Configure a Security Context for a Pod or Container

    • Discretionary Access Control: Permission to access an object, like a file, is based on user ID (UID) and group ID (GID).

    • : Objects are assigned security labels.

    • Running as privileged or unprivileged.

    • Linux Capabilities: Give a process some privileges, but not all the privileges of the root user.

    • : Use program profiles to restrict the capabilities of individual programs.

    • Seccomp: Filter a process’s system calls.

    • AllowPrivilegeEscalation: Controls whether a process can gain more privileges than its parent process. This bool directly controls whether the flag gets set on the container process. AllowPrivilegeEscalation is true always when the container is: 1) run as Privileged OR 2) has .

    • readOnlyRootFilesystem: Mounts the container’s root filesystem as read-only.

    The above bullets are not a complete set of security context settings — please see SecurityContext for a comprehensive list.

    For more information about security mechanisms in Linux, see

    You need to have a Kubernetes cluster, and the kubectl command-line tool must be configured to communicate with your cluster. It is recommended to run this tutorial on a cluster with at least two nodes that are not acting as control plane hosts. If you do not already have a cluster, you can create one by using minikube or you can use one of these Kubernetes playgrounds:

    To check the version, enter kubectl version.

    Set the security context for a Pod

    To specify security settings for a Pod, include the securityContext field in the Pod specification. The securityContext field is a PodSecurityContext object. The security settings that you specify for a Pod apply to all Containers in the Pod. Here is a configuration file for a Pod that has a securityContext and an emptyDir volume:

    In the configuration file, the runAsUser field specifies that for any Containers in the Pod, all processes run with user ID 1000. The runAsGroup field specifies the primary group ID of 3000 for all processes within any containers of the Pod. If this field is omitted, the primary group ID of the containers will be root(0). Any files created will also be owned by user 1000 and group 3000 when runAsGroup is specified. Since fsGroup field is specified, all processes of the container are also part of the supplementary group ID 2000. The owner for volume /data/demo and any files created in that volume will be Group ID 2000.

    Create the Pod:

    1. kubectl apply -f https://k8s.io/examples/pods/security/security-context.yaml

    Verify that the Pod’s Container is running:

    1. kubectl get pod security-context-demo

    Get a shell to the running Container:

    1. kubectl exec -it security-context-demo -- sh

    In your shell, list the running processes:

    1. ps

    The output shows that the processes are running as user 1000, which is the value of runAsUser:

    1. PID   USER     TIME  COMMAND
    2.     1 1000      0:00 sleep 1h
    3.     6 1000      0:00 sh
    4. ...

    In your shell, navigate to /data, and list the one directory:

    1. cd /data
    2. ls -l

    The output shows that the /data/demo directory has group ID 2000, which is the value of fsGroup.

    1. drwxrwsrwx 2 root 2000 4096 Jun  6 20:08 demo

    In your shell, navigate to /data/demo, and create a file:

    1. cd demo
    2. echo hello > testfile

    List the file in the /data/demo directory:

    1. ls -l
    1. -rw-r--r-- 1 1000 2000 6 Jun  6 20:08 testfile

    Run the following command:

    1. $ id
    2. uid=1000 gid=3000 groups=2000

    You will see that gid is 3000 which is same as runAsGroup field. If the runAsGroup was omitted the gid would remain as 0(root) and the process will be able to interact with files that are owned by root(0) group and that have the required group permissions for root(0) group.

    Exit your shell:

    1. exit

    Configure volume permission and ownership change policy for Pods

    FEATURE STATE: Kubernetes v1.23 [stable]

    By default, Kubernetes recursively changes ownership and permissions for the contents of each volume to match the fsGroup specified in a Pod’s securityContext when that volume is mounted. For large volumes, checking and changing ownership and permissions can take a lot of time, slowing Pod startup. You can use the fsGroupChangePolicy field inside a securityContext to control the way that Kubernetes checks and manages ownership and permissions for a volume.

    fsGroupChangePolicy - fsGroupChangePolicy defines behavior for changing ownership and permission of the volume before being exposed inside a Pod. This field only applies to volume types that support fsGroup controlled ownership and permissions. This field has two possible values:

    • OnRootMismatch: Only change permissions and ownership if permission and ownership of root directory does not match with expected permissions of the volume. This could help shorten the time it takes to change ownership and permission of a volume.
    • Always: Always change permission and ownership of the volume when volume is mounted.

