About Security Context Constraints

Similar to the way that RBAC resources control user access, administrators can use Security Context Constraints (SCCs) to control permissions for pods. These permissions include actions that a pod, a collection of containers, can perform and what resources it can access. You can use SCCs to define a set of conditions that a pod must run with in order to be accepted into the system.

SCCs allow an administrator to control:

  • Whether a pod can run rivileged containers.

  • The capabilities that a container can request.

  • The use of host directories as volumes.

  • The SELinux context of the container.

  • The container user ID.

  • The use of host namespaces and networking.

  • The allocation of an FSGroup that owns the pod’s volumes.

  • The configuration of allowable supplemental groups.

  • Whether a container requires the use of a read only root file system.

  • The usage of volume types.

  • The configuration of allowable seccomp profiles.

Docker has a default list of capabilities that are allowed for each container of a pod. The containers use the capabilities from this default list, but pod manifest authors can alter it by requesting additional capabilities or removing some of the default behaviors. Use the allowedCapabilities, defaultAddCapabilities, and requiredDropCapabilities parameters to control such requests from the pods and to dictate which capabilities can be requested, which ones must be added to each container, and which ones must be forbidden.

The cluster contains eight default SCCs:

  • anyuid

  • hostaccess

  • hostmount-anyuid

  • hostnetwork

  • node-exporter

  • nonroot

  • privileged

  • restricted

Do not modify the default SCCs. Customizing the default SCCs can lead to issues when OpenShift Container Platform is upgraded. Instead, create new SCCs.

The privileged SCC allows:

  • Users to run privileged pods

  • Pods to mount host directories as volumes

  • Pods to run as any user

  • Pods to run with any MCS label

  • Pods to use the host’s IPC namespace

  • Pods to use the host’s PID namespace

  • Pods to use any FSGroup

  • Pods to use any supplemental group

  • Pods to use any seccomp profiles

  • Pods to request any capabilities

The restricted SCC:

  • Ensures that pods cannot run as privileged.

  • Ensures that pods cannot mount host directory volumes.

  • Requires that a pod run as a user in a pre-allocated range of UIDs.

  • Requires that a pod run with a pre-allocated MCS label.

  • Allows pods to use any FSGroup.

  • Allows pods to use any supplemental group.

For more information about each SCC, see the kubernetes.io/description annotation available on the SCC.

SCCs are composed of settings and strategies that control the security features a pod has access to. These settings fall into three categories:

Controlled by a boolean

Fields of this type default to the most restrictive value. For example, AllowPrivilegedContainer is always set to false if unspecified.

Controlled by an allowable set

Fields of this type are checked against the set to ensure their value is allowed.

Controlled by a strategy

Items that have a strategy to generate a value provide:

  • A mechanism to generate the value, and

  • A mechanism to ensure that a specified value falls into the set of allowable values.

SCC Strategies

RunAsUser
  1. MustRunAs - Requires a runAsUser to be configured. Uses the configured runAsUser as the default. Validates against the configured runAsUser.

  2. MustRunAsRange - Requires minimum and maximum values to be defined if not using pre-allocated values. Uses the minimum as the default. Validates against the entire allowable range.

  3. MustRunAsNonRoot - Requires that the pod be submitted with a non-zero runAsUser or have the USER directive defined in the image. No default provided.

  4. RunAsAny - No default provided. Allows any runAsUser to be specified.

SELinuxContext
  1. MustRunAs - Requires seLinuxOptions to be configured if not using pre-allocated values. Uses seLinuxOptions as the default. Validates against seLinuxOptions.

  2. RunAsAny - No default provided. Allows any seLinuxOptions to be specified.

SupplementalGroups
  1. MustRunAs - Requires at least one range to be specified if not using pre-allocated values. Uses the minimum value of the first range as the default. Validates against all ranges.

  2. RunAsAny - No default provided. Allows any supplementalGroups to be specified.

FSGroup
  1. MustRunAs - Requires at least one range to be specified if not using pre-allocated values. Uses the minimum value of the first range as the default. Validates against the first ID in the first range.

