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subdirectory for authorization

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Jordan Liggitt
2017-02-14 16:55:28 -05:00
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---
assignees:
- erictune
- lavalamp
- deads2k
- liggitt
title: Using Authorization Plugins
---
In Kubernetes, authorization happens as a separate step from authentication.
See the [Accessing Control Overview](/docs/admin/accessing-the-api/) for an
overview of how authentication and authorization are applied to requests.
Authorization applies to all HTTP accesses on the main (secure) apiserver port.
The authorization check for any request compares attributes of the context of
the request, (such as user, resource, and namespace) with access
policies. An API call must be allowed by some policy in order to proceed.
The following implementations are available, and are selected by flag:
- `--authorization-mode=AlwaysDeny` blocks all requests (used in tests).
- `--authorization-mode=AlwaysAllow` allows all requests; use if you don't
need authorization.
- `--authorization-mode=ABAC` allows for a simple local-file-based user-configured
authorization policy. ABAC stands for Attribute-Based Access Control.
authorization policy.
- `--authorization-mode=RBAC` is an experimental implementation which allows
for authorization to be driven by the Kubernetes API.
RBAC stands for Roles-Based Access Control.
- `--authorization-mode=Webhook` allows for authorization to be driven by a
remote service using REST.
If multiple modes are provided the set is unioned, and only a single authorizer is required to admit the action. This means the flag:
```
--authorization-mode=AlwaysDeny,AlwaysAllow
```
will always allow.
## Request Attributes
A request has the following attributes that can be considered for authorization:
- user (the user-string which a user was authenticated as).
- group (the list of group names the authenticated user is a member of).
- "extra" (a map of arbitrary string keys to string values, provided by the authentication layer)
- whether the request is for an API resource.
- the request path.
- allows authorizing access to miscellaneous non-resource endpoints like `/api` or `/healthz` (see [kubectl](#kubectl)).
- the request verb.
- API verbs `get`, `list`, `create`, `update`, `patch`, `watch`, `proxy`, `redirect`, `delete`, and `deletecollection` are used for resource requests
- HTTP verbs `get`, `post`, `put`, and `delete` are used for non-resource requests
- what resource is being accessed (for resource requests only)
- what subresource is being accessed (for resource requests only)
- the namespace of the object being accessed (for namespaced resource requests only)
- the API group being accessed (for resource requests only); an empty string designates the [core API group](/docs/api/)
The request verb for a resource API endpoint can be determined by the HTTP verb used and whether or not the request acts on an individual resource or a collection of resources:
HTTP verb | request verb
----------|---------------
POST | create
GET, HEAD | get (for individual resources), list (for collections)
PUT | update
PATCH | patch
DELETE | delete (for individual resources), deletecollection (for collections)
## ABAC Mode
### Policy File Format
For mode `ABAC`, also specify `--authorization-policy-file=SOME_FILENAME`.
The file format is [one JSON object per line](http://jsonlines.org/). There
should be no enclosing list or map, just one map per line.
Each line is a "policy object". A policy object is a map with the following
properties:
- Versioning properties:
- `apiVersion`, type string; valid values are "abac.authorization.kubernetes.io/v1beta1". Allows versioning and conversion of the policy format.
- `kind`, type string: valid values are "Policy". Allows versioning and conversion of the policy format.
- `spec` property set to a map with the following properties:
- Subject-matching properties:
- `user`, type string; the user-string from `--token-auth-file`. If you specify `user`, it must match the username of the authenticated user.
- `group`, type string; if you specify `group`, it must match one of the groups of the authenticated user. `system:authenticated` matches all authenticated requests. `system:unauthenticated` matches all unauthenticated requests.
- `readonly`, type boolean, when true, means that the policy only applies to get, list, and watch operations.
- Resource-matching properties:
- `apiGroup`, type string; an API group, such as `extensions`. `*` matches all API groups.
- `namespace`, type string; a namespace string. `*` matches all resource requests.
- `resource`, type string; a resource, such as `pods`. `*` matches all resource requests.