    For example:

    Note: This field has no effect on ephemeral volume types such as , configMap, and .

    FEATURE STATE: Kubernetes v1.23 [beta]

    If you deploy a Container Storage Interface (CSI) driver which supports the VOLUME_MOUNT_GROUP NodeServiceCapability, the process of setting file ownership and permissions based on the fsGroup specified in the securityContext will be performed by the CSI driver instead of Kubernetes, provided that the DelegateFSGroupToCSIDriver Kubernetes feature gate is enabled. In this case, since Kubernetes doesn’t perform any ownership and permission change, fsGroupChangePolicy does not take effect, and as specified by CSI, the driver is expected to mount the volume with the provided fsGroup, resulting in a volume that is readable/writable by the fsGroup.

    Please refer to the and the description of the VolumeCapability.MountVolume.volume_mount_group field in the CSI spec for more information.

    Set the security context for a Container

    To specify security settings for a Container, include the field in the Container manifest. The securityContext field is a SecurityContext object. Security settings that you specify for a Container apply only to the individual Container, and they override settings made at the Pod level when there is overlap. Container settings do not affect the Pod’s Volumes.

    Here is the configuration file for a Pod that has one Container. Both the Pod and the Container have a securityContext field:

    Configure a Security Context for a Pod or Container - 图2

    1. kind: Pod
    2. metadata:
    3.   name: security-context-demo-2
    4. spec:
    5.   securityContext:
    6.     runAsUser: 1000
    7.   containers:
    8.   - name: sec-ctx-demo-2
    9.     image: gcr.io/google-samples/node-hello:1.0
    10.     securityContext:
    11.       runAsUser: 2000
    12.       allowPrivilegeEscalation: false

    Create the Pod:

    1. kubectl apply -f https://k8s.io/examples/pods/security/security-context-2.yaml

    Verify that the Pod’s Container is running:

    1. kubectl get pod security-context-demo-2

    Get a shell into the running Container:

    1. kubectl exec -it security-context-demo-2 -- sh

    In your shell, list the running processes:

    1. ps aux

    The output shows that the processes are running as user 2000. This is the value of runAsUser specified for the Container. It overrides the value 1000 that is specified for the Pod.

    1. USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
    2. 2000         1  0.0  0.0   4336   764 ?        Ss   20:36   0:00 /bin/sh -c node server.js
    3. 2000         8  0.1  0.5 772124 22604 ?        Sl   20:36   0:00 node server.js
    4. ...

    Exit your shell:

    1. exit

    Set capabilities for a Container

    With , you can grant certain privileges to a process without granting all the privileges of the root user. To add or remove Linux capabilities for a Container, include the capabilities field in the securityContext section of the Container manifest.

    First, see what happens when you don’t include a capabilities field. Here is configuration file that does not add or remove any Container capabilities:

    pods/security/security-context-3.yaml 

    1. apiVersion: v1
    2. kind: Pod
    3. metadata:
    4.   name: security-context-demo-3
    5. spec:
    6.   containers:
    7.   - name: sec-ctx-3
    8.     image: gcr.io/google-samples/node-hello:1.0

    Create the Pod:

    1. kubectl apply -f https://k8s.io/examples/pods/security/security-context-3.yaml
    1. kubectl get pod security-context-demo-3

    Get a shell into the running Container:

    1. kubectl exec -it security-context-demo-3 -- sh

    In your shell, list the running processes:

    1. ps aux

    The output shows the process IDs (PIDs) for the Container:

    In your shell, view the status for process 1:

    1. cd /proc/1
    2. cat status

    The output shows the capabilities bitmap for the process:

    1. ...
    2. CapPrm:    00000000a80425fb
    3. CapEff:    00000000a80425fb
    4. ...

