fix(k8spspreadonlyrootfilesystem): CEL support wildcard in exemptImages

Despite it being non-sensical to put a `*` in exemptImages (functionally
disabling the policy), this is supported in the existing rego
implementation of the template.

Thus, not doing it in the CEL implementation is an inconsistency and a
breaking change.

This PR upholds the contract by adding support for `*` as an
exemptImage.

Signed-off-by: juliankatz <[email protected]>

artifacthub/library/pod-security-policy/read-only-root-filesystem/1.1.1/artifacthub-pkg.yml

@@ -0,0 +1,22 @@
+version: 1.1.1
+name: k8spspreadonlyrootfilesystem
+displayName: Read Only Root Filesystem
+createdAt: "2024-08-26T22:42:23Z"
+description: Requires the use of a read-only root file system by pod containers. Corresponds to the `readOnlyRootFilesystem` field in a PodSecurityPolicy. For more information, see https://kubernetes.io/docs/concepts/policy/pod-security-policy/#volumes-and-file-systems
+digest: 29a934504c9589a3b88a923f0a52bc4e96ae585cbe5d8e6d8d342b6f8c86928d
+license: Apache-2.0
+homeURL: https://open-policy-agent.github.io/gatekeeper-library/website/read-only-root-filesystem
+keywords:
+    - gatekeeper
+    - open-policy-agent
+    - policies
+readme: |-
+    # Read Only Root Filesystem
+    Requires the use of a read-only root file system by pod containers. Corresponds to the `readOnlyRootFilesystem` field in a PodSecurityPolicy. For more information, see https://kubernetes.io/docs/concepts/policy/pod-security-policy/#volumes-and-file-systems
+install: |-
+    ### Usage
+    ```shell
+    kubectl apply -f https://raw.githubusercontent.com/open-policy-agent/gatekeeper-library/master/artifacthub/library/pod-security-policy/read-only-root-filesystem/1.1.1/template.yaml
+    ```
+provider:
+    name: Gatekeeper Library

artifacthub/library/pod-security-policy/read-only-root-filesystem/1.1.1/kustomization.yaml

@@ -0,0 +1,2 @@
+resources:
+  - template.yaml

artifacthub/library/pod-security-policy/read-only-root-filesystem/1.1.1/samples/psp-readonlyrootfilesystem/constraint.yaml

@@ -0,0 +1,9 @@
+apiVersion: constraints.gatekeeper.sh/v1beta1
+kind: K8sPSPReadOnlyRootFilesystem
+metadata:
+  name: psp-readonlyrootfilesystem
+spec:
+  match:
+    kinds:
+      - apiGroups: [""]
+        kinds: ["Pod"]

artifacthub/library/pod-security-policy/read-only-root-filesystem/1.1.1/samples/psp-readonlyrootfilesystem/disallowed_ephemeral.yaml

@@ -0,0 +1,12 @@
+apiVersion: v1
+kind: Pod
+metadata:
+  name: nginx-readonlyrootfilesystem-disallowed
+  labels:
+    app: nginx-readonlyrootfilesystem
+spec:
+  ephemeralContainers:
+  - name: nginx
+    image: nginx
+    securityContext:
+      readOnlyRootFilesystem: false

artifacthub/library/pod-security-policy/read-only-root-filesystem/1.1.1/samples/psp-readonlyrootfilesystem/example_allowed.yaml

@@ -0,0 +1,12 @@
+apiVersion: v1
+kind: Pod
+metadata:
+  name: nginx-readonlyrootfilesystem-allowed
+  labels:
+    app: nginx-readonlyrootfilesystem
+spec:
+  containers:
+  - name: nginx
+    image: nginx
+    securityContext:
+      readOnlyRootFilesystem: true

artifacthub/library/pod-security-policy/read-only-root-filesystem/1.1.1/samples/psp-readonlyrootfilesystem/example_disallowed.yaml

@@ -0,0 +1,12 @@
+apiVersion: v1
+kind: Pod
+metadata:
+  name: nginx-readonlyrootfilesystem-disallowed
+  labels:
+    app: nginx-readonlyrootfilesystem
+spec:
+  containers:
+  - name: nginx
+    image: nginx
+    securityContext:
+      readOnlyRootFilesystem: false

artifacthub/library/pod-security-policy/read-only-root-filesystem/1.1.1/samples/psp-readonlyrootfilesystem/update.yaml

