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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 can perform and what resources it can access. You can use SCCs to define a set of conditions that a pod must run with to be accepted into the system.

Security context constraints allow an administrator to control:

  • Whether a pod can run privileged containers with the allowPrivilegedContainer flag.

  • Whether a pod is constrained with the allowPrivilegeEscalation flag.

  • 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 volumes

  • The configuration of allowable supplemental groups

  • Whether a container requires write access to its root file system

  • The usage of volume types

  • The configuration of allowable seccomp profiles

Do not set the openshift.io/run-level label on any namespaces in OpenShift Container Platform. This label is for use by internal OpenShift Container Platform components to manage the startup of major API groups, such as the Kubernetes API server and OpenShift API server. If the openshift.io/run-level label is set, no SCCs are applied to pods in that namespace, causing any workloads running in that namespace to be highly privileged.

Default security context constraints

The cluster contains several default security context constraints (SCCs) as described in the table below. Additional SCCs might be installed when you install Operators or other components to OpenShift Container Platform.

Do not modify the default SCCs. Customizing the default SCCs can lead to issues when some of the platform pods deploy or OpenShift Container Platform is upgraded. During upgrades between some versions of OpenShift Container Platform, the values of the default SCCs are reset to the default values, which discards all customizations to those SCCs.

Instead, create new SCCs as needed.

Table 1. Default security context constraints
Security context constraint Description

anyuid

Provides all features of the restricted SCC, but allows users to run with any UID and any GID.

hostaccess

Allows access to all host namespaces but still requires pods to be run with a UID and SELinux context that are allocated to the namespace.

This SCC allows host access to namespaces, file systems, and PIDs. It should only be used by trusted pods. Grant with caution.

hostmount-anyuid

Provides all the features of the restricted SCC, but allows host mounts and running as any UID and any GID on the system.

This SCC allows host file system access as any UID, including UID 0. Grant with caution.

hostnetwork

Allows using host networking and host ports but still requires pods to be run with a UID and SELinux context that are allocated to the namespace.

If additional workloads are run on control plane hosts, use caution when providing access to hostnetwork. A workload that runs hostnetwork on a control plane host is effectively root on the cluster and must be trusted accordingly.

hostnetwork-v2

Like the hostnetwork SCC, but with the following differences:

  • ALL capabilities are dropped from containers.

  • The NET_BIND_SERVICE capability can be added explicitly.

  • seccompProfile is set to runtime/default by default.

  • allowPrivilegeEscalation must be unset or set to false in security contexts.

node-exporter

Used for the Prometheus node exporter.

This SCC allows host file system access as any UID, including UID 0. Grant with caution.

nonroot

Provides all features of the restricted SCC, but allows users to run with any non-root UID. The user must specify the UID or it must be specified in the manifest of the container runtime.

nonroot-v2

Like the nonroot SCC, but with the following differences:

  • ALL capabilities are dropped from containers.

  • The NET_BIND_SERVICE capability can be added explicitly.

  • seccompProfile is set to runtime/default by default.

  • allowPrivilegeEscalation must be unset or set to false in security contexts.

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.

This is the most relaxed SCC and should be used only for cluster administration. Grant with caution.

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

Setting privileged: true in the pod specification does not necessarily select the privileged SCC. The SCC that has allowPrivilegedContainer: true and has the highest prioritization will be chosen if the user has the permissions to use it.

restricted

Denies access to all host features and requires pods to be run with a UID, and SELinux context that are allocated to the namespace.

The restricted SCC:

  • Ensures that pods cannot run as privileged

  • Ensures that pods cannot mount host directory volumes

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

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

  • Allows pods to use any FSGroup

  • Allows pods to use any supplemental group

In clusters that were upgraded from OpenShift Container Platform 4.10 or earlier, this SCC is available for use by any authenticated user. The restricted SCC is no longer available to users of new OpenShift Container Platform 4.11 installations, unless the access is explicitly granted.

restricted-v2

Like the restricted SCC, but with the following differences:

  • ALL capabilities are dropped from containers.

  • The NET_BIND_SERVICE capability can be added explicitly.

  • seccompProfile is set to runtime/default by default.

  • allowPrivilegeEscalation must be unset or set to false in security contexts.

This is the most restrictive SCC provided by a new installation and will be used by default for authenticated users.

The restricted-v2 SCC is the most restrictive of the SCCs that is included by default with the system. However, you can create a custom SCC that is even more restrictive. For example, you can create an SCC that restricts readOnlyRootFilesystem to true.

Security context constraints settings

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

Category Description

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.

CRI-O has the following default list of capabilities that are allowed for each container of a pod:

  • CHOWN

  • DAC_OVERRIDE

  • FSETID

  • FOWNER

  • SETGID

  • SETUID

  • SETPCAP

  • NET_BIND_SERVICE

  • KILL

The containers use the capabilities from this default list, but pod manifest authors can alter the list 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. With these parameters you can specify which capabilities can be requested, which ones must be added to each container, and which ones must be forbidden, or dropped, from each container.

You can drop all capabilites from containers by setting the requiredDropCapabilities parameter to ALL. This is what the restricted-v2 SCC does.

Security context constraints strategies

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

  • 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.

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

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

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

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

SupplementalGroups
  • 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.

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

FSGroup
  • 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.

  • 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.

This 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 control

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 pre-allocated values.

An 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.

Security context constraints prioritization

Security context constraints (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 va