Chapter 15. Managing security context constraints
In OpenShift Container Platform, you can use security context constraints (SCCs) to control permissions for the pods in your cluster.
Default SCCs are created during installation and when you install some Operators or other components. As a cluster administrator, you can also create your own SCCs by using the OpenShift CLI (oc).
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. Additionally, the default SCC values are reset to the defaults during some cluster upgrades, which discards all customizations to those SCCs.
Instead of modifying the default SCCs, create and modify your own SCCs as needed. For detailed steps, see Creating security context constraints.
15.1. 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 determine the 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
allowPrivilegedContainerflag -
Whether a pod is constrained with the
allowPrivilegeEscalationflag - 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
FSGroupthat 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
seccompprofiles
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.
15.1.1. 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. Additionally, the default SCC values are reset to the defaults during some cluster upgrades, which discards all customizations to those SCCs.
Instead of modifying the default SCCs, create and modify your own SCCs as needed. For detailed steps, see Creating security context constraints.
| Security context constraint | Description |
|---|---|
|
|
Provides all features of the |
|
| 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. Warning This SCC allows host access to namespaces, file systems, and PIDs. It should only be used by trusted pods. Grant with caution. |
|
|
Provides all the features of the Warning This SCC allows host file system access as any UID, including UID 0. Grant with caution. |
|
| 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. Warning
If additional workloads are run on control plane hosts, use caution when providing access to |
|
|
Like the
|
|
| Used for the Prometheus node exporter. Warning This SCC allows host file system access as any UID, including UID 0. Grant with caution. |
|
|
Provides all features of the |
|
|
Like the
|
|
| 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.
The
Note
Setting |
|
| 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
In clusters that were upgraded from OpenShift Container Platform 4.10 or earlier, this SCC is available for use by any authenticated user. The |
|
|
Like the
This is the most restrictive SCC provided by a new installation and will be used by default for authenticated users. Note
The |
15.1.2. 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, |
| 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:
|
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.
15.1.3. Security context constraints strategies
RunAsUser
MustRunAs- Requires arunAsUserto be configured. Uses the configuredrunAsUseras the default. Validates against the configuredrunAsUser.Example
MustRunAssnippet... runAsUser: type: MustRunAs uid: <id> ...
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.Example
MustRunAsRangesnippet... runAsUser: type: MustRunAsRange uidRangeMax: <maxvalue> uidRangeMin: <minvalue> ...
MustRunAsNonRoot- Requires that the pod be submitted with a non-zerorunAsUseror have theUSERdirective defined in the image. No default provided.Example
MustRunAsNonRootsnippet... runAsUser: type: MustRunAsNonRoot ...
RunAsAny- No default provided. Allows anyrunAsUserto be specified.Example
RunAsAnysnippet... runAsUser: type: RunAsAny ...
SELinuxContext
-
MustRunAs- RequiresseLinuxOptionsto be configured if not using pre-allocated values. UsesseLinuxOptionsas the default. Validates againstseLinuxOptions. -
RunAsAny- No default provided. Allows anyseLinuxOptionsto 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 anysupplementalGroupsto 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 anyfsGroupID to be specified.
15.1.4. 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:
-
awsElasticBlockStore -
azureDisk -
azureFile -
cephFS -
cinder -
configMap -
downwardAPI -
emptyDir -
fc -
flexVolume -
flocker -
gcePersistentDisk -
ephemeral -
gitRepo -
glusterfs -
hostPath -
iscsi -
nfs -
persistentVolumeClaim -
photonPersistentDisk -
portworxVolume -
projected -
quobyte -
rbd -
scaleIO -
secret -
storageos -
vsphereVolume - * (A special value to allow the use of all volume types.)
-
none(A special value to disallow the use of all volumes types. Exists only for backwards compatibility.)
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.
15.1.5. 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.
When you create a workload resource, such as deployment, only the service account is used to find the SCCs and admit the pods when they are created.
Admission uses the following approach to create the final security context for the pod:
- Retrieve all SCCs available for use.
- Generate field values for security context settings that were not specified on the request.
- 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.
15.1.6. 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 priority value of 0 is the lowest possible priority. A nil priority is considered a 0, or lowest, priority. Higher priority SCCs are moved to the front of the set when sorting.