  2. RunAsAny - No default provided. Allows any fsGroup ID to be specified.

Controlling volumes

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

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

The list of allowable volume types is not exhaustive because new types are added with each release of OpenShift Container Platform.

For backwards compatibility, the usage of allowHostDirVolumePlugin overrides settings in the volumes field. For example, if allowHostDirVolumePlugin is set to false but allowed in the volumes field, then the hostPath value will be removed from volumes.

Admission

Admission control with SCCs allows for control over the creation of resources based on the capabilities granted to a user.

In terms of the SCCs, this means that an admission controller can inspect the user information made available in the context to retrieve an appropriate set of SCCs. Doing so ensures the pod is authorized to make requests about its operating environment or to generate a set of constraints to apply to the pod.

The set of SCCs that admission uses to authorize a pod are determined by the user identity and groups that the user belongs to. Additionally, if the pod specifies a service account, the set of allowable SCCs includes any constraints accessible to the service account.

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

  1. Retrieve all SCCs 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 constraints.

If a matching set of constraints is found, then the pod is accepted. If the request cannot be matched to an SCC, the pod is rejected.

A pod must validate every field against the SCC. The following are examples for just two of the fields that must be validated:

These examples are in the context of a strategy using the preallocated values.

A FSGroup SCC strategy of MustRunAs

If the pod defines a fsGroup ID, then that ID must equal the default fsGroup ID. Otherwise, the pod is not validated by that SCC and the next SCC is evaluated.

If the SecurityContextConstraints.fsGroup field has value RunAsAny and the pod specification omits the Pod.spec.securityContext.fsGroup, then this field is considered valid. Note that it is possible that during validation, other SCC settings will reject other pod fields and thus cause the pod to fail.

A SupplementalGroups SCC strategy of MustRunAs

If the pod specification defines one or more supplementalGroups IDs, then the pod’s IDs must equal one of the IDs in the namespace’s openshift.io/sa.scc.supplemental-groups annotation. Otherwise, the pod is not validated by that SCC and the next SCC is evaluated.

If the SecurityContextConstraints.supplementalGroups field has value RunAsAny and the pod specification omits the Pod.spec.securityContext.supplementalGroups, then this field is considered valid. Note that it is possible that during validation, other SCC settings will reject other pod fields and thus cause the pod to fail.

SCC prioritization

SCCs have a priority field that affects the ordering when attempting to validate a request by the admission controller. A higher priority SCC is moved to the front of the set when sorting. When the complete set of available SCCs are determined they are ordered by:

  1. Highest priority first, nil is considered a 0 priority

  2. If priorities are equal, the SCCs will be sorted from most restrictive to least restrictive

  3. If both priorities and restrictions are equal the SCCs will be sorted by name

By default, the anyuid SCC granted to cluster administrators is given priority in their SCC set. This allows cluster administrators to run pods as any user by without specifying a RunAsUser on the pod’s SecurityContext. The administrator may still specify a RunAsUser if they wish.

About pre-allocated Security Context Constraints values

The admission controller is aware of certain conditions in the Security Context Constraints (SCCs) that trigger it to look up pre-allocated values from a namespace and populate the SCC before processing the pod. Each SCC strategy is evaluated independently of other strategies, with the pre-allocated values, where allowed, for each policy aggregated with pod specification values to make the final values for the various IDs defined in the running pod.

The following SCCs cause the admission controller to look for pre-allocated values when no ranges are defined in the pod specification:

  1. A RunAsUser strategy of MustRunAsRange with no minimum or maximum set. Admission looks for the openshift.io/sa.scc.uid-range annotation to populate range fields.

  2. An SELinuxContext strategy of MustRunAs with no level set. Admission looks for the openshift.io/sa.scc.mcs annotation to populate the level.

  3. A FSGroup strategy of MustRunAs. Admission looks for the openshift.io/sa.scc.supplemental-groups annotation.

  4. A SupplementalGroups strategy of MustRunAs. Admission looks for the openshift.io/sa.scc.supplemental-groups annotation.