    Make a note of the capabilities bitmap, and then exit your shell:

    1. exit

    Next, run a Container that is the same as the preceding container, except that it has additional capabilities set.

    Here is the configuration file for a Pod that runs one Container. The configuration adds the CAP_NET_ADMIN and CAP_SYS_TIME capabilities:

    Configure a Security Context for a Pod or Container - 图4

    1. apiVersion: v1
    2. metadata:
    3.   name: security-context-demo-4
    4. spec:
    5.   containers:
    7.     image: gcr.io/google-samples/node-hello:1.0
    8.     securityContext:
    9.       capabilities:
    10.         add: ["NET_ADMIN", "SYS_TIME"]

    Create the Pod:

    1. kubectl apply -f https://k8s.io/examples/pods/security/security-context-4.yaml

    Get a shell into the running Container:

    1. kubectl exec -it security-context-demo-4 -- sh

    In your shell, view the capabilities for process 1:

    1. cd /proc/1
    2. cat status

    The output shows capabilities bitmap for the process:

    1. ...
    2. CapPrm:    00000000aa0435fb
    3. CapEff:    00000000aa0435fb
    4. ...

    Compare the capabilities of the two Containers:

    1. 00000000a80425fb
    2. 00000000aa0435fb

    In the capability bitmap of the first container, bits 12 and 25 are clear. In the second container, bits 12 and 25 are set. Bit 12 is CAP_NET_ADMIN, and bit 25 is CAP_SYS_TIME. See capability.h for definitions of the capability constants.

    Note: Linux capability constants have the form CAP_XXX. But when you list capabilities in your Container manifest, you must omit the CAP_ portion of the constant. For example, to add CAP_SYS_TIME, include SYS_TIME in your list of capabilities.

    To set the Seccomp profile for a Container, include the seccompProfile field in the securityContext section of your Pod or Container manifest. The seccompProfile field is a object consisting of type and localhostProfile. Valid options for type include RuntimeDefault, Unconfined, and Localhost. localhostProfile must only be set set if type: Localhost. It indicates the path of the pre-configured profile on the node, relative to the kubelet’s configured Seccomp profile location (configured with the --root-dir flag).

    Here is an example that sets the Seccomp profile to the node’s container runtime default profile:

    1. ...
    2. securityContext:
    3.   seccompProfile:
    4.     type: RuntimeDefault

    Here is an example that sets the Seccomp profile to a pre-configured file at <kubelet-root-dir>/seccomp/my-profiles/profile-allow.json:

    1. ...
    2. securityContext:
    3.   seccompProfile:
    4.     type: Localhost
    5.     localhostProfile: my-profiles/profile-allow.json

    Assign SELinux labels to a Container

    To assign SELinux labels to a Container, include the seLinuxOptions field in the securityContext section of your Pod or Container manifest. The seLinuxOptions field is an object. Here’s an example that applies an SELinux level:

    1. ...
    2. securityContext:
    3.   seLinuxOptions:
    4.     level: "s0:c123,c456"

    Note: To assign SELinux labels, the SELinux security module must be loaded on the host operating system.

    Discussion

    The security context for a Pod applies to the Pod’s Containers and also to the Pod’s Volumes when applicable. Specifically fsGroup and seLinuxOptions are applied to Volumes as follows:

    • fsGroup: Volumes that support ownership management are modified to be owned and writable by the GID specified in fsGroup. See the for more details.

    • seLinuxOptions: Volumes that support SELinux labeling are relabeled to be accessible by the label specified under seLinuxOptions. Usually you only need to set the section. This sets the Multi-Category Security (MCS) label given to all Containers in the Pod as well as the Volumes.

    Delete the Pod:

    What’s next