Seccomp and Kubernetes

Seccomp stands for secure computing mode and has been a feature of the Linux kernel since version 2.6.12. It can be used to sandbox the privileges of a process, restricting the calls it is able to make from userspace into the kernel. Kubernetes lets you automatically apply seccomp profiles loaded onto a node to your Pods and containers.

Seccomp fields

There are four ways to specify a seccomp profile for a pod:

• for the whole Pod using spec.securityContext.seccompProfile
• for a single container using spec.containers[*].securityContext.seccompProfile
• for an (restartable / sidecar) init container using spec.initContainers[*].securityContext.seccompProfile
• for an ephemeral container using spec.ephemeralContainers[*].securityContext.seccompProfile

pods/security/seccomp/fields.yaml

apiVersion: v1
kind: Pod
metadata:
  name: pod
spec:
  securityContext:
    seccompProfile:
      type: Unconfined
  ephemeralContainers:
  - name: ephemeral-container
    image: debian
    securityContext:
      seccompProfile:
        type: RuntimeDefault
  initContainers:
  - name: init-container
    image: debian
    securityContext:
      seccompProfile:
        type: RuntimeDefault
  containers:
  - name: container
    image: docker.io/library/debian:stable
    securityContext:
      seccompProfile:
        type: Localhost
        localhostProfile: my-profile.json

The Pod in the example above runs as Unconfined, while the ephemeral-container and init-container specifically defines RuntimeDefault. If the ephemeral or init container would not have set the securityContext.seccompProfile field explicitly, then the value would be inherited from the Pod. The same applies to the container, which runs a Localhost profile my-profile.json.

Generally speaking, fields from (ephemeral) containers have a higher priority than the Pod level value, while containers which do not set the seccomp field inherit the profile from the Pod.

The following values are possible for the seccompProfile.type:

Unconfined

The workload runs without any seccomp restrictions.

RuntimeDefault

A default seccomp profile defined by the container runtime is applied. The default profiles aim to provide a strong set of security defaults while preserving the functionality of the workload. It is possible that the default profiles differ between container runtimes and their release versions, for example when comparing those from CRI-O and containerd.

Localhost

The localhostProfile will be applied, which has to be available on the node disk (on Linux it's /var/lib/kubelet/seccomp). The availability of the seccomp profile is verified by the container runtime on container creation. If the profile does not exist, then the container creation will fail with a CreateContainerError.

Localhost profiles

Seccomp profiles are JSON files following the scheme defined by the OCI runtime specification. A profile basically defines actions based on matched syscalls, but also allows to pass specific values as arguments to syscalls. For example:

{
  "defaultAction": "SCMP_ACT_ERRNO",
  "defaultErrnoRet": 38,
  "syscalls": [
    {
      "names": [
        "adjtimex",
        "alarm",
        "bind",
        "waitid",
        "waitpid",
        "write",
        "writev"
      ],
      "action": "SCMP_ACT_ALLOW"
    }
  ]
}

The defaultAction in the profile above is defined as SCMP_ACT_ERRNO and will return as fallback to the actions defined in syscalls. The error is defined as code 38 via the defaultErrnoRet field.

The following actions are generally possible:

SCMP_ACT_ERRNO

Return the specified error code.

SCMP_ACT_ALLOW

Allow the syscall to be executed.

SCMP_ACT_KILL_PROCESS

Kill the process.

SCMP_ACT_KILL_THREAD and SCMP_ACT_KILL

Kill only the thread.

SCMP_ACT_TRAP

Throw a SIGSYS signal.

SCMP_ACT_NOTIFY and SECCOMP_RET_USER_NOTIF.

Notify the user space.

SCMP_ACT_TRACE

Notify a tracing process with the specified value.

SCMP_ACT_LOG

Allow the syscall to be executed after the action has been logged to syslog or auditd.

Some actions like SCMP_ACT_NOTIFY or SECCOMP_RET_USER_NOTIF may be not supported depending on the container runtime, OCI runtime or Linux kernel version being used. There may be also further limitations, for example that SCMP_ACT_NOTIFY cannot be used as defaultAction or for certain syscalls like write. All those limitations are defined by either the OCI runtime (runc, crun) or libseccomp.

The syscalls JSON array contains a list of objects referencing syscalls by their respective names. For example, the action SCMP_ACT_ALLOW can be used to create a whitelist of allowed syscalls as outlined in the example above. It would also be possible to define another list using the action SCMP_ACT_ERRNO but a different return (errnoRet) value.

It is also possible to specify the arguments (args) passed to certain syscalls. More information about those advanced use cases can be found in the OCI runtime spec and the Seccomp Linux kernel documentation.

Further reading

• Restrict a Container's Syscalls with seccomp
• Pod Security Standards