Author: Sascha Grunert
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.
But distributing those seccomp profiles is a major challenge in Kubernetes,
because the JSON files have to be available on all nodes where a workload can
possibly run. Projects like the Security Profiles
Operator solve that problem by
running as a daemon within the cluster, which makes me wonder which part of that
distribution could be done by the container
runtime.
Runtimes usually apply the profiles from a local path, for example:
apiVersion: v1
kind: Pod
metadata:
name: pod
spec:
containers:
- name: container
image: nginx:1.25.3
securityContext:
seccompProfile:
type: Localhost
localhostProfile: nginx-1.25.3.json
The profile nginx-1.25.3.json has to be available in the root directory of the
kubelet, appended by the seccomp directory. This means the default location
for the profile on-disk would be /var/lib/kubelet/seccomp/nginx-1.25.3.json.
If the profile is not available, then runtimes will fail on container creation
like this:
kubectl get pods
NAME READY STATUS RESTARTS AGE
pod 0/1 CreateContainerError 0 38s
kubectl describe pod/pod | tail
Tolerations: node.kubernetes.io/not-ready:NoExecute op=Exists for 300s
node.kubernetes.io/unreachable:NoExecute op=Exists for 300s
Events:
Type Reason Age From Message
---- ------ ---- ---- -------
Normal Scheduled 117s default-scheduler Successfully assigned default/pod to 127.0.0.1
Normal Pulling 117s kubelet Pulling image "nginx:1.25.3"
Normal Pulled 111s kubelet Successfully pulled image "nginx:1.25.3" in 5.948s (5.948s including waiting)
Warning Failed 7s (x10 over 111s) kubelet Error: setup seccomp: unable to load local profile "/var/lib/kubelet/seccomp/nginx-1.25.3.json": open /var/lib/kubelet/seccomp/nginx-1.25.3.json: no such file or directory
Normal Pulled 7s (x9 over 111s) kubelet Container image "nginx:1.25.3" already present on machine
The major obstacle of having to manually distribute the Localhost profiles
will lead many end-users to fall back to RuntimeDefault or even running their
workloads as Unconfined (with disabled seccomp).
CRI-O to the rescue
The Kubernetes container runtime CRI-O
provides various features using custom annotations. The v1.30 release
adds support for a new set of
annotations called seccomp-profile.kubernetes.cri-o.io/POD and
seccomp-profile.kubernetes.cri-o.io/<CONTAINER>. Those annotations allow you
to specify:
- a seccomp profile for a specific container, when used as:
seccomp-profile.kubernetes.cri-o.io/<CONTAINER>(example:
seccomp-profile.kubernetes.cri-o.io/webserver: 'registry.example/example/webserver:v1') - a seccomp profile for every container within a pod, when used without the
container name suffix but the reserved namePOD:
seccomp-profile.kubernetes.cri-o.io/POD - a seccomp profile for a whole container image, if the image itself contains
the annotationseccomp-profile.kubernetes.cri-o.io/PODor
seccomp-profile.kubernetes.cri-o.io/<CONTAINER>.
CRI-O will only respect the annotation if the runtime is configured to allow it,
as well as for workloads running as Unconfined. All other workloads will still
use the value from the securityContext with a higher priority.
The annotations alone will not help much with the distribution of the profiles,
but the way they can be referenced will! For example, you can now specify
seccomp profiles like regular container images by using OCI artifacts:
apiVersion: v1
kind: Pod
metadata:
name: pod
annotations:
seccomp-profile.kubernetes.cri-o.io/POD: quay.io/crio/seccomp:v2
spec: …
The image quay.io/crio/seccomp:v2 contains a seccomp.json file, which
contains the actual profile content. Tools like ORAS or
Skopeo can be used to inspect the
contents of the image:
oras pull quay.io/crio/seccomp:v2
Downloading 92d8ebfa89aa seccomp.json
Downloaded 92d8ebfa89aa seccomp.json
Pulled [registry] quay.io/crio/seccomp:v2
Digest: sha256:f0205dac8a24394d9ddf4e48c7ac201ca7dcfea4c554f7ca27777a7f8c43ec1b
jq . seccomp.json | head
{
"defaultAction": "SCMP_ACT_ERRNO",
"defaultErrnoRet": 38,
"defaultErrno": "ENOSYS",
"archMap": [
{
"architecture": "SCMP_ARCH_X86_64",
"subArchitectures": [
"SCMP_ARCH_X86",
"SCMP_ARCH_X32"
# Inspect the plain manifest of the image
skopeo inspect --raw docker://quay.io/crio/seccomp:v2 | jq .