When the complete set of available SCCs is determined, the SCCs are ordered in the following manner:
- The highest priority SCCs are ordered first.
- If the priorities are equal, the SCCs are sorted from most restrictive to least restrictive.
- If both the priorities and restrictions are equal, the SCCs are 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 specifying RunAsUser in the pod’s SecurityContext.
15.2. 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:
-
A
RunAsUserstrategy ofMustRunAsRangewith no minimum or maximum set. Admission looks for theopenshift.io/sa.scc.uid-rangeannotation to populate range fields. -
An
SELinuxContextstrategy ofMustRunAswith no level set. Admission looks for theopenshift.io/sa.scc.mcsannotation to populate the level. -
A
FSGroupstrategy ofMustRunAs. Admission looks for theopenshift.io/sa.scc.supplemental-groupsannotation. -
A
SupplementalGroupsstrategy ofMustRunAs. Admission looks for theopenshift.io/sa.scc.supplemental-groupsannotation.
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:
-
RunAsAnyandMustRunAsNonRootstrategies 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. -
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. -
MustRunAsRangeandMustRunAs(range-based) strategies provide the minimum value of the range. As with a single valueMustRunAsstrategy, 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.
15.3. Example security context constraints
The following examples show the security context constraints (SCC) format and annotations:
Annotated privileged SCC
allowHostDirVolumePlugin: true allowHostIPC: true allowHostNetwork: true allowHostPID: true allowHostPorts: true allowPrivilegedContainer: true allowedCapabilities: 1 - '*' apiVersion: security.openshift.io/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 - KILL - MKNOD - SETUID - SETGID 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: 10 - '*'
- 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
FSGroupstrategy, which dictates the allowable values for the security context. - 4
- The groups that can access this SCC.
- 5
- A list of capabilities to drop from a pod. Or, specify
ALLto drop all capabilities. - 6
- The
runAsUserstrategy type, which dictates the allowable values for the security context. - 7
- The
seLinuxContextstrategy type, which dictates the allowable values for the security context. - 8
- The
supplementalGroupsstrategy, which dictates the allowable supplemental groups for the security context. - 9
- The users who can access this SCC.
- 10
- The allowable volume types for the security context. In the example,
*allows the use of all volume types.
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-v2 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 the
restricted-v2SCC is granted to all authenticated users by default, it will be available to all users and service accounts and used in most cases. Therestricted-v2SCC usesMustRunAsRangestrategy for constraining and defaulting the possible values of thesecurityContext.runAsUserfield. The admission plugin will look for theopenshift.io/sa.scc.uid-rangeannotation on the current project to populate range fields, as it does not provide this range. In the end, a container will haverunAsUserequal 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.
15.4. Creating security context constraints
If the default security context constraints (SCCs) do not satisfy your application workload requirements, you can create a custom SCC by using the OpenShift CLI (oc).
Creating and modifying your own SCCs are advanced operations that might cause instability to your cluster. If you have questions about using your own SCCs, contact Red Hat Support. For information about contacting Red Hat support, see Getting support.
Prerequisites
-
Install the OpenShift CLI (
oc). -
Log in to the cluster as a user with the
cluster-adminrole.
Procedure
Define the SCC in a YAML file named
scc-admin.yaml:kind: SecurityContextConstraints apiVersion: security.openshift.io/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 drop specific capabilities for an SCC by setting the
requiredDropCapabilitiesfield with the desired values. Any specified capabilities are dropped from the container. To drop all capabilities, specifyALL. For example, to create an SCC that drops theKILL,MKNOD, andSYS_CHROOTcapabilities, add the following to the SCC object:requiredDropCapabilities: - KILL - MKNOD - SYS_CHROOT
NoteYou cannot list a capability in both
allowedCapabilitiesandrequiredDropCapabilities.CRI-O supports the same list of capability values that are found in the Docker documentation.
Create the SCC by passing in the file:
$ oc create -f scc-admin.yaml
Example output
securitycontextconstraints "scc-admin" created
Verification
Verify that the SCC was created:
$ oc get scc scc-admin
Example output
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]
15.5. 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.
You cannot assign a SCC to pods created in one of the default namespaces: default, kube-system, kube-public, openshift-node, openshift-infra, openshift. These namespaces should not be used for running pods or services.
