fix: procmount should allow empty values

There is a mismatch between existing Rego and CEL implementation. Rego allows not providing proc mount value at all while CEL needs a value. Both fail if procMount is set to `null` despite it being a valid value equivalent to not defining the key at all.

artifacthub/library/pod-security-policy/proc-mount/1.1.2/README.md

@@ -0,0 +1,12 @@
+# ProcMount security context policy
+
+`procMount` denotes the type of proc mount to use for the containers. The default is `DefaultProcMount` which uses the container runtime defaults for readonly paths and masked paths.
+
+Types of proc mount are:
+
+- `DefaultProcMount` uses the container runtime default ProcType. Most container runtimes mask certain paths in /proc to avoid accidental security exposure of special devices or information.
+
+- `UnmaskedProcMount` bypasses the default masking behavior of the container runtime and ensures the newly created /proc the container stays in tact with no modifications.
+
+This requires the `ProcMountType` feature flag to be enabled. Set `--feature-gates=ProcMountType=true` in Kubernetes API Server to be able to use `Unmasked` procMount type (requires v1.12 and above). For more information, see
+https://kubernetes.io/docs/reference/command-line-tools-reference/kube-apiserver/#options and https://kubernetes.io/docs/reference/command-line-tools-reference/feature-gates/.

artifacthub/library/pod-security-policy/proc-mount/1.1.2/artifacthub-pkg.yml

@@ -0,0 +1,22 @@
+version: 1.1.2
+name: k8spspprocmount
+displayName: Proc Mount
+createdAt: "2025-01-10T16:19:17Z"
+description: Controls the allowed `procMount` types for the container. Corresponds to the `allowedProcMountTypes` field in a PodSecurityPolicy. For more information, see https://kubernetes.io/docs/concepts/policy/pod-security-policy/#allowedprocmounttypes
+digest: bdec708081728bb3d1e48168ee36f4112b7fec284cb0482843cad793fff38b4f
+license: Apache-2.0
+homeURL: https://open-policy-agent.github.io/gatekeeper-library/website/proc-mount
+keywords:
+    - gatekeeper
+    - open-policy-agent
+    - policies
+readme: |-
+    # Proc Mount
+    Controls the allowed `procMount` types for the container. Corresponds to the `allowedProcMountTypes` field in a PodSecurityPolicy. For more information, see https://kubernetes.io/docs/concepts/policy/pod-security-policy/#allowedprocmounttypes
+install: |-
+    ### Usage
+    ```shell
+    kubectl apply -f https://raw.githubusercontent.com/open-policy-agent/gatekeeper-library/master/artifacthub/library/pod-security-policy/proc-mount/1.1.2/template.yaml
+    ```
+provider:
+    name: Gatekeeper Library

artifacthub/library/pod-security-policy/proc-mount/1.1.2/kustomization.yaml

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

artifacthub/library/pod-security-policy/proc-mount/1.1.2/samples/psp-proc-mount/constraint.yaml

@@ -0,0 +1,13 @@
+apiVersion: constraints.gatekeeper.sh/v1beta1
+kind: K8sPSPProcMount
+metadata:
+  name: psp-proc-mount
+spec:
+  match:
+    kinds:
+      - apiGroups: [""]
+        kinds: ["Pod"]
+  parameters:
+    procMount: Default
+    exemptImages:
+    - "safeimages.com/*"

artifacthub/library/pod-security-policy/proc-mount/1.1.2/samples/psp-proc-mount/disallowed_ephemeral.yaml

@@ -0,0 +1,13 @@
+apiVersion: v1
+kind: Pod
+metadata:
+  name: nginx-proc-mount-disallowed
+  labels:
+    app: nginx-proc-mount
+spec:
+  hostUsers: false
+  ephemeralContainers:
+  - name: nginx
+    image: nginx
+    securityContext:
+      procMount: Unmasked #Default

artifacthub/library/pod-security-policy/proc-mount/1.1.2/samples/psp-proc-mount/example_allowed.yaml

@@ -0,0 +1,13 @@
+apiVersion: v1
+kind: Pod
+metadata:
+  name: nginx-proc-mount-allowed
+  labels:
+    app: nginx-proc-mount
+spec:
+  hostUsers: false
+  containers:
+  - name: nginx
+    image: nginx
+    securityContext:
+      procMount: Default

artifacthub/library/pod-security-policy/proc-mount/1.1.2/samples/psp-proc-mount/example_allowed_exempt_image.yaml