{
"schemaVersion": 2,
"mediaType": "application/vnd.oci.image.manifest.v1+json",
"config":
{
"mediaType": "application/vnd.cncf.seccomp-profile.config.v1+json",
"digest": "sha256:ca3d163bab055381827226140568f3bef7eaac187cebd76878e0b63e9e442356",
"size": 3,
},
"layers":
[
{
"mediaType": "application/vnd.oci.image.layer.v1.tar",
"digest": "sha256:92d8ebfa89aa6dd752c6443c27e412df1b568d62b4af129494d7364802b2d476",
"size": 18853,
"annotations": { "org.opencontainers.image.title": "seccomp.json" },
},
],
"annotations": { "org.opencontainers.image.created": "2024-02-26T09:03:30Z" },
}
The image manifest contains a reference to a specific required config media type
(application/vnd.cncf.seccomp-profile.config.v1+json) and a single layer
(application/vnd.oci.image.layer.v1.tar) pointing to the seccomp.json file.
But now, let’s give that new feature a try!
Using the annotation for a specific container or whole pod
CRI-O needs to be configured adequately before it can utilize the annotation. To
do this, add the annotation to the allowed_annotations array for the runtime.
This can be done by using a drop-in configuration
/etc/crio/crio.conf.d/10-crun.conf like this:
[crio.runtime]
default_runtime = "crun"
[crio.runtime.runtimes.crun]
allowed_annotations = [
"seccomp-profile.kubernetes.cri-o.io",
]
Now, let’s run CRI-O from the latest main commit. This can be done by either
building it from source, using the static binary bundles
or the prerelease packages.
To demonstrate this, I ran the crio binary from my command line using a single
node Kubernetes cluster via local-up-cluster.sh.
Now that the cluster is up and running, let’s try a pod without the annotation
running as seccomp Unconfined:
cat pod.yaml
apiVersion: v1
kind: Pod
metadata:
name: pod
spec:
containers:
- name: container
image: nginx:1.25.3
securityContext:
seccompProfile:
type: Unconfined
kubectl apply -f pod.yaml
The workload is up and running:
kubectl get pods
NAME READY STATUS RESTARTS AGE
pod 1/1 Running 0 15s
And no seccomp profile got applied if I inspect the container using
crictl:
export CONTAINER_ID=$(sudo crictl ps --name container -q)
sudo crictl inspect $CONTAINER_ID | jq .info.runtimeSpec.linux.seccomp
null
Now, let’s modify the pod to apply the profile quay.io/crio/seccomp:v2 to the
container:
apiVersion: v1
kind: Pod
metadata:
name: pod
annotations:
seccomp-profile.kubernetes.cri-o.io/container: quay.io/crio/seccomp:v2
spec:
containers:
- name: container
image: nginx:1.25.3
I have to delete and recreate the Pod, because only recreation will apply a new
seccomp profile:
kubectl delete pod/pod
pod "pod" deleted
kubectl apply -f pod.yaml
pod/pod created
The CRI-O logs will now indicate that the runtime pulled the artifact:
WARN[…] Allowed annotations are specified for workload [seccomp-profile.kubernetes.cri-o.io]
INFO[…] Found container specific seccomp profile annotation: seccomp-profile.kubernetes.cri-o.io/container=quay.io/crio/seccomp:v2 id=26ddcbe6-6efe-414a-88fd-b1ca91979e93 name=/runtime.v1.RuntimeService/CreateContainer
INFO[…] Pulling OCI artifact from ref: quay.io/crio/seccomp:v2 id=26ddcbe6-6efe-414a-88fd-b1ca91979e93 name=/runtime.v1.RuntimeService/CreateContainer
INFO[…] Retrieved OCI artifact seccomp profile of len: 18853 id=26ddcbe6-6efe-414a-88fd-b1ca91979e93 name=/runtime.v1.RuntimeService/CreateContainer
And the container is finally using the profile:
export CONTAINER_ID=$(sudo crictl ps --name container -q)
sudo crictl inspect $CONTAINER_ID | jq .info.runtimeSpec.linux.seccomp | head
{
"defaultAction": "SCMP_ACT_ERRNO",
"defaultErrnoRet": 38,
"architectures": [
"SCMP_ARCH_X86_64",
"SCMP_ARCH_X86",
"SCMP_ARCH_X32"
],
"syscalls": [
{
The same would work for every container in the pod, if users replace the
/container suffix with the reserved name /POD, for example:
apiVersion: v1
kind: Pod
metadata:
name: pod
annotations:
seccomp-profile.kubernetes.cri-o.io/POD: quay.io/crio/seccomp:v2
spec:
containers:
- name: container
image: nginx:1.25.3
Using the annotation for a container image
While specifying seccomp profiles as OCI artifacts on certain workloads is a
cool feature, the majority of end users would like to link seccomp profiles to
published container images. This can be done by using a container image
annotation; instead of being applied to a Kubernetes Pod, the annotation is some
metadata applied at the container image itself. For example,
Podman can be used to add the image annotation directly
during image build:
podman build
--annotation seccomp-profile.kubernetes.cri-o.io=quay.io/crio/seccomp:v2
-t quay.io/crio/nginx-seccomp:v2 .