During the generation phase, the security context provider uses default values for any parameter values that are not specifically set in the pod. Default values are based on the selected strategy:

  1. RunAsAny and MustRunAsNonRoot strategies do not provide default values. If the pod needs a parameter value, such as a group ID, you must define the value in the pod specification.

  2. MustRunAs (single value) strategies provide a default value that is always used. For example, for group IDs, even if the pod specification defines its own ID value, the namespace’s default parameter value also appears in the pod’s groups.

  3. MustRunAsRange and MustRunAs (range-based) strategies provide the minimum value of the range. As with a single value MustRunAs strategy, the namespace’s default parameter value appears in the running pod. If a range-based strategy is configurable with multiple ranges, it provides the minimum value of the first configured range.

FSGroup and SupplementalGroups strategies fall back to the openshift.io/sa.scc.uid-range annotation if the openshift.io/sa.scc.supplemental-groups annotation does not exist on the namespace. If neither exists, the SCC is not created.

By default, the annotation-based FSGroup strategy configures itself with a single range based on the minimum value for the annotation. For example, if your annotation reads 1/3, the FSGroup strategy configures itself with a minimum and maximum value of 1. If you want to allow more groups to be accepted for the FSGroup field, you can configure a custom SCC that does not use the annotation.

The openshift.io/sa.scc.supplemental-groups annotation accepts a comma-delimited list of blocks in the format of <start>/<length or <start>-<end>. The openshift.io/sa.scc.uid-range annotation accepts only a single block.

Example Security Context Constraints

The following examples show the Security Context Constraint (SCC) format and annotations:

Annotated priviledged SCC
allowHostDirVolumePlugin: true
allowHostIPC: true
allowHostNetwork: true
allowHostPID: true
allowHostPorts: true
allowPrivilegedContainer: true
allowedCapabilities: (1)
- '*'
apiVersion: v1
defaultAddCapabilities: [] (2)
fsGroup: (3)
  type: RunAsAny
groups: (4)
- system:cluster-admins
- system:nodes
kind: SecurityContextConstraints
metadata:
  annotations:
    kubernetes.io/description: 'privileged allows access to all privileged and host
      features and the ability to run as any user, any group, any fsGroup, and with
      any SELinux context.  WARNING: this is the most relaxed SCC and should be used
      only for cluster administration. Grant with caution.'
  creationTimestamp: null
  name: privileged
priority: null
readOnlyRootFilesystem: false
requiredDropCapabilities: [] (5)
runAsUser: (6)
  type: RunAsAny
seLinuxContext: (7)
  type: RunAsAny
seccompProfiles:
- '*'
supplementalGroups: (8)
  type: RunAsAny
users: (9)
- system:serviceaccount:default:registry
- system:serviceaccount:default:router
- system:serviceaccount:openshift-infra:build-controller
volumes:
- '*'
1 A list of capabilities that a pod can request. An empty list means that none of capabilities can be requested while the special symbol * allows any capabilities.
2 A list of additional capabilities that are added to any pod.
3 The FSGroup strategy, which dictates the allowable values for the Security Context.
4 The groups that can access this SCC.
5 A list of capabilities that are be dropped from a pod.
6 The runAsUser strategy type, which dictates the allowable values for the Security Context.
7 The seLinuxContext strategy type, which dictates the allowable values for the Security Context.
8 The supplementalGroups strategy, which dictates the allowable supplemental groups for the Security Context.
9 The users who can access this SCC.

The users and groups fields on the SCC control which users can access the SCC. By default, cluster administrators, nodes, and the build controller are granted access to the privileged SCC. All authenticated users are granted access to the restricted SCC.

