Forensic container checkpointing in Kubernetes

Authors: Adrian Reber (Red Hat)

Forensic container checkpointing is based on Checkpoint/Restore In Userspace (CRIU) and allows the creation of stateful copies of a running container without the container knowing that it is being checkpointed. The copy of the container can be analyzed and restored in a sandbox environment multiple times without the original container being aware of it. Forensic container checkpointing was introduced as an alpha feature in Kubernetes v1.25.

How does it work?

With the help of CRIU it is possible to checkpoint and restore containers. CRIU is integrated in runc, crun, CRI-O and containerd and forensic container checkpointing as implemented in Kubernetes uses these existing CRIU integrations.

Why is it important?

With the help of CRIU and the corresponding integrations it is possible to get all information and state about a running container on disk for later forensic analysis. Forensic analysis might be important to inspect a suspicious container without stopping or influencing it. If the container is really under attack, the attacker might detect attempts to inspect the container. Taking a checkpoint and analysing the container in a sandboxed environment offers the possibility to inspect the container without the original container and maybe attacker being aware of the inspection.

In addition to the forensic container checkpointing use case, it is also possible to migrate a container from one node to another node without loosing the internal state. Especially for stateful containers with long initialization times restoring from a checkpoint might save time after a reboot or enable much faster startup times.

How do I use container checkpointing?

The feature is behind a feature gate, so make sure to enable the ContainerCheckpoint gate before you can use the new feature.

The runtime must also support container checkpointing:

• containerd: support is currently under discussion. See containerd pull request #6965 for more details.

• CRI-O: v1.25 has support for forensic container checkpointing.

Usage example with CRI-O

To use forensic container checkpointing in combination with CRI-O, the runtime needs to be started with the command-line option --enable-criu-support=true. For Kubernetes, you need to run your cluster with the ContainerCheckpoint feature gate enabled. As the checkpointing functionality is provided by CRIU it is also necessary to install CRIU. Usually runc or crun depend on CRIU and therefore it is installed automatically.

It is also important to mention that at the time of writing the checkpointing functionality is to be considered as an alpha level feature in CRI-O and Kubernetes and the security implications are still under consideration.

Once containers and pods are running it is possible to create a checkpoint. Checkpointing is currently only exposed on the kubelet level. To checkpoint a container, you can run curl on the node where that container is running, and trigger a checkpoint:

curl -X POST "https://localhost:10250/checkpoint/namespace/podId/container"

For a container named counter in a pod named counters in a namespace named default the kubelet API endpoint is reachable at:

curl -X POST "https://localhost:10250/checkpoint/default/counters/counter"

For completeness the following curl command-line options are necessary to have curl accept the kubelet's self signed certificate and authorize the use of the kubelet checkpoint API:

--insecure --cert /var/run/kubernetes/client-admin.crt --key /var/run/kubernetes/client-admin.key

Triggering this kubelet API will request the creation of a checkpoint from CRI-O. CRI-O requests a checkpoint from your low-level runtime (for example, runc). Seeing that request, runc invokes the criu tool to do the actual checkpointing.

Once the checkpointing has finished the checkpoint should be available at /var/lib/kubelet/checkpoints/checkpoint-<pod-name>_<namespace-name>-<container-name>-<timestamp>.tar

You could then use that tar archive to restore the container somewhere else.

Restore a checkpointed container outside of Kubernetes (with CRI-O)

With the checkpoint tar archive it is possible to restore the container outside of Kubernetes in a sandboxed instance of CRI-O. For better user experience during restore, I recommend that you use the latest version of CRI-O from the main CRI-O GitHub branch. If you're using CRI-O v1.25, you'll need to manually create certain directories Kubernetes would create before starting the container.

The first step to restore a container outside of Kubernetes is to create a pod sandbox using crictl:

crictl runp pod-config.json

Then you can restore the previously checkpointed container into the newly created pod sandbox:

crictl create <POD_ID> container-config.json pod-config.json

Instead of specifying a container image in a registry in container-config.json you need to specify the path to the checkpoint archive that you created earlier:

{
  "metadata": {
      "name": "counter"
  },
  "image":{
      "image": "/var/lib/kubelet/checkpoints/<checkpoint-archive>.tar"
  }
}

Next, run crictl start <CONTAINER_ID> to start that container, and then a copy of the previously checkpointed container should be running.

Restore a checkpointed container within of Kubernetes

To restore the previously checkpointed container directly in Kubernetes it is necessary to convert the checkpoint archive into an image that can be pushed to a registry.

One possible way to convert the local checkpoint archive consists of the following steps with the help of buildah:

newcontainer=$(buildah from scratch)
buildah add $newcontainer /var/lib/kubelet/checkpoints/checkpoint-<pod-name>_<namespace-name>-<container-name>-<timestamp>.tar /
buildah config --annotation=io.kubernetes.cri-o.annotations.checkpoint.name=<container-name> $newcontainer
buildah commit $newcontainer checkpoint-image:latest
buildah rm $newcontainer

The resulting image is not standardized and only works in combination with CRI-O. Please consider this image format as pre-alpha. There are ongoing discussions to standardize the format of checkpoint images like this. Important to remember is that this not yet standardized image format only works if CRI-O has been started with --enable-criu-support=true. The security implications of starting CRI-O with CRIU support are not yet clear and therefore the functionality as well as the image format should be used with care.

Now, you'll need to push that image to a container image registry. For example:

buildah push localhost/checkpoint-image:latest container-image-registry.example/user/checkpoint-image:latest

To restore this checkpoint image (container-image-registry.example/user/checkpoint-image:latest), the image needs to be listed in the specification for a Pod. Here's an example manifest:

apiVersion: v1
kind: Pod
metadata:
  namePrefix: example-
spec:
  containers:
  - name: <container-name>
    image: container-image-registry.example/user/checkpoint-image:latest
  nodeName: <destination-node>

Kubernetes schedules the new Pod onto a node. The kubelet on that node instructs the container runtime (CRI-O in this example) to create and start a container based on an image specified as registry/user/checkpoint-image:latest. CRI-O detects that registry/user/checkpoint-image:latest is a reference to checkpoint data rather than a container image. Then, instead of the usual steps to create and start a container, CRI-O fetches the checkpoint data and restores the container from that specified checkpoint.

The application in that Pod would continue running as if the checkpoint had not been taken; within the container, the application looks and behaves like any other container that had been started normally and not restored from a checkpoint.

With these steps, it is possible to replace a Pod running on one node with a new equivalent Pod that is running on a different node, and without losing the state of the containers in that Pod.

How do I get involved?

You can reach SIG Node by several means:

• Slack: #sig-node
• Mailing list

Further reading

Please see the follow-up article Forensic container analysis for details on how a container checkpoint can be analyzed.

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