@@ -0,0 +1,13 @@
+apiVersion: v1
+kind: Pod
+metadata:
+  name: nginx-proc-mount-exempt-image
+  labels:
+    app: nginx-proc-mount
+spec:
+  hostUsers: false
+  containers:
+  - name: nginx
+    image: safeimages.com/nginx
+    securityContext:
+      procMount: Unmasked #Default

artifacthub/library/pod-security-policy/proc-mount/1.1.2/samples/psp-proc-mount/example_allowed_missing.yaml

@@ -0,0 +1,18 @@
+apiVersion: v1
+kind: Pod
+metadata:
+  name: nginx-proc-mount-disallowed
+  labels:
+    app: nginx-proc-mount
+spec:
+  hostUsers: false
+  containers:
+  - name: no-proc-mount-value
+    image: nginx
+    securityContext:
+      procMount: null
+  - name: no-proc-mount
+    image: nginx
+    securityContext: {}
+  - name: no-context
+    image: nginx

artifacthub/library/pod-security-policy/proc-mount/1.1.2/samples/psp-proc-mount/example_disallowed.yaml

@@ -0,0 +1,13 @@
+apiVersion: v1
+kind: Pod
+metadata:
+  name: nginx-proc-mount-disallowed
+  labels:
+    app: nginx-proc-mount
+spec:
+  hostUsers: false
+  containers:
+  - name: nginx
+    image: nginx
+    securityContext:
+      procMount: Unmasked #Default

artifacthub/library/pod-security-policy/proc-mount/1.1.2/samples/psp-proc-mount/update.yaml

@@ -0,0 +1,17 @@
+kind: AdmissionReview
+apiVersion: admission.k8s.io/v1beta1
+request:
+  operation: "UPDATE"
+  object:
+    apiVersion: v1
+    kind: Pod
+    metadata:
+      name: nginx-proc-mount-disallowed
+      labels:
+        app: nginx-proc-mount
+    spec:
+      containers:
+      - name: nginx
+        image: nginx
+        securityContext:
+          procMount: Unmasked #Default

artifacthub/library/pod-security-policy/proc-mount/1.1.2/suite.yaml

@@ -0,0 +1,33 @@
+kind: Suite
+apiVersion: test.gatekeeper.sh/v1alpha1
+metadata:
+  name: proc-mount
+tests:
+- name: default-proc-mount-required
+  template: template.yaml
+  constraint: samples/psp-proc-mount/constraint.yaml
+  cases:
+  - name: example-disallowed
+    object: samples/psp-proc-mount/example_disallowed.yaml
+    assertions:
+    - violations: yes
+  - name: example-allowed
+    object: samples/psp-proc-mount/example_allowed.yaml
+    assertions:
+    - violations: no
+  - name: example-allowed-missing
+    object: samples/psp-proc-mount/example_allowed_missing.yaml
+    assertions:
+    - violations: no
+  - name: disallowed-ephemeral
+    object: samples/psp-proc-mount/disallowed_ephemeral.yaml
+    assertions:
+    - violations: yes
+  - name: update
+    object: samples/psp-proc-mount/update.yaml
+    assertions:
+    - violations: no
+  - name: image-exempt-prefix-match
+    object: samples/psp-proc-mount/example_allowed_exempt_image.yaml
+    assertions:
+    - violations: no