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
defaultif not specified. - 3
- The API group that includes the
SecurityContextConstraintsresource. Automatically defined whensccis 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
resourceNamesfield. - 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 role binding or a cluster role binding to use the user-defined SCC called scc-name.
Because RBAC is designed to prevent escalation, even project administrators are unable to grant access to an SCC. By default, they are not allowed to use the verb use on SCC resources, including the restricted-v2 SCC.
15.6. Reference of security context constraints commands
You can manage security context constraints (SCCs) in your instance as normal API objects by using the OpenShift CLI (oc).
You must have cluster-admin privileges to manage SCCs.
15.6.1. Listing security context constraints
To get a current list of SCCs:
$ oc get scc
Example output
NAME PRIV CAPS SELINUX RUNASUSER FSGROUP SUPGROUP PRIORITY READONLYROOTFS VOLUMES anyuid false <no value> MustRunAs RunAsAny RunAsAny RunAsAny 10 false ["configMap","downwardAPI","emptyDir","persistentVolumeClaim","projected","secret"] hostaccess false <no value> MustRunAs MustRunAsRange MustRunAs RunAsAny <no value> false ["configMap","downwardAPI","emptyDir","hostPath","persistentVolumeClaim","projected","secret"] hostmount-anyuid false <no value> MustRunAs RunAsAny RunAsAny RunAsAny <no value> false ["configMap","downwardAPI","emptyDir","hostPath","nfs","persistentVolumeClaim","projected","secret"] hostnetwork false <no value> MustRunAs MustRunAsRange MustRunAs MustRunAs <no value> false ["configMap","downwardAPI","emptyDir","persistentVolumeClaim","projected","secret"] hostnetwork-v2 false ["NET_BIND_SERVICE"] MustRunAs MustRunAsRange MustRunAs MustRunAs <no value> false ["configMap","downwardAPI","emptyDir","persistentVolumeClaim","projected","secret"] node-exporter true <no value> RunAsAny RunAsAny RunAsAny RunAsAny <no value> false ["*"] nonroot false <no value> MustRunAs MustRunAsNonRoot RunAsAny RunAsAny <no value> false ["configMap","downwardAPI","emptyDir","persistentVolumeClaim","projected","secret"] nonroot-v2 false ["NET_BIND_SERVICE"] MustRunAs MustRunAsNonRoot RunAsAny RunAsAny <no value> false ["configMap","downwardAPI","emptyDir","persistentVolumeClaim","projected","secret"] privileged true ["*"] RunAsAny RunAsAny RunAsAny RunAsAny <no value> false ["*"] restricted false <no value> MustRunAs MustRunAsRange MustRunAs RunAsAny <no value> false ["configMap","downwardAPI","emptyDir","persistentVolumeClaim","projected","secret"] restricted-v2 false ["NET_BIND_SERVICE"] MustRunAs MustRunAsRange MustRunAs RunAsAny <no value> false ["configMap","downwardAPI","emptyDir","persistentVolumeClaim","projected","secret"]
15.6.2. Examining security context constraints
You can view information about a particular SCC, including which users, service accounts, and groups the SCC is applied to.
For example, to examine the restricted SCC:
$ oc describe scc restricted
Example output
Name: restricted Priority: <none> Access: Users: <none> 1 Groups: <none> 2 Settings: Allow Privileged: false Allow Privilege Escalation: true Default Add Capabilities: <none> Required Drop Capabilities: KILL,MKNOD,SETUID,SETGID Allowed Capabilities: <none> Allowed Seccomp Profiles: <none> Allowed Volume Types: configMap,downwardAPI,emptyDir,persistentVolumeClaim,projected,secret Allowed Flexvolumes: <all> Allowed Unsafe Sysctls: <none> Forbidden Sysctls: <none> 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.
15.6.3. Updating security context constraints
If your custom SCC no longer satisfies your application workloads requirements, you can update your SCC by using the OpenShift CLI (oc).
To update an existing SCC:
$ oc edit scc <scc_name>
To preserve customized SCCs during upgrades, do not edit settings on the default SCCs.
15.6.4. Deleting security context constraints
If you no longer require your custom SCC, you can delete the SCC by using the OpenShift CLI (oc).
To delete an SCC:
$ oc delete scc <scc_name>
Do not delete default SCCs. If you delete a default SCC, it is regenerated by the Cluster Version Operator.
