The Kinvolk Seccomp Agent is receiving seccomp file descriptors from container runtimes and handling system calls on behalf of the containers. Its goal is to support different use cases:

• unprivileged container builds (procfs mounts with masked entries)
• support of safe mknod (e.g. /dev/null)

It is possible to write your own seccomp agent with a different behaviour by reusing the packages in the pkg/ directory. The Kinvolk Seccomp Agent is only about 100 lines of code. It relies on different packages:

• pkg/agent: listens on a unix socket to receive new seccomp file descriptors from the container runtime and associates a registry to them
• pkg/handlers: basic implementations of system call handlers, such as mkdir, mount...
• pkg/kuberesolver: allows users to assign a custom registry to the seccomp fd depending on the Kubernetes pod.
• pkg/nsenter: allows handlers implementations to execute code in different namespaces
• pkg/readarg: allows handlers implementations to dereference system call arguments.
• pkg/registry: a set of system call handlers associated to a seccomp file descriptor.

• Run the Seccomp Agent with the "demo-basic" container resolver.

sudo ./seccompagent -resolver=demo-basic

Demo of mount in a container without CAP_SYS_ADMIN:

/ # mount -t proc proc root
/ # ls /root/self/cmdline
/root/self/cmdline

• Demo of overriding a mkdir path:

/ # mkdir /abc
/ # ls -1d /ab*
/abc-pid-4072889

• Demo of overriding a chmod error:

/ # chmod 777 /
chmod: /: Bad message

This demo shows that the Seccomp Agent can have different behaviour depending on the Kubernetes pod (in this case, the pod's namespace and name).

• Install a seccomp policy: /var/lib/kubelet/seccomp/notify.json

{
   "architectures" : [
      "SCMP_ARCH_X86",
      "SCMP_ARCH_X32"
   ],
   "defaultAction" : "SCMP_ACT_ALLOW",
   "listenerPath": "/run/seccomp-agent.socket",
   "listenerMetadata": "MKDIR_TMPL=-{{.Namespace}}-{{.Pod}}-{{.Container}}\nEXEC_PATTERN=/bin/true\nEXEC_DURATION=2s\nMOUNT_PROC=true",
   "syscalls" : [
      {
         "action" : "SCMP_ACT_NOTIFY",
         "names" : [
            "openat",
            "open",
            "mkdir",
            "mount",
	    "chmod"
         ]
      }
   ]
}

• Deploy the seccomp agent:

kubectl apply -f deploy/seccompagent.yaml

• Deploy a pod with the seccomp policy:

apiVersion: v1
kind: Pod
metadata:
  name: mynotifypod
  # /var/lib/kubelet/seccomp/notify.json
  annotations:
    seccomp.security.alpha.kubernetes.io/pod: localhost/notify.json
spec:
  restartPolicy: Never
  containers:
  - name: container1
    image: busybox
    command: ["sh"]
    args: ["-c", "sleep infinity"]

• Run commands in the pod:

$ kubectl exec -it mynotifypod -- /bin/sh
/ # mkdir /abc
/ # ls -1d /abc*
/abc-default-mynotifypod-TODO
/ # mount -t proc proc root
/ # mount|grep /root
proc on /root type proc (rw,relatime)
/ # time -f %E /bin/echo -n ""
0m 0.00s
/ # time -f %E /bin/true
0m 2.00s