- Non-resource-matching properties:
- `nonResourcePath`, type string; matches the non-resource request paths (like `/version` and `/apis`). `*` matches all non-resource requests. `/foo/*` matches `/foo/` and all of its subpaths.
An unset property is the same as a property set to the zero value for its type
(e.g. empty string, 0, false). However, unset should be preferred for
readability.
In the future, policies may be expressed in a JSON format, and managed via a
REST interface.
### Authorization Algorithm
A request has attributes which correspond to the properties of a policy object.
When a request is received, the attributes are determined. Unknown attributes
are set to the zero value of its type (e.g. empty string, 0, false).
A property set to `"*"` will match any value of the corresponding attribute.
The tuple of attributes is checked for a match against every policy in the
policy file. If at least one line matches the request attributes, then the
request is authorized (but may fail later validation).
To permit any authenticated user to do something, write a policy with the
group property set to `"system:authenticated"`.
To permit any unauthenticated user to do something, write a policy with the
group property set to `"system:unauthenticated"`.
To permit a user to do anything, write a policy with the apiGroup, namespace,
resource, and nonResourcePath properties set to `"*"`.
### Kubectl
Kubectl uses the `/api` and `/apis` endpoints of api-server to negotiate
client/server versions. To validate objects sent to the API by create/update
operations, kubectl queries certain swagger resources. For API version `v1`
those would be `/swaggerapi/api/v1` & `/swaggerapi/experimental/v1`.
When using ABAC authorization, those special resources have to be explicitly
exposed via the `nonResourcePath` property in a policy (see [examples](#examples) below):
* `/api`, `/api/*`, `/apis`, and `/apis/*` for API version negotiation.
* `/version` for retrieving the server version via `kubectl version`.
* `/swaggerapi/*` for create/update operations.
To inspect the HTTP calls involved in a specific kubectl operation you can turn
up the verbosity:
kubectl --v=8 version
### Examples
1. Alice can do anything to all resources:
```json
{"apiVersion": "abac.authorization.kubernetes.io/v1beta1", "kind": "Policy", "spec": {"user": "alice", "namespace": "*", "resource": "*", "apiGroup": "*"}}
```
2. Kubelet can read any pods:
```json
{"apiVersion": "abac.authorization.kubernetes.io/v1beta1", "kind": "Policy", "spec": {"user": "kubelet", "namespace": "*", "resource": "pods", "readonly": true}}
```
3. Kubelet can read and write events:
```json
{"apiVersion": "abac.authorization.kubernetes.io/v1beta1", "kind": "Policy", "spec": {"user": "kubelet", "namespace": "*", "resource": "events"}}
```
4. Bob can just read pods in namespace "projectCaribou":
```json
{"apiVersion": "abac.authorization.kubernetes.io/v1beta1", "kind": "Policy", "spec": {"user": "bob", "namespace": "projectCaribou", "resource": "pods", "readonly": true}}
```
5. Anyone can make read-only requests to all non-resource paths:
```json
{"apiVersion": "abac.authorization.kubernetes.io/v1beta1", "kind": "Policy", "spec": {"group": "system:authenticated", "readonly": true, "nonResourcePath": "*"}}
{"apiVersion": "abac.authorization.kubernetes.io/v1beta1", "kind": "Policy", "spec": {"group": "system:unauthenticated", "readonly": true, "nonResourcePath": "*"}}
```
[Complete file example](http://releases.k8s.io/{{page.githubbranch}}/pkg/auth/authorizer/abac/example_policy_file.jsonl)
### A quick note on service accounts
A service account automatically generates a user. The user's name is generated
according to the naming convention:
```shell
system:serviceaccount:<namespace>:<serviceaccountname>
```
Creating a new namespace also causes a new service account to be created, of
this form:
```shell
system:serviceaccount:<namespace>:default
```
For example, if you wanted to grant the default service account in the
kube-system full privilege to the API, you would add this line to your policy
file:
```json
{"apiVersion":"abac.authorization.kubernetes.io/v1beta1","kind":"Policy","spec":{"user":"system:serviceaccount:kube-system:default","namespace":"*","resource":"*","apiGroup":"*"}}
```
The apiserver will need to be restarted to pickup the new policy lines.