@@ -0,0 +1,17 @@
+kind: AdmissionReview
+apiVersion: admission.k8s.io/v1beta1
+request:
+  operation: "UPDATE"
+  object:
+    apiVersion: v1
+    kind: Pod
+    metadata:
+      name: nginx-readonlyrootfilesystem-disallowed
+      labels:
+        app: nginx-readonlyrootfilesystem
+    spec:
+      containers:
+      - name: nginx
+        image: nginx
+        securityContext:
+          readOnlyRootFilesystem: false

artifacthub/library/pod-security-policy/read-only-root-filesystem/1.1.1/samples/wildcard-support/constraint.yaml

@@ -0,0 +1,12 @@
+apiVersion: constraints.gatekeeper.sh/v1beta1
+kind: K8sPSPReadOnlyRootFilesystem
+metadata:
+  name: psp-readonlyrootfilesystem
+spec:
+  match:
+    kinds:
+      - apiGroups: [""]
+        kinds: ["Pod"]
+  parameters:
+    exemptImages:
+      - "*"

artifacthub/library/pod-security-policy/read-only-root-filesystem/1.1.1/suite.yaml

@@ -0,0 +1,47 @@
+kind: Suite
+apiVersion: test.gatekeeper.sh/v1alpha1
+metadata:
+  name: read-only-root-filesystem
+tests:
+- name: require-read-only-root-filesystem
+  template: template.yaml
+  constraint: samples/psp-readonlyrootfilesystem/constraint.yaml
+  cases:
+  - name: example-disallowed
+    object: samples/psp-readonlyrootfilesystem/example_disallowed.yaml
+    assertions:
+    - violations: yes
+      message: "only read-only root filesystem container is allowed: nginx"
+  - name: example-allowed
+    object: samples/psp-readonlyrootfilesystem/example_allowed.yaml
+    assertions:
+    - violations: no
+  - name: disallowed-ephemeral
+    object: samples/psp-readonlyrootfilesystem/disallowed_ephemeral.yaml
+    assertions:
+    - violations: yes
+      message: "only read-only root filesystem container is allowed: nginx"
+  - name: update
+    object: samples/psp-readonlyrootfilesystem/update.yaml
+    assertions:
+    - violations: no
+- name: wildcard-support
+  template: template.yaml
+  constraint: samples/wildcard-support/constraint.yaml
+  cases:
+  - name: example-disallowed
+    object: samples/psp-readonlyrootfilesystem/example_disallowed.yaml
+    assertions:
+    - violations: no
+  - name: example-allowed
+    object: samples/psp-readonlyrootfilesystem/example_allowed.yaml
+    assertions:
+    - violations: no
+  - name: disallowed-ephemeral
+    object: samples/psp-readonlyrootfilesystem/disallowed_ephemeral.yaml
+    assertions:
+    - violations: no
+  - name: update
+    object: samples/psp-readonlyrootfilesystem/update.yaml
+    assertions:
+    - violations: no

artifacthub/library/pod-security-policy/read-only-root-filesystem/1.1.1/template.yaml