Without explicit runAsUser setting
apiVersion: v1
kind: Pod
metadata:
  name: security-context-demo
spec:
  securityContext: (1)
  containers:
  - name: sec-ctx-demo
    image: gcr.io/google-samples/node-hello:1.0
1 When a container or pod does not request a user ID under which it should be run, the effective UID depends on the SCC that emits this pod. Because restricted SCC is granted to all authenticated users by default, it will be available to all users and service accounts and used in most cases. The restricted SCC uses MustRunAsRange strategy for constraining and defaulting the possible values of the securityContext.runAsUser field. The admission plug-in will look for the openshift.io/sa.scc.uid-range annotation on the current project to populate range fields, as it does not provide this range. In the end, a container will have runAsUser equal to the first value of the range that is hard to predict because every project has different ranges.
With explicit runAsUser setting
apiVersion: v1
kind: Pod
metadata:
  name: security-context-demo
spec:
  securityContext:
    runAsUser: 1000 (1)
  containers:
    - name: sec-ctx-demo
      image: gcr.io/google-samples/node-hello:1.0
1 A container or pod that requests a specific user ID will be accepted by OpenShift Container Platform only when a service account or a user is granted access to a SCC that allows such a user ID. The SCC can allow arbitrary IDs, an ID that falls into a range, or the exact user ID specific to the request.

This configuration is valid for SELinux, fsGroup, and Supplemental Groups.

Creating Security Context Constraints

You can create a Security Context Constraint (SCC) by using the CLI.

Prerequisites
  • You must install the oc command line.

  • Your account must have cluster-admin privileges to create SCCs.

Procedure
  1. Define the SCC in a JSON or YAML file:

    Security Context Constraint Object Definition
    kind: SecurityContextConstraints
    apiVersion: v1
    metadata:
      name: scc-admin
    allowPrivilegedContainer: true
    runAsUser:
      type: RunAsAny
    seLinuxContext:
      type: RunAsAny
    fsGroup:
      type: RunAsAny
    supplementalGroups:
      type: RunAsAny
    users:
    - my-admin-user
    groups:
    - my-admin-group

    Optionally, you can add drop capabilities to an SCC by setting the requiredDropCapabilities field with the desired values. Any specified capabilities will be dropped from the container. For example, to create an SCC with the KILL, MKNOD, and SYS_CHROOT required drop capabilities, add the following to the SCC object:

    requiredDropCapabilities:
    - KILL
    - MKNOD
    - SYS_CHROOT

    You can see the list of possible values in the Docker documentation.

    Because capabilities are passed to the Docker, you can use a special ALL value to drop all possible capabilities.

  2. Then, run oc create passing the file to create it:

    $ oc create -f scc_admin.yaml
    securitycontextconstraints "scc-admin" created
  3. Verify that the SCC was created:

    $ oc get scc scc-admin
    NAME        PRIV      CAPS      SELINUX    RUNASUSER   FSGROUP    SUPGROUP   PRIORITY   READONLYROOTFS   VOLUMES
    scc-admin   true      []        RunAsAny   RunAsAny    RunAsAny   RunAsAny   <none>     false            [awsElasticBlockStore azureDisk azureFile cephFS cinder configMap downwardAPI emptyDir fc flexVolume flocker gcePersistentDisk gitRepo glusterfs iscsi nfs persistentVolumeClaim photonPersistentDisk quobyte rbd secret vsphere]

Role-based access to Security Context Constraints

You can specify SCCs as resources that are handled by RBAC. This allows you to scope access to your SCCs to a certain project or to the entire cluster. Assigning users, groups, or service accounts directly to an SCC retains cluster-wide scope.

To include access to SCCs for your role, specify the scc resource when creating a role.

$ oc create role <role-name> --verb=use --resource=scc --resource-name=<scc-name> -n <namespace>

This results in the following role definition:

apiVersion: rbac.authorization.k8s.io/v1
kind: Role
metadata:
...
  name: role-name (1)
  namespace: namespace (2)
...
rules:
- apiGroups:
  - security.openshift.io (3)
  resourceNames:
  - scc-name (4)
  resources:
  - securitycontextconstraints (5)
  verbs: (6)
  - use
1 The role’s name.
2 Namespace of the defined role. Defaults to default if not specified.
3 The API group that includes the SecurityContextConstraint resource. Automatically defined when scc is specified as a resource.
4 An example name for an SCC you want to have access.
5 Name of the resource group that allows users to specify SCC names in the resourceNames field.
6 A list of verbs to apply to the role.