## RBAC Mode
When specified "RBAC" (Role-Based Access Control) uses the
"rbac.authorization.k8s.io" API group to drive authorization decisions,
allowing admins to dynamically configure permission policies through the
Kubernetes API.
As of 1.3 RBAC mode is in alpha and considered experimental.
To use RBAC, you must both enable the authorization module with `--authorization-mode=RBAC`,
and [enable the API version](
/docs/admin/cluster-management/#turn-on-or-off-an-api-version-for-your-cluster),
with a `--runtime-config=` that includes `rbac.authorization.k8s.io/v1alpha1`.
### Privilege Escalation Prevention and Bootstrapping
The `rbac.authorization.k8s.io` API group inherently attempts to prevent users
from escalating privileges. Simply put, __a user can't grant permissions they
don't already have even when the RBAC authorizer it disabled__. If "user-1"
does not have the ability to read secrets in "namespace-a", they cannot create
a binding that would grant that permission to themselves or any other user.
When bootstrapping, superuser credentials should include the `system:masters`
group, for example by creating a client cert with `/O=system:masters`. This
gives those credentials full access to the API and allows an admin to then set
up bindings for other users.
In Kubernetes versions 1.4 and 1.5, there was a similar flag that gave a user
full access:
```
--authorization-rbac-super-user=admin
```
__This flag will be removed in 1.6__. Admins should prefer the `system:masters`
group when setting up clusters.
### Roles, RolesBindings, ClusterRoles, and ClusterRoleBindings
The RBAC API Group declares four top level types which will be covered in this
section. Users can interact with these resources as they would with any other
API resource. Through `kubectl`, direct calls to the API, etc. For instance,
`kubectl create -f (resource).yml` can be used with any of these examples,
though readers who wish to follow along should review the following section on
bootstrapping first.
In the RBAC API Group, roles hold a logical grouping of permissions. These
permissions map very closely to ABAC policies, but only contain information
about requests being made. Permission are purely additive, rules may only omit
permissions they do not wish to grant.
Here's an example of a role which grants read access to pods within the
"default" namespace.
```yaml
kind: Role
apiVersion: rbac.authorization.k8s.io/v1alpha1
metadata:
namespace: default
name: pod-reader
rules:
- apiGroups: [""] # The API group "" indicates the core API Group.
resources: ["pods"]
verbs: ["get", "watch", "list"]
```
`ClusterRoles` hold the same information as a `Role` but can apply to any
namespace as well as non-namespaced resources (such as `Nodes`,
`PersistentVolume`, etc.). The following `ClusterRole` can grant permissions to
read secrets in any namespace.
```yaml
kind: ClusterRole
apiVersion: rbac.authorization.k8s.io/v1alpha1
metadata:
# "namespace" omitted since ClusterRoles are not namespaced.
name: secret-reader
rules:
- apiGroups: [""]
resources: ["secrets"]
verbs: ["get", "watch", "list"]
nonResourceURLs: []
```
`RoleBindings` perform the task of granting the permission to a user or set of
users. They hold a list of subjects which they apply to, and a reference to the
`Role` being assigned.
The following `RoleBinding` assigns the "pod-reader" role to the user "jane"
within the "default" namespace, and allows jane to read pods.
```yaml
# This role binding allows "jane" to read pods in the namespace "default"
kind: RoleBinding
apiVersion: rbac.authorization.k8s.io/v1alpha1
metadata:
name: read-pods
namespace: default
subjects:
- kind: User # May be "User", "Group" or "ServiceAccount"
name: jane
roleRef:
kind: Role
name: pod-reader
apiGroup: rbac.authorization.k8s.io
```
`RoleBindings` may also refer to a `ClusterRole`. However, a `RoleBinding` that
refers to a `ClusterRole` only applies in the `RoleBinding`'s namespace, not at
the cluster level. This allows admins to define a set of common roles for the
entire cluster, then reuse them in multiple namespaces.