@@ -0,0 +1,144 @@
+apiVersion: templates.gatekeeper.sh/v1
+kind: ConstraintTemplate
+metadata:
+  name: k8spspreadonlyrootfilesystem
+  annotations:
+    metadata.gatekeeper.sh/title: "Read Only Root Filesystem"
+    metadata.gatekeeper.sh/version: 1.1.1
+    description: >-
+      Requires the use of a read-only root file system by pod containers.
+      Corresponds to the `readOnlyRootFilesystem` field in a
+      PodSecurityPolicy. For more information, see
+      https://kubernetes.io/docs/concepts/policy/pod-security-policy/#volumes-and-file-systems
+spec:
+  crd:
+    spec:
+      names:
+        kind: K8sPSPReadOnlyRootFilesystem
+      validation:
+        # Schema for the `parameters` field
+        openAPIV3Schema:
+          type: object
+          description: >-
+            Requires the use of a read-only root file system by pod containers.
+            Corresponds to the `readOnlyRootFilesystem` field in a
+            PodSecurityPolicy. For more information, see
+            https://kubernetes.io/docs/concepts/policy/pod-security-policy/#volumes-and-file-systems
+          properties:
+            exemptImages:
+              description: >-
+                Any container that uses an image that matches an entry in this list will be excluded
+                from enforcement. Prefix-matching can be signified with `*`. For example: `my-image-*`.
+
+                It is recommended that users use the fully-qualified Docker image name (e.g. start with a domain name)
+                in order to avoid unexpectedly exempting images from an untrusted repository.
+              type: array
+              items:
+                type: string
+  targets:
+    - target: admission.k8s.gatekeeper.sh
+      code:
+      - engine: K8sNativeValidation
+        source:
+          variables:
+          - name: containers
+            expression: 'has(variables.anyObject.spec.containers) ? variables.anyObject.spec.containers : []'
+          - name: initContainers
+            expression: 'has(variables.anyObject.spec.initContainers) ? variables.anyObject.spec.initContainers : []'
+          - name: ephemeralContainers
+            expression: 'has(variables.anyObject.spec.ephemeralContainers) ? variables.anyObject.spec.ephemeralContainers : []'
+          - name: exemptImagePrefixes
+            expression: |
+              !has(variables.params.exemptImages) ? [] :
+                variables.params.exemptImages.filter(image, image.endsWith("*")).map(image, string(image).replace("*", ""))
+          - name: exemptImageExplicit
+            expression: |
+              !has(variables.params.exemptImages) ? [] : 
+                variables.params.exemptImages.filter(image, !image.endsWith("*"))
+          - name: exemptImageWildcard
+            expression: |
+              !has(variables.params.exemptImages) ? false :
+                variables.params.exemptImages.exists_one(image, string(image) == "*")
+          - name: exemptImages
+            expression: |
+              (variables.containers + variables.initContainers + variables.ephemeralContainers).filter(container,
+                container.image in variables.exemptImageExplicit ||
+                variables.exemptImagePrefixes.exists(exemption, string(container.image).startsWith(exemption)))
+          - name: badContainers
+            expression: |
+              (variables.containers + variables.initContainers + variables.ephemeralContainers).filter(container,
+                (!(container.image in variables.exemptImages) && !variables.exemptImageWildcard) &&
+                (!has(container.securityContext) ||
+                !has(container.securityContext.readOnlyRootFilesystem) ||
+                container.securityContext.readOnlyRootFilesystem != true)
+              ).map(container, container.name)
+          validations:
+          - expression: '(has(request.operation) && request.operation == "UPDATE") || size(variables.badContainers) == 0'
+            messageExpression: '"only read-only root filesystem container is allowed: " + variables.badContainers.join(", ")'
+
+      - engine: Rego
+        source:
+          rego: |
+            package k8spspreadonlyrootfilesystem
+
+            import data.lib.exclude_update.is_update
+            import data.lib.exempt_container.is_exempt
+
+            violation[{"msg": msg, "details": {}}] {
+                # spec.containers.readOnlyRootFilesystem field is immutable.
+                not is_update(input.review)
+
+                c := input_containers[_]
+                not is_exempt(c)
+                input_read_only_root_fs(c)
+                msg := sprintf("only read-only root filesystem container is allowed: %v", [c.name])
+            }
+
+            input_read_only_root_fs(c) {
+                not has_field(c, "securityContext")
+            }
+            input_read_only_root_fs(c) {
+                not c.securityContext.readOnlyRootFilesystem == true
+            }
+
+            input_containers[c] {
+                c := input.review.object.spec.containers[_]
+            }
+            input_containers[c] {
+                c := input.review.object.spec.initContainers[_]
+            }
+            input_containers[c] {
+                c := input.review.object.spec.ephemeralContainers[_]
+            }
+
+            # has_field returns whether an object has a field
+            has_field(object, field) = true {
+                object[field]
+            }
+          libs:
+            - |
+              package lib.exclude_update
+
+              is_update(review) {
+                  review.operation == "UPDATE"
+              }
+            - |
+              package lib.exempt_container
+
+              is_exempt(container) {
+                  exempt_images := object.get(object.get(input, "parameters", {}), "exemptImages", [])
+                  img := container.image
+                  exemption := exempt_images[_]
+                  _matches_exemption(img, exemption)
+              }
+
+              _matches_exemption(img, exemption) {
+                  not endswith(exemption, "*")
+                  exemption == img
+              }
+
+              _matches_exemption(img, exemption) {
+                  endswith(exemption, "*")
+                  prefix := trim_suffix(exemption, "*")
+                  startswith(img, prefix)
+              }

library/pod-security-policy/read-only-root-filesystem/samples/psp-readonlyrootfilesystem/constraint.yaml

