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Concepts

Detailed explanations of Kubernetes system concepts and abstractions.

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Pod Security Policies

Objects of type PodSecurityPolicy govern the ability to make requests on a pod that affect the SecurityContext that will be applied to a pod and container.

See PodSecurityPolicy proposal for more information.

What is a Pod Security Policy?

A Pod Security Policy is a cluster-level resource that controls the actions that a pod can perform and what it has the ability to access. The PodSecurityPolicy objects define a set of conditions that a pod must run with in order to be accepted into the system. They allow an administrator to control the following:

Control Aspect Field Name
Running of privileged containers privileged
Default set of capabilities that will be added to a container defaultAddCapabilities
Capabilities that will be dropped from a container requiredDropCapabilities
Capabilities a container can request to be added allowedCapabilities
Controlling the usage of volume types volumes
The use of host networking hostNetwork
The use of host ports hostPorts
The use of host’s PID namespace hostPID
The use of host’s IPC namespace hostIPC
The SELinux context of the container seLinux
The user ID runAsUser
Configuring allowable supplemental groups supplementalGroups
Allocating an FSGroup that owns the pod’s volumes fsGroup
Requiring the use of a read only root file system readOnlyRootFilesystem

Pod Security Policies are comprised of settings and strategies that control the security features a pod has access to. These settings fall into three categories:

Strategies

RunAsUser

SELinux

SupplementalGroups

FSGroup

Controlling Volumes

The usage of specific volume types can be controlled by setting the volumes field of the PSP. The allowable values of this field correspond to the volume sources that are defined when creating a volume:

  1. azureFile
  2. azureDisk
  3. flocker
  4. flexVolume
  5. hostPath
  6. emptyDir
  7. gcePersistentDisk
  8. awsElasticBlockStore
  9. gitRepo
  10. secret
  11. nfs
  12. iscsi
  13. glusterfs
  14. persistentVolumeClaim
  15. rbd
  16. cinder
  17. cephFS
  18. downwardAPI
  19. fc
  20. configMap
  21. vsphereVolume
  22. quobyte
  23. photonPersistentDisk
  24. projected
  25. portworxVolume
  26. scaleIO
  27. * (allow all volumes)

The recommended minimum set of allowed volumes for new PSPs are configMap, downwardAPI, emptyDir, persistentVolumeClaim, secret, and projected.

Host Network

Admission

Admission control with PodSecurityPolicy allows for control over the creation and modification of resources based on the capabilities allowed in the cluster.

Admission uses the following approach to create the final security context for the pod:

  1. Retrieve all PSPs available for use.
  2. Generate field values for security context settings that were not specified on the request.
  3. Validate the final settings against the available policies.

If a matching policy is found, then the pod is accepted. If the request cannot be matched to a PSP, the pod is rejected.

A pod must validate every field against the PSP.

Creating a Pod Security Policy

Here is an example Pod Security Policy. It has permissive settings for all fields

psp.yaml 
apiVersion: extensions/v1beta1
kind: PodSecurityPolicy
metadata:
  name: permissive
spec:
  seLinux:
    rule: RunAsAny
  supplementalGroups:
    rule: RunAsAny
  runAsUser:
    rule: RunAsAny
  fsGroup:
    rule: RunAsAny
  hostPorts:
  - min: 8000
    max: 8080
  volumes:
  - '*'

Create the policy by downloading the example file and then running this command:

$ kubectl create -f ./psp.yaml
podsecuritypolicy "permissive" created

Getting a list of Pod Security Policies

To get a list of existing policies, use kubectl get:

$ kubectl get psp
NAME        PRIV   CAPS  SELINUX   RUNASUSER         FSGROUP   SUPGROUP  READONLYROOTFS  VOLUMES
permissive  false  []    RunAsAny  RunAsAny          RunAsAny  RunAsAny  false           [*]
privileged  true   []    RunAsAny  RunAsAny          RunAsAny  RunAsAny  false           [*]
restricted  false  []    RunAsAny  MustRunAsNonRoot  RunAsAny  RunAsAny  false           [emptyDir secret downwardAPI configMap persistentVolumeClaim projected]

Editing a Pod Security Policy

To modify policy interactively, use kubectl edit:

$ kubectl edit psp permissive

This command will open a default text editor where you will be able to modify policy.

Deleting a Pod Security Policy

Once you don’t need a policy anymore, simply delete it with kubectl:

$ kubectl delete psp permissive
podsecuritypolicy "permissive" deleted

Enabling Pod Security Policies

In order to use Pod Security Policies in your cluster you must ensure the following

  1. You have enabled the api type extensions/v1beta1/podsecuritypolicy (only for versions prior 1.6)
  2. You have enabled the admission controller PodSecurityPolicy
  3. You have defined your policies

Working With RBAC

In Kubernetes 1.5 and newer, you can use PodSecurityPolicy to control access to privileged containers based on user role and groups. Access to different PodSecurityPolicy objects can be controlled via authorization. To limit access to PodSecurityPolicy objects for pods created via a Deployment, ReplicaSet, etc, the Controller Manager must be run against the secured API port, and must not have superuser permissions.

PodSecurityPolicy authorization uses the union of all policies available to the user creating the pod and the service account specified on the pod. When pods are created via a Deployment, ReplicaSet, etc, it is Controller Manager that creates the pod, so if it is running against the unsecured API port, all PodSecurityPolicy objects would be allowed, and you could not effectively subdivide access. Access to given PSP policies for a user will be effective only when deploying Pods directly. For more details, see the PodSecurityPolicy RBAC example of applying PodSecurityPolicy to control access to privileged containers based on role and groups when deploying Pods directly.

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