For instance, even though the following `RoleBinding` refers to a `ClusterRole`,
"dave" (the subject) will only be able read secrets in the "development"
namespace, the namespace of the `RoleBinding`.
```yaml
# This role binding allows "dave" to read secrets in the namespace "development"
kind: RoleBinding
apiVersion: rbac.authorization.k8s.io/v1alpha1
metadata:
name: read-secrets
namespace: development # This binding only applies in the "development" namespace
subjects:
- kind: User # May be "User", "Group" or "ServiceAccount"
name: dave
roleRef:
kind: ClusterRole
name: secret-reader
apiGroup: rbac.authorization.k8s.io
```
Finally a `ClusterRoleBinding` may be used to grant permissions in all
namespaces. The following `ClusterRoleBinding` allows any user in the group
"manager" to read secrets in any namespace.
```yaml
# This cluster role binding allows anyone in the "manager" group to read secrets in any namespace.
kind: ClusterRoleBinding
apiVersion: rbac.authorization.k8s.io/v1alpha1
metadata:
name: read-secrets-global
subjects:
- kind: Group # May be "User", "Group" or "ServiceAccount"
name: manager
roleRef:
kind: ClusterRole
 name: secret-reader
apiGroup: rbac.authorization.k8s.io
```
### Referring to Resources
Most resources are represented by a string representation of their name, such as "pods", just as it
appears in the URL for the relevant API endpoint. However, some Kubernetes APIs involve a
"subresource" such as the logs for a pod. The URL for the pods logs endpoint is:
```
GET /api/v1/namespaces/{namespace}/pods/{name}/log
```
In this case, "pods" is the namespaced resource, and "log" is a subresource of pods. To represent
this in an RBAC role, use a slash to delimit the resource and subresource names. To allow a subject
to read both pods and pod logs, you would write:
```yaml
kind: Role
apiVersion: rbac.authorization.k8s.io/v1alpha1
metadata:
namespace: default
name: pod-and-pod-logs-reader
rules:
- apiGroups: [""]
resources: ["pods", "pods/log"]
verbs: ["get", "list"]
```
### Referring to Subjects
RoleBindings and ClusterRoleBindings bind "subjects" to "roles".
Subjects can be groups, users or service accounts.
Users are represented by strings. These can be plain usernames, like
"alice", or email style names, like "bob@example.com", or numeric ids
as string. It is up to the Kubernetes admin to configure
the [authentication modules](/docs/admin/authentication/) to produce
usernames in the desired format. The RBAC authorization system does
not require any particular format. However, the prefix `system:` is
reserved for Kubernetes system use, and so the admin should ensure
usernames should not contain this prefix by accident.
Groups information in Kubernetes is currently provided by the Authenticator
modules. (In the future we may add a separate way for the RBAC Authorizer
to query groups information for users.) Groups, like users, are represented
by a string, and that string has no format requirements, other than that the
prefix `system:` is reserved.
Service Accounts have usernames with the `system:` prefix and belong
to groups with the `system:` prefix.
#### Role Binding Examples
Only the `subjects` section of a RoleBinding object shown in the following examples.
For a user called `alice@example.com`, specify
```yaml
subjects:
- kind: User
name: "alice@example.com"
```
For a group called `frontend-admins`, specify:
```yaml
subjects:
- kind: Group
name: "frontend-admins"
```
For the default service account in the kube-system namespace:
```yaml
subjects:
- kind: ServiceAccount
name: default
namespace: kube-system
```
For all service accounts in the `qa` namespace:
```yaml
subjects:
- kind: Group
name: system:serviceaccounts:qa
```
For all service accounts everywhere:
```yaml
subjects:
- kind: Group
name: system:serviceaccounts
```
For all authenticated users:
```yaml
subjects:
- kind: Group
name: system:authenticated
```
For all unauthenticated users:
```yaml
subjects:
- kind: Group
name: system:unauthenticated
```
For all users:
```yaml
subjects:
- kind: Group
name: system:authenticated
- kind: Group
name: system:unauthenticated
```
### `system:*` prefixed RBAC resources
If an RBAC resource (`ClusterRole`, `ClusterRoleBinding`, etc) is prefixed with `system:*`, that indicates that
the resource is "owned" by the infrastructure. Modifications to these resources can result in non-functional clusters.