The pushed image then contains the annotation:
skopeo inspect --raw docker://quay.io/crio/nginx-seccomp:v2 |
jq '.annotations."seccomp-profile.kubernetes.cri-o.io"'
"quay.io/crio/seccomp:v2"
If I now use that image in an CRI-O test pod definition:
apiVersion: v1
kind: Pod
metadata:
name: pod
# no Pod annotations set
spec:
containers:
- name: container
image: quay.io/crio/nginx-seccomp:v2
Then the CRI-O logs will indicate that the image annotation got evaluated and
the profile got applied:
kubectl delete pod/pod
pod "pod" deleted
kubectl apply -f pod.yaml
pod/pod created
INFO[…] Found image specific seccomp profile annotation: seccomp-profile.kubernetes.cri-o.io=quay.io/crio/seccomp:v2 id=c1f22c59-e30e-4046-931d-a0c0fdc2c8b7 name=/runtime.v1.RuntimeService/CreateContainer
INFO[…] Pulling OCI artifact from ref: quay.io/crio/seccomp:v2 id=c1f22c59-e30e-4046-931d-a0c0fdc2c8b7 name=/runtime.v1.RuntimeService/CreateContainer
INFO[…] Retrieved OCI artifact seccomp profile of len: 18853 id=c1f22c59-e30e-4046-931d-a0c0fdc2c8b7 name=/runtime.v1.RuntimeService/CreateContainer
INFO[…] Created container 116a316cd9a11fe861dd04c43b94f45046d1ff37e2ed05a4e4194fcaab29ee63: default/pod/container id=c1f22c59-e30e-4046-931d-a0c0fdc2c8b7 name=/runtime.v1.RuntimeService/CreateContainer
export CONTAINER_ID=$(sudo crictl ps --name container -q)
sudo crictl inspect $CONTAINER_ID | jq .info.runtimeSpec.linux.seccomp | head
{
"defaultAction": "SCMP_ACT_ERRNO",
"defaultErrnoRet": 38,
"architectures": [
"SCMP_ARCH_X86_64",
"SCMP_ARCH_X86",
"SCMP_ARCH_X32"
],
"syscalls": [
{
For container images, the annotation seccomp-profile.kubernetes.cri-o.io will
be treated in the same way as seccomp-profile.kubernetes.cri-o.io/POD and
applies to the whole pod. In addition to that, the whole feature also works when
using the container specific annotation on an image, for example if a container
is named container1:
skopeo inspect --raw docker://quay.io/crio/nginx-seccomp:v2-container |
jq '.annotations."seccomp-profile.kubernetes.cri-o.io/container1"'
"quay.io/crio/seccomp:v2"
The cool thing about this whole feature is that users can now create seccomp
profiles for specific container images and store them side by side in the same
registry. Linking the images to the profiles provides a great flexibility to
maintain them over the whole application’s life cycle.
Pushing profiles using ORAS
The actual creation of the OCI object that contains a seccomp profile requires a
bit more work when using ORAS. I have the hope that tools like Podman will
simplify the overall process in the future. Right now, the container registry
needs to be OCI compatible,
which is also the case for Quay.io. CRI-O expects the seccomp
profile object to have a container image media type
(application/vnd.cncf.seccomp-profile.config.v1+json), while ORAS uses
application/vnd.oci.empty.v1+json per default. To achieve all of that, the
following commands can be executed:
echo "{}" > config.json
oras push
--config config.json:application/vnd.cncf.seccomp-profile.config.v1+json
quay.io/crio/seccomp:v2 seccomp.json
The resulting image contains the mediaType that CRI-O expects. ORAS pushes a
single layer seccomp.json to the registry. The name of the profile does not
matter much. CRI-O will pick the first layer and check if that can act as a
seccomp profile.
Future work
CRI-O internally manages the OCI artifacts like regular files. This provides the
benefit of moving them around, removing them if not used any more or having any
other data available than seccomp profiles. This enables future enhancements in
CRI-O on top of OCI artifacts, but also allows thinking about stacking seccomp
profiles as part of having multiple layers in an OCI artifact. The limitation
that it only works for Unconfined workloads for v1.30.x releases is something
different CRI-O would like to address in the future. Simplifying the overall
user experience by not compromising security seems to be the key for a
successful future of seccomp in container workloads.
The CRI-O maintainers will be happy to listen to any feedback or suggestions on
the new feature! Thank you for reading this blog post, feel free to reach out
to the maintainers via the Kubernetes Slack channel #crio
or create an issue in the GitHub repository.
Originally posted on Kubernetes Blog
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