@@ -7,3 +7,6 @@ spec:
     kinds:
       - apiGroups: [""]
         kinds: ["Pod"]
+  parameters:
+    exemptImages:
+    - "specialprogram"

library/pod-security-policy/read-only-root-filesystem/samples/psp-readonlyrootfilesystem/example_allowed_exempted.yaml

@@ -0,0 +1,12 @@
+apiVersion: v1
+kind: Pod
+metadata:
+  name: nginx-readonlyrootfilesystem-disallowed
+  labels:
+    app: nginx-readonlyrootfilesystem
+spec:
+  containers:
+  - name: specialprogram
+    image: specialprogram
+    securityContext:
+      readOnlyRootFilesystem: false

library/pod-security-policy/read-only-root-filesystem/samples/wildcard_support/example_safe_image_prefix.yaml

@@ -0,0 +1,12 @@
+apiVersion: v1
+kind: Pod
+metadata:
+  name: nginx-readonlyrootfilesystem-allowed
+  labels:
+    app: nginx-readonlyrootfilesystem
+spec:
+  containers:
+  - name: nginx
+    image: "safe-images.com/nginx"
+    securityContext:
+      readOnlyRootFilesystem: false

library/pod-security-policy/read-only-root-filesystem/samples/wildcard_support/example_unsafe_image_prefix.yaml

@@ -0,0 +1,12 @@
+apiVersion: v1
+kind: Pod
+metadata:
+  name: nginx-readonlyrootfilesystem-allowed
+  labels:
+    app: nginx-readonlyrootfilesystem
+spec:
+  containers:
+  - name: nginx
+    image: "unsafe-images.com/nginx"
+    securityContext:
+      readOnlyRootFilesystem: false

library/pod-security-policy/read-only-root-filesystem/samples/wildcard_support/full_wildcard_constraint.yaml

@@ -0,0 +1,12 @@
+apiVersion: constraints.gatekeeper.sh/v1beta1
+kind: K8sPSPReadOnlyRootFilesystem
+metadata:
+  name: psp-readonlyrootfilesystem
+spec:
+  match:
+    kinds:
+      - apiGroups: [""]
+        kinds: ["Pod"]
+  parameters:
+    exemptImages:
+      - "*"

library/pod-security-policy/read-only-root-filesystem/samples/wildcard_support/prefix_matching_constraint.yaml

@@ -0,0 +1,12 @@
+apiVersion: constraints.gatekeeper.sh/v1beta1
+kind: K8sPSPReadOnlyRootFilesystem
+metadata:
+  name: psp-readonlyrootfilesystem
+spec:
+  match:
+    kinds:
+      - apiGroups: [""]
+        kinds: ["Pod"]
+  parameters:
+    exemptImages:
+      - "safe-images.com/*"

library/pod-security-policy/read-only-root-filesystem/suite.yaml

@@ -25,3 +25,27 @@ tests:
     object: samples/psp-readonlyrootfilesystem/update.yaml
     assertions:
     - violations: no
+  - name: exact exemption
+    object: samples/psp-readonlyrootfilesystem/example_allowed_exempted.yaml
+    assertions:
+    - violations: no
+- name: full-wildcard
+  template: template.yaml
+  constraint: samples/wildcard_support/full_wildcard_constraint.yaml
+  cases:
+  - name: allow-normally-disallowed
+    object: samples/psp-readonlyrootfilesystem/example_disallowed.yaml
+    assertions:
+    - violations: no
+- name: wildcard-suffix
+  template: template.yaml
+  constraint: samples/wildcard_support/prefix_matching_constraint.yaml
+  cases:
+  - name: image-with-exempt-prefix-readOnlyRootFilesystem-not-required
+    object: samples/wildcard_support/example_safe_image_prefix.yaml
+    assertions:
+    - violations: no
+  - name: image-with-different-prefix-must-set-readOnlyRootFilesystem
+    object: samples/wildcard_support/example_unsafe_image_prefix.yaml
+    assertions:
+    - violations: yes