates that the user has access permission to the workspace
Required groups authorizer validates that the user is in the annotation-based list of groups required for a workspace
System CRD authorizer prevents undesired updates to certain core resources, like the status subresource on APIBindings
Maximal permission policy authorizer validates the maximal permission policy RBAC policy in the API exporter workspace
Local Policy authorizer validates the RBAC policy in the workspace that is accessed
Global Policy authorizer validates the RBAC policy in the workspace that is accessed across shards
Kubernetes Bootstrap Policy authorizer validates the RBAC Kubernetes standard policy

Required Groups Authorizer

A authorization.kcp.io/required-groups annotation can be added to a LogicalCluster to specify additional groups that are required to access a workspace for a user to be member of. The syntax is a disjunction (separator ,) of conjunctions (separator ;).

For example, <group1>;<group2>,<group3> means that a user must be member of <group1> AND <group2>, OR of <group3>.

The annotation is copied onto sub-workspaces during workspace creation, but is then not updated automatically if it's changed.

Workspace Content Authorizer

The workspace content authorizer checks whether the user is granted access to the workspace. Access is granted access through verb=access non-resource permission to / inside of the workspace.

The ClusterRole system:kcp:workspace:access is pre-defined which makes it easy to give a user access through a ClusterRoleBinding inside of the workspace.

For example, to give a user user1 access, create the following ClusterRoleBinding:

apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRoleBinding
metadata:
  name: example-access
subjects:
- kind: User
  name: user1
roleRef:
  apiGroup: rbac.authorization.k8s.io
  kind: ClusterRole
  name: system:kcp:workspace:access

To give a user user1 admin access, create the following ClusterRoleBinding:

apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRoleBinding
metadata:
  name: example-admin
subjects:
- kind: User
  name: user1
roleRef:
  apiGroup: rbac.authorization.k8s.io
  kind: ClusterRole
  name: cluster-admin

A service-account defined in a workspace implicitly is granted access to it.

A service-account defined in a different workspace is NOT given access to it.

Note

By default, workspaces are only accessible to a user if they are in Ready phase. Workspaces that are initializing can be accessed only by users that are granted admin verb on the workspaces/content resource in the parent workspace.

Service accounts declared within a workspace don't have access to initializing workspaces.

System CRD Authorizer

This small authorizer simply prevents updates to the status subresource on APIExports or APIBindings. Note that this authorizer does not validate changes to the CustomResourceDefitions themselves, but to objects from those CRDs instead.

Maximal Permission Policy Authorizer

If the requested resource type is part of an API binding, then this authorizer verifies that the request is not exceeding the maximum permission policy of the related API export. Currently, the "local policy" maximum permission policy type is supported.

Local Policy

The local maximum permission policy delegates the decision to the RBAC of the related API export. To distinguish between local RBAC role bindings in that workspace and those for this these maximum permission policy, every name and group is prefixed with apis.kcp.io:binding:.

Example: Given an APIBinding for type foo declared in workspace consumer that refers to an APIExport declared in workspace provider and a user user-1 having the group group-1 requesting a create of foo in the default namespace in the consumer workspace, this authorizer verifies that user-1 is allowed to execute this request by delegating to provider's RBAC using prefixed attributes.

Here, this authorizer prepends the apis.kcp.io:binding: prefix to the username and all groups the user belongs to. Using prefixed attributes prevents RBAC collisions i.e. if user-1 is granted to execute requests within the provider workspace directly.

For the given example RBAC request looks as follows:

  • Username: apis.kcp.io:binding:user-1
  • Groups: [apis.kcp.io:binding:group-1]
  • Resource: foo
  • Namespace: default
  • Workspace: provider
  • Verb: create

The following Role and RoleBinding declared within the provider workspace will grant access to the request:

apiVersion: rbac.authorization.k8s.io/v1
kind: Role
metadata:
  name: foo-creator
  clusterName: provider
rules:
- apiGroups:
  - foo.api
  resources:
  - foos
  verbs:
  - create
---
apiVersion: rbac.authorization.k8s.io/v1
kind: RoleBinding
metadata:
  name: user-1-foo-creator
  namespace: default
  clusterName: provider
subjects:
- kind: User
  name: apis.kcp.io:binding:user-1
roleRef:
  apiGroup: rbac.authorization.k8s.io
  kind: Role
  name: foo-creator

Note

The same authorization scheme is enforced when executing the request of a claimed resource via the virtual APIExport API server, i.e. a claimed resource is bound to the same maximal permission policy. Only the actual owner of that resources can go beyond that policy.

TBD: Example

Local Policy Authorizer

This authorizer ensures that RBAC rules contained within a workspace are being applied and work just like in a regular Kubernetes cluster.

It is possible to bind to Roles and ClusterRoles in the bootstrap policy from a local policy's RoleBinding or ClusterRoleBinding, for example the system:kcp:workspace:access ClusterRole exists in the system:admin logical cluster, but can still be bound from without any other logical cluster.

Global Policy Authorizer

This authorizer works identically to the Local Policy Authorizer, just with the difference that it uses a global (i.e. across shards) getter for Roles and RoleBindings.

Bootstrap Policy Authorizer

The bootstrap policy authorizer works just like the local authorizer but references RBAC rules defined in the system:admin system workspace. This workspace is where the classic Kubernetes RBAC like the cluster-admin ClusterRole is being defined and the policy defined in this workspace applies to every workspace in a kcp shard.

Webhook Authorizer

This authorizer can be enabled by providing the --authorization-webhook-config-file flag to the kcp process and works identically to how it works in vanilla Kubernetes.

The given configuration file must be of the kubeconfg format and point to an HTTPS server, potentially including certificate information as needed:

apiVersion: v1
kind: Config
clusters:
  - name: webhook
    cluster:
      server: https://localhost:8080/
current-context: webhook
contexts:
  - name: webhook
    context:
      cluster: webhook

The webhook will receive every authorization request made in kcp, including internal ones. This means if the webhook is badly configured, it can even prevent kcp from starting up successfully, but on the other hand this allows a lot of influence over the authorization in kcp.

The webhook will receive JSON-marshalled SubjectAccessReview objects, that (compared to vanilla Kubernetes) include the name of target logical cluster as an extra field, like so:

{
  "apiVersion": "authorization.k8s.io/v1beta1",
  "kind": "SubjectAccessReview",
  "spec": {
    "resourceAttributes": {
      "namespace": "kittensandponies",
      "verb": "get",
      "group": "unicorn.example.org",
      "resource": "pods"
    },
    "user": "jane",
    "group": [
      "group1",
      "group2"
    ],
    "extra": {
      "authorization.kubernetes.io/cluster-name": ["root"]
    }
  }
}

Note

The extra field will contain the logical cluster name (e.g. o43u2gh528rtfg721rg92), not the human-readable path. Webhooks need to resolve the name to a path themselves if necessary.