One example is the `clusterrole/system:nodes`. This role is used to provide limited permissions to kubelets. If the
role is modified, its possible to prevent your kubelet from working.
### Default ClusterRoles and ClusterRoleBindings
When starting up an API server without any ClusterRoles or ClusterRoleBindings, the API server will bootstrap itself
with a set of default ClusterRoles and ClusterRoleBindings. Most of these are `system:*` prefixed, but some are not.
They are all labeled with `kubernetes.io/bootstrapping=rbac-defaults`. These are the most commonly used:
|Default Role |Description
|*admin* |A project manager. If used in a `RoleBinding`, an *admin* user will have
rights to view any Kubernetes resource in the project and modify any resource in the
project except for quota.
|*cluster-admin* |A super-user role that can perform any action in any project. When
granted to a user within a local policy, they have full control over quota and
roles and every action on every resource in the project.
|*cluster-status* |A role that can get basic cluster status information.
|*edit* |A role that can modify most objects in a project, but does not have the
power to view or modify roles or bindings.
|*view* |A role who cannot make any modifications, but can see most objects in a
project. They cannot view or modify roles or bindings. They cannot view secrets,
since those are escalating.
|*system:auth-delegator*|A role which allows delegated authentication and authorization
checks. This is commonly used by add-on API servers for a unified authentication and
authorization experience.
|*system:basic-user* |A role that can get basic information about himself.
|*system:discovery*|A role which provides just enough power to access discovery and
negotiate an API level.
### CLI helpers
In order to ease the binding of `ClusterRoles`, two CLI helpers were created. `kubectl create rolebinding` and
`kubectl create clusterrolebinding`. See the CLI help for detailed usage, but they allow for usage like
`kubectl create clusterrolebinding cluster-admins --clusterrole=cluster-admin --user=root` and
`kubectl create rolebinding -n my-namespace viewers --clusterrole=view --user=collaborator --serviceaccount=other-ns:default`.
## Webhook Mode
When specified, mode `Webhook` causes Kubernetes to query an outside REST
service when determining user privileges.
### Configuration File Format
Mode `Webhook` requires a file for HTTP configuration, specify by the
`--authorization-webhook-config-file=SOME_FILENAME` flag.
The configuration file uses the [kubeconfig](/docs/user-guide/kubeconfig-file/)
file format. Within the file "users" refers to the API Server webhook and
"clusters" refers to the remote service.
A configuration example which uses HTTPS client auth:
```yaml
# clusters refers to the remote service.
clusters:
- name: name-of-remote-authz-service
cluster:
certificate-authority: /path/to/ca.pem # CA for verifying the remote service.
server: https://authz.example.com/authorize # URL of remote service to query. Must use 'https'.
# users refers to the API Server's webhook configuration.
users:
- name: name-of-api-server
user:
client-certificate: /path/to/cert.pem # cert for the webhook plugin to use
client-key: /path/to/key.pem # key matching the cert
# kubeconfig files require a context. Provide one for the API Server.
current-context: webhook
contexts:
- context:
cluster: name-of-remote-authz-service
user: name-of-api-sever
name: webhook
```
### Request Payloads
When faced with an authorization decision, the API Server POSTs a JSON
serialized api.authorization.v1beta1.SubjectAccessReview object describing the
action. This object contains fields describing the user attempting to make the
request, and either details about the resource being accessed or requests
attributes.
Note that webhook API objects are subject to the same [versioning compatibility rules](/docs/api/)
as other Kubernetes API objects. Implementers should be aware of looser
compatibility promises for beta objects and check the "apiVersion" field of the
request to ensure correct deserialization. Additionally, the API Server must
enable the `authorization.k8s.io/v1beta1` API extensions group (`--runtime-config=authorization.k8s.io/v1beta1=true`).
