Mutating policies

Mutating policies receive an object request and rebuild it (mutate it) into a new request, according to the defined values in the policy settings. The request proceeds through the Kubernetes API, potentially being evaluated by other policies.

To permit mutating request behavior in a policy set the ClusterAdmissionPolicy.mutating field to true.

Why mutating policies can be dangerous

Unreviewed mutating policies can introduce vulnerabilities

danger

To prevent system abuse, Kubewarden administrators should review mutating policies. Mutating policies could, for example, modify a workload, such that it permits privileged container creation.

Solution

If in doubt, split policies into mutating and validating policies, instead of writing or deploying policies that both validate and mutate. This is particularly important when using a DSL, such as Rego, to build complex policies.

Misconfigured mutating policies together with third party Kubernetes Controllers can get stuck in an infinite loop

danger

Mutating policies return requests that proceed through the Kubernetes API. If there are other Kubernetes Controllers that listen for those same resources, they may mutate them back in a follow-up request. This could lead to an infinite feedback loop of mutations.

Solution

Perform the mutation against:

1. The lower type of resource (for example, Pod).
2. The highest type of resource (for example, Deployment). Note: this could still lead to loops if a controller is managing those resources. For example, controllers of GitOps solutions (like fleet, flux, argo, ...) or other third party controllers that translate their own CRDs into Deployment objects.

Examples

You can see a mutating policy at work. Create the following ClusterAdmissionPolicy with the mutating field set to true:

# Command
kubectl apply -f - <<EOF
apiVersion: policies.kubewarden.io/v1alpha2
kind: ClusterAdmissionPolicy
metadata:
  name: psp-user-group
spec:
  module: "registry://ghcr.io/kubewarden/policies/user-group-psp:v0.1.5"
  rules:
  - apiGroups: [""]
    apiVersions: ["v1"]
    resources: ["pods"]
    operations:
    - CREATE
    - UPDATE
  mutating: true
  settings:
    run_as_user:
      rule: "MustRunAs"
      ranges:
        - min: 1000
          max: 2000
        - min: 3000
          max: 4000
    run_as_group:
      rule: "RunAsAny"
    supplemental_groups:
      rule: "RunAsAny"
EOF

# Output
clusteradmissionpolicy.policies.kubewarden.io/psp-user-group created

You use the psp-user-group policy to control users and groups in containers and mutate the requests. In the previous example, you set the run_as_user field and it's added to the container securityContext section.

As the mutating field is true, the following request succeeds:

# Command
kubectl apply -f - <<EOF
apiVersion: v1
kind: Pod
metadata:
  name: pause-user-group
spec:
  containers:
    - name: pause
      image: registry.k8s.io/pause
EOF

# Output
pod/pause-user-group created

You can see the results of the container's securityContext after the request application:

# Command
kubectl get pods pause-user-group -o jsonpath='{ .spec.containers[].securityContext }'

# Output
{"runAsUser":1000}

Now, modify the ClusterAdmissionPolicy by setting the field mutating to false:

# Command
kubectl apply -f - <<EOF
apiVersion: policies.kubewarden.io/v1alpha2
kind: ClusterAdmissionPolicy
metadata:
  name: psp-user-group
spec:
  module: "registry://ghcr.io/kubewarden/policies/user-group-psp:v0.1.5"
  rules:
  - apiGroups: [""]
    apiVersions: ["v1"]
    resources: ["pods"]
    operations:
    - CREATE
    - UPDATE
  mutating: false
  settings:
    run_as_user:
      rule: "MustRunAs"
      ranges:
        - min: 1000
          max: 2000
        - min: 3000
          max: 4000
    run_as_group:
      rule: "RunAsAny"
    supplemental_groups:
      rule: "RunAsAny"
EOF

# Output
clusteradmissionpolicy.policies.kubewarden.io/psp-user-group configured

As the mutating field is false, the following request fails:

# Command
kubectl apply -f - <<EOF
apiVersion: v1
kind: Pod
metadata:
  name: pause-user-group
spec:
  containers:
    - name: pause
      image: registry.k8s.io/pause
EOF

# Output
Error from server: error when creating ".\\pause-user-group.yaml": admission webhook "psp-user-group.kubewarden.admission" denied the request: Request rejected by policy psp-user-group. The policy attempted to mutate the request, but it is currently configured to not allow mutations.

In conclusion, you can see Kubewarden replicates the same behavior as the deprecated Kubernetes Pod Security Policies (PSP).