A local or cluster role with such a rule allows the subjects that are bound to it with a RoleBinding or a ClusterRoleBinding to use the user-defined SCC called scc-name.

Because RBAC is designed to prevent escalation, even project administrators will be unable to grant access to an SCC. By defualt, they are not allowed to use the verb use on SCC resources, including the restricted SCC.

Security Context Constraints reference commands

You can manage SCCs in your instance as normal API objects using the CLI.

You must have cluster-admin privileges to manage SCCs.

Do not modify the default SCCs. Customizing the default SCCs can lead to issues when upgrading. Instead, create new SCCs.

Listing SCCs

To get a current list of SCCs:

$ oc get scc

NAME               PRIV    CAPS   SELINUX     RUNASUSER          FSGROUP     SUPGROUP    PRIORITY   READONLYROOTFS   VOLUMES
anyuid             false   []     MustRunAs   RunAsAny           RunAsAny    RunAsAny    10         false            [configMap downwardAPI emptyDir persistentVolumeClaim projected secret]
hostaccess         false   []     MustRunAs   MustRunAsRange     MustRunAs   RunAsAny    <none>     false            [configMap downwardAPI emptyDir hostPath persistentVolumeClaim projected secret]
hostmount-anyuid   false   []     MustRunAs   RunAsAny           RunAsAny    RunAsAny    <none>     false            [configMap downwardAPI emptyDir hostPath nfs persistentVolumeClaim projected secret]
hostnetwork        false   []     MustRunAs   MustRunAsRange     MustRunAs   MustRunAs   <none>     false            [configMap downwardAPI emptyDir persistentVolumeClaim projected secret]
node-exporter      false   []     RunAsAny    RunAsAny           RunAsAny    RunAsAny    <none>     false            [*]
nonroot            false   []     MustRunAs   MustRunAsNonRoot   RunAsAny    RunAsAny    <none>     false            [configMap downwardAPI emptyDir persistentVolumeClaim projected secret]
privileged         true    [*]    RunAsAny    RunAsAny           RunAsAny    RunAsAny    <none>     false            [*]
restricted         false   []     MustRunAs   MustRunAsRange     MustRunAs   RunAsAny    <none>     false            [configMap downwardAPI emptyDir persistentVolumeClaim projected secret]

Examining an SCC

To examine a particular SCC, use oc get, oc describe, or oc edit. For example, to examine the restricted SCC:

$ oc describe scc restricted
Name:					restricted
Priority:				<none>
Access:
  Users:				<none>
  Groups:				system:authenticated
Settings:
  Allow Privileged:			false
  Default Add Capabilities:		<none>
  Required Drop Capabilities:		KILL,MKNOD,SYS_CHROOT,SETUID,SETGID
  Allowed Capabilities:			<none>
  Allowed Seccomp Profiles:		<none>
  Allowed Volume Types:			configMap,downwardAPI,emptyDir,persistentVolumeClaim,projected,secret
  Allow Host Network:			false
  Allow Host Ports:			false
  Allow Host PID:			false
  Allow Host IPC:			false
  Read Only Root Filesystem:		false
  Run As User Strategy: MustRunAsRange
    UID:				<none>
    UID Range Min:			<none>
    UID Range Max:			<none>
  SELinux Context Strategy: MustRunAs
    User:				<none>
    Role:				<none>
    Type:				<none>
    Level:				<none>
  FSGroup Strategy: MustRunAs
    Ranges:				<none>
  Supplemental Groups Strategy: RunAsAny
    Ranges:				<none>

To preserve customized SCCs during upgrades, do not edit settings on the default SCCs.

Deleting an SCC

To delete an SCC:

$ oc delete scc <scc_name>

If you delete a default SCC, it will regenerate when you restart the cluster.

Updating an SCC

To update an existing SCC:

$ oc edit scc <scc_name>

To preserve customized SCCs during upgrades, do not edit settings on the default SCCs.