feat: Add gatekeeper policy (#67)

README.md

@@ -17,6 +17,9 @@ The `github-app-operator` is a Kubernetes operator that generates an access toke
 - Stores the access token in a secret specified by `accessTokenSecret`.
 
 ### Private Key Retrieval Options
+> [!TIP]
+> There is a sample constraint template and constraint for Gatekeeper to restrict the type of private key source in the `gatekeeper-policy` folder since we can't restrict it to be unique in the GithubApp CRD.
+
 
 #### 1. Using a Kubernetes Secret
 - **Configuration:**

gatekeeper-policy/constraint-template.yaml

@@ -0,0 +1,22 @@
+apiVersion: templates.gatekeeper.sh/v1
+kind: ConstraintTemplate
+metadata:
+  name: githubappprivatekey
+spec:
+  crd:
+    spec:
+      names:
+        kind: GithubAppPrivateKey
+  targets:
+    - target: admission.k8s.gatekeeper.sh
+      rego: |
+        package githubappsecrets
+
+        violation[{"msg": msg}] {
+          target_keys := {"privateKeySecret", "googlePrivateKeySecret", "vaultPrivateKey"}
+          provided_keys := {key | _ = input.review.object.spec[key]}
+          intersection := target_keys & provided_keys
+          count(intersection) != 1
+          invalid := provided_keys - target_keys
+          msg := "Exactly one of privateKeySecret, googlePrivateKeySecret or vaultPrivateKey are allowed"
+        }

gatekeeper-policy/constraint.yaml

@@ -0,0 +1,9 @@
+apiVersion: constraints.gatekeeper.sh/v1beta1
+kind: GithubAppPrivateKey
+metadata:
+  name: githubapp-unique-key-constraint
+spec:
+  match:
+    kinds:
+      - apiGroups: ["githubapp.samir.io"]
+        kinds: ["GithubApp"]