An example request body:
```json
{
"apiVersion": "authorization.k8s.io/v1beta1",
"kind": "SubjectAccessReview",
"spec": {
"resourceAttributes": {
"namespace": "kittensandponies",
"verb": "GET",
"group": "unicorn.example.org",
"resource": "pods"
},
"user": "jane",
"group": [
"group1",
"group2"
]
}
}
```
The remote service is expected to fill the SubjectAccessReviewStatus field of
the request and respond to either allow or disallow access. The response body's
"spec" field is ignored and may be omitted. A permissive response would return:
```json
{
"apiVersion": "authorization.k8s.io/v1beta1",
"kind": "SubjectAccessReview",
"status": {
"allowed": true
}
}
```
To disallow access, the remote service would return:
```json
{
"apiVersion": "authorization.k8s.io/v1beta1",
"kind": "SubjectAccessReview",
"status": {
"allowed": false,
"reason": "user does not have read access to the namespace"
}
}
```
Access to non-resource paths are sent as:
```json
{
"apiVersion": "authorization.k8s.io/v1beta1",
"kind": "SubjectAccessReview",
"spec": {
"nonResourceAttributes": {
"path": "/debug",
"verb": "GET"
},
"user": "jane",
"group": [
"group1",
"group2"
]
}
}
```
Non-resource paths include: `/api`, `/apis`, `/metrics`, `/resetMetrics`,
`/logs`, `/debug`, `/healthz`, `/swagger-ui/`, `/swaggerapi/`, `/ui`, and
`/version.` Clients require access to `/api`, `/api/*`, `/apis`, `/apis/*`,
and `/version` to discover what resources and versions are present on the server.
Access to other non-resource paths can be disallowed without restricting access
to the REST api.
For further documentation refer to the authorization.v1beta1 API objects and
plugin/pkg/auth/authorizer/webhook/webhook.go.
## Module Development
Other implementations can be developed fairly easily.
The APIserver calls the Authorizer interface:
```go
type Authorizer interface {
Authorize(a Attributes) error
}
```
to determine whether or not to allow each API action.
An authorization plugin is a module that implements this interface.
Authorization plugin code goes in `pkg/auth/authorizer/$MODULENAME`.
An authorization module can be completely implemented in go, or can call out
to a remote authorization service. Authorization modules can implement
their own caching to reduce the cost of repeated authorization calls with the
same or similar arguments. Developers should then consider the interaction
between caching and revocation of permissions.
### Checking API Access
Kubernetes exposes the `subjectaccessreviews.v1beta1.authorization.k8s.io` resource as a
normal resource that allows external access to API authorizer decisions. No matter which authorizer
you choose to use, you can issue a `POST` with a `SubjectAccessReview` just like the webhook
authorizer to the `apis/authorization.k8s.io/v1beta1/subjectaccessreviews` endpoint and
get back a response. For instance:
```bash
kubectl create --v=8 -f - << __EOF__
{
"apiVersion": "authorization.k8s.io/v1beta1",
"kind": "SubjectAccessReview",
"spec": {
"resourceAttributes": {
"namespace": "kittensandponies",
"verb": "GET",
"group": "unicorn.example.org",
"resource": "pods"
},
"user": "jane",
"group": [
"group1",
"group2"
]
}
}
__EOF__
--- snip lots of output ---
I0913 08:12:31.362873 27425 request.go:908] Response Body: {"kind":"SubjectAccessReview","apiVersion":"authorization.k8s.io/v1beta1","metadata":{"creationTimestamp":null},"spec":{"resourceAttributes":{"namespace":"kittensandponies","verb":"GET","group":"unicorn.example.org","resource":"pods"},"user":"jane","group":["group1","group2"]},"status":{"allowed":true}}
subjectaccessreview "" created
```
This is useful for debugging access problems, in that you can use this resource
to determine what access an authorizer is granting.