artifacthub/library/pod-security-policy/proc-mount/1.1.2/template.yaml

@@ -0,0 +1,185 @@
+apiVersion: templates.gatekeeper.sh/v1
+kind: ConstraintTemplate
+metadata:
+  name: k8spspprocmount
+  annotations:
+    metadata.gatekeeper.sh/title: "Proc Mount"
+    metadata.gatekeeper.sh/version: 1.1.2
+    description: >-
+      Controls the allowed `procMount` types for the container. Corresponds to
+      the `allowedProcMountTypes` field in a PodSecurityPolicy. For more
+      information, see
+      https://kubernetes.io/docs/concepts/policy/pod-security-policy/#allowedprocmounttypes
+spec:
+  crd:
+    spec:
+      names:
+        kind: K8sPSPProcMount
+      validation:
+        # Schema for the `parameters` field
+        openAPIV3Schema:
+          type: object
+          description: >-
+            Controls the allowed `procMount` types for the container. Corresponds to
+            the `allowedProcMountTypes` field in a PodSecurityPolicy. For more
+            information, see
+            https://kubernetes.io/docs/concepts/policy/pod-security-policy/#allowedprocmounttypes
+          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
+            procMount:
+              type: string
+              description: >-
+                Defines the strategy for the security exposure of certain paths
+                in `/proc` by the container runtime. Setting to `Default` uses
+                the runtime defaults, where `Unmasked` bypasses the default
+                behavior.
+              enum:
+                - Default
+                - Unmasked
+  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: exemptImages
+            expression: |
+              (variables.containers + variables.initContainers + variables.ephemeralContainers).filter(
+                container,
+                container.image in variables.exemptImageExplicit ||
+                  variables.exemptImagePrefixes.exists(
+                    exemption,
+                    string(container.image).startsWith(exemption)
+                  )
+              ).map(container, container.image)
+          - name: allowedProcMount
+            expression: |
+              !has(variables.params) ? "default" : 
+                !has(variables.params.procMount) ? "default" : 
+                  (variables.params.procMount.lowerAscii() == "default" || variables.params.procMount.lowerAscii() == "unmasked") ? variables.params.procMount.lowerAscii() : "default"
+          - name: badContainers
+            expression: |
+              (variables.containers + variables.initContainers + variables.ephemeralContainers).filter(container,
+                !(container.image in variables.exemptImages) &&
+                !(
+                  (variables.allowedProcMount == "unmasked") ||
+                  (variables.allowedProcMount == "default" && (!has(container.securityContext) || !has(container.securityContext.procMount) || container.securityContext.procMount == null || container.securityContext.procMount.lowerAscii() == "default"))
+                )
+              ).map(container, "ProcMount type is not allowed, container: " + container.name +". Allowed procMount types: " + variables.allowedProcMount)
+          validations:
+          - expression: '(has(request.operation) && request.operation == "UPDATE") || size(variables.badContainers) == 0'
+            messageExpression: 'variables.badContainers.join("\n")' 
+      - engine: Rego
+        source:
+          rego: |
+            package k8spspprocmount
+
+            import data.lib.exclude_update.is_update
+            import data.lib.exempt_container.is_exempt
+
+            violation[{"msg": msg, "details": {}}] {
+                # spec.containers.securityContext.procMount field is immutable.
+                not is_update(input.review)
+
+                c := input_containers[_]
+                not is_exempt(c)
+                allowedProcMount := get_allowed_proc_mount(input)
+                not input_proc_mount_type_allowed(allowedProcMount, c)
+                msg := sprintf("ProcMount type is not allowed, container: %v. Allowed procMount types: %v", [c.name, allowedProcMount])
+            }
+
+            input_proc_mount_type_allowed(allowedProcMount, c) {
+                allowedProcMount == "default"
+                lower(c.securityContext.procMount) == "default"
+            }
+            input_proc_mount_type_allowed(allowedProcMount, _) {
+                allowedProcMount == "unmasked"
+            }
+
+            input_containers[c] {
+                c := input.review.object.spec.containers[_]
+                c.securityContext.procMount != null
+            }
+            input_containers[c] {
+                c := input.review.object.spec.initContainers[_]
+                c.securityContext.procMount != null
+            }
+            input_containers[c] {
+                c := input.review.object.spec.ephemeralContainers[_]
+                c.securityContext.procMount != null
+            }
+
+            get_allowed_proc_mount(arg) = out {
+                not arg.parameters
+                out = "default"
+            }
+            get_allowed_proc_mount(arg) = out {
+                not arg.parameters.procMount
+                out = "default"
+            }
+            get_allowed_proc_mount(arg) = out {
+                arg.parameters.procMount
+                not valid_proc_mount(arg.parameters.procMount)
+                out = "default"
+            }
+            get_allowed_proc_mount(arg) = out {
+                valid_proc_mount(arg.parameters.procMount)
+                out = lower(arg.parameters.procMount)
+            }
+
+            valid_proc_mount(str) {
+                lower(str) == "default"
+            }
+            valid_proc_mount(str) {
+                lower(str) == "unmasked"
+            }
+          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/proc-mount/samples/psp-proc-mount/example_allowed_missing.yaml

@@ -0,0 +1,18 @@
+apiVersion: v1
+kind: Pod
+metadata:
+  name: nginx-proc-mount-disallowed
+  labels:
+    app: nginx-proc-mount
+spec:
+  hostUsers: false
+  containers:
+  - name: no-proc-mount-value
+    image: nginx
+    securityContext:
+      procMount: null
+  - name: no-proc-mount
+    image: nginx
+    securityContext: {}
+  - name: no-context
+    image: nginx

library/pod-security-policy/proc-mount/suite.yaml

@@ -15,6 +15,10 @@ tests:
     object: samples/psp-proc-mount/example_allowed.yaml
     assertions:
     - violations: no
+  - name: example-allowed-missing
+    object: samples/psp-proc-mount/example_allowed_missing.yaml
+    assertions:
+    - violations: no
   - name: disallowed-ephemeral
     object: samples/psp-proc-mount/disallowed_ephemeral.yaml
     assertions: