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Managing Security Context Constraints | Cluster Administration | OpenShift Container Platform 3.5
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Managing Security Context Constraints

Overview

Security context constraints allow administrators to control permissions for pods. To learn more about this API type, see the security context constraints (SCCs) architecture documentation. 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 Security Context Constraints

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 secret]
hostaccess         false     []        MustRunAs   MustRunAsRange     MustRunAs   RunAsAny    <none>     false            [configMap downwardAPI emptyDir hostPath persistentVolumeClaim secret]
hostmount-anyuid   false     []        MustRunAs   RunAsAny           RunAsAny    RunAsAny    <none>     false            [configMap downwardAPI emptyDir hostPath persistentVolumeClaim secret]
hostnetwork        false     []        MustRunAs   MustRunAsRange     MustRunAs   MustRunAs   <none>     false            [configMap downwardAPI emptyDir persistentVolumeClaim secret]
nonroot            false     []        MustRunAs   MustRunAsNonRoot   RunAsAny    RunAsAny    <none>     false            [configMap downwardAPI emptyDir persistentVolumeClaim secret]
privileged         true      []        RunAsAny    RunAsAny           RunAsAny    RunAsAny    <none>     false            [*]
restricted         false     []        MustRunAs   MustRunAsRange     MustRunAs   RunAsAny    <none>     false            [configMap downwardAPI emptyDir persistentVolumeClaim secret]

Examining a Security Context Constraints Object

To examine a particular SCC, use oc get, oc describe, oc export, 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:			<none>
  Allowed Capabilities:				<none>
  Allowed Volume Types:				awsElasticBlockStore,azureFile,cephFS,cinder,configMap,downwardAPI,emptyDir,fc,flexVolume,flocker,gcePersistentDisk,gitRepo,glusterfs,iscsi,nfs,persistentVolumeClaim,rbd,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: RunAsAny
    Ranges:					<none>
  Supplemental Groups Strategy: RunAsAny
    Ranges:					<none>

In order to preserve customized SCCs during upgrades, do not edit settings on the default SCCs other than priority, users, groups, labels, and annotations.

Creating New Security Context Constraints

To create a new SCC:

  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.

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

    $ oc create -f scc_admin.yaml
securitycontextconstraints/scc-admin

  3. Verify that the SCC was created:

    $ oc get scc
NAME         PRIV      CAPS      HOSTDIR   SELINUX     RUNASUSER
privileged   true      []        true      RunAsAny    RunAsAny
restricted   false     []        false     MustRunAs   MustRunAsRange
scc-admin    true      []        false     RunAsAny    RunAsAny

Deleting Security Context Constraints

To delete an SCC:

$ oc delete scc <scc_name>

If you delete a default SCC, it will be regenerated upon restart.

Updating Security Context Constraints

To update an existing SCC:

$ oc edit scc <scc_name>

In order to preserve customized SCCs during upgrades, do not edit settings on the default SCCs other than priority, users, and groups.

Example Security Context Constraints Settings

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. See Understanding Pre-allocated Values and Security Context Constraints for more information.
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 works with SELinux, fsGroup, and Supplemental Groups. See Volume Security for more information.

Updating the Default Security Context Constraints

Default SCCs will be created when the master is started if they are missing. To reset SCCs to defaults, or update existing SCCs to new default definitions after an upgrade you may:

  1. Delete any SCC you would like to be reset and let it be recreated by restarting the master

  2. Use the oc adm policy reconcile-sccs command

The oc adm policy reconcile-sccs command will set all SCC policies to the default values but retain any additional users, groups, labels, and annotations as well as priorities you may have already set. To view which SCCs will be changed you may run the command with no options or by specifying your preferred output with the -o <format> option.

After reviewing it is recommended that you back up your existing SCCs and then use the --confirm option to persist the data.

If you would like to reset priorities and grants, use the --additive-only=false option.

If you have customized settings other than priority, users, groups, labels, or annotations in an SCC, you will lose those settings when you reconcile.

How Do I?

The following describe common scenarios and procedures using SCCs.

Grant Access to the Privileged SCC

In some cases, an administrator might want to allow users or groups outside the administrator group access to create more privileged pods. To do so, you can:

  1. Determine the user or group you would like to have access to the SCC.

    Granting access to a user only works when the user directly creates a pod. For pods created on behalf of a user, in most cases by the system itself, access should be given to a service account under which related controller is operated upon. Examples of resources that create pods on behalf of a user are Deployments, StatefulSets, DaemonSets, etc.

  2. Run:

    $ oc adm policy add-scc-to-user <scc_name> <user_name>
$ oc adm policy add-scc-to-group <scc_name> <group_name>

    For example, to allow the e2e-user access to the privileged SCC, run:

    $ oc adm policy add-scc-to-user privileged e2e-user

  3. Modify SecurityContext of a container to request a privileged mode.

Grant a service Account Access to the Privileged SCC

First, create a service account. For example, to create service account mysvcacct in project myproject:

$ oc create serviceaccount mysvcacct -n myproject

Then, add the service account to the privileged SCC.

