Mutating policies

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

If you want to allow the behavior of mutating requests, set the ClusterAdmissionPolicy.mutating field to true.

However, if you set the ClusterAdmissionPolicy.mutating field to false, the mutated requests will be rejected.

Why mutating policies can be dangerous

Unreviewed mutating policies can introduce vulnerabilities

To prevent system abuse, Kubewarden administrators should review mutating policies: mutating policies could for example modify a workload, such that it allows for 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 3rd party Kubernetes Controllers can get stuck in an infinite loop

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:

The lower type of resource (e.g: Pod).
The highest type of resource (e.g: 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 3rd party controllers that translate their own CRDs into Deployment objects.

Examples

Let’s 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

The psp-user-group policy is used to control users and groups in containers and can mutate the requests. In the example above, the runAsUser field is set and it will be added to the container securityContext section.

As the mutating field is set to true, the following request will be applied successfully:

# 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

Once the request is applied, you can see the results of the container’s securityContext:

# 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 set to false, the following request will fail:

# 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).