$ oc adm policy add-scc-to-user privileged system:serviceaccount:myproject:mysvcacct

Then, ensure that the resource is being created on behalf of the service account. To do so, set the spec.serviceAccountName field to a service account name. Leaving the service account name blank will result in the default service account being used.

Then, ensure that at least one of the pod’s containers is requesting a privileged mode in the security context.

Enable Images to Run with USER in the Dockerfile

To relax the security in your cluster so that images are not forced to run as a pre-allocated UID, without granting everyone access to the privileged SCC:

  1. Grant all authenticated users access to the anyuid SCC:

    $ oc adm policy add-scc-to-group anyuid system:authenticated

This allows images to run as the root UID if no USER is specified in the Dockerfile.

Enable Container Images that Require Root

Some container images (examples: postgres and redis) require root access and have certain expectations about how volumes are owned. For these images, add the service account to the anyuid SCC.

$ oc adm policy add-scc-to-user anyuid system:serviceaccount:myproject:mysvcacct

Use --mount-host on the Registry

It is recommended that persistent storage using PersistentVolume and PersistentVolumeClaim objects be used for registry deployments. If you are testing and would like to instead use the oc adm registry command with the --mount-host option, you must first create a new service account for the registry and add it to the privileged SCC. See the Administrator Guide for full instructions.

Provide Additional Capabilities

In some cases, an image may require capabilities that Docker does not provide out of the box. You can provide the ability to request additional capabilities in the pod specification which will be validated against an SCC.

This allows images to run with elevated capabilities and should be used only if necessary. You should not edit the default restricted SCC to enable additional capabilities.

When used in conjunction with a non-root user, you must also ensure that the file that requires the additional capability is granted the capabilities using the setcap command. For example, in the Dockerfile of the image:

setcap cap_net_raw,cap_net_admin+p /usr/bin/ping

Further, if a capability is provided by default in Docker, you do not need to modify the pod specification to request it. For example, NET_RAW is provided by default and capabilities should already be set on ping, therefore no special steps should be required to run ping.

To provide additional capabilities:

  1. Create a new SCC

  2. Add the allowed capability using the allowedCapabilities field.

  3. When creating the pod, request the capability in the securityContext.capabilities.add field.

Modify Cluster Default Behavior

When you grant access to the anyuid SCC for everyone, your cluster:

  • Does not pre-allocate UIDs

  • Allows containers to run as any user

  • Prevents privileged containers

 $ oc adm policy add-scc-to-group anyuid system:authenticated

To modify your cluster so that it does not pre-allocate UIDs and does not allow containers to run as root, grant access to the nonroot SCC for everyone:

 $ oc adm policy add-scc-to-group nonroot system:authenticated

Be very careful with any modifications that have a cluster-wide impact. When you grant an SCC to all authenticated users, as in the previous example, or modify an SCC that applies to all users, such as the restricted SCC, it also affects Kubernetes and OpenShift Container Platform components, including the web console and integrated docker registry. Changes made with these SCCs can cause these components to stop functioning.

Instead, create a custom SCC and target it to only specific users or groups. This way potential issues are confined to the affected users or groups and do not impact critical cluster components.

Use the hostPath Volume Plug-in

To relax the security in your cluster so that pods are allowed to use the hostPath volume plug-in without granting everyone access to more privileged SCCs such as privileged, hostaccess, or hostmount-anyuid, perform the following actions:

  1. Create a new SCC named hostpath

  2. Set the allowHostDirVolumePlugin parameter to true for the new SCC:

    $ oc patch scc hostpath -p '{"allowHostDirVolumePlugin": true}'

  3. Grant access to this SCC to all users:

    $ oc adm policy add-scc-to-group hostpath system:authenticated

Now, all the pods that request hostPath volumes are admitted by the hostpath SCC.

Ensure That Admission Attempts to Use a Specific SCC First

You may control the sort ordering of SCCs in admission by setting the Priority field of the SCCs. Please see the SCC Prioritization section for more information on sorting.

Add an SCC to a User, Group, or Project

Before adding an SCC to a user or group, you can first use the scc-review option to check if the user or group can create a pod. See the Authorization topic for more information.

SCCs are not granted directly to a project. Instead, you add a service account to an SCC and either specify the service account name on your pod or, when unspecified, run as the default service account.

To add an SCC to a user:

$ oc adm policy add-scc-to-user <scc_name> <user_name>

To add an SCC to a service account:

$ oc adm policy add-scc-to-user <scc_name> \
    system:serviceaccount:<serviceaccount_namespace>:<serviceaccount_name>

If you are currently in the project to which the service account belongs, you can use the -z flag and just specify the <serviceaccount_name>.

$ oc adm policy add-scc-to-user <scc_name> -z <serviceaccount_name>

Usage of the -z flag as described above is highly recommended, as it helps prevent typos and ensures that access is granted only to the specified service account. If not in the project, use the -n option to indicate the project namespace it applies to.

To add an SCC to a group:

$ oc adm policy add-scc-to-group <scc_name> <group_name>

To add an SCC to all service accounts in a namespace:

$ oc adm policy add-scc-to-group <scc_name> \
    system:serviceaccounts:<serviceaccount_namespace>