impersonator: run as a distinct SA with minimal permissions

This change updates the impersonation proxy code to run as a
distinct service account that only has permission to impersonate
identities.  Thus any future vulnerability that causes the
impersonation headers to be dropped will fail closed instead of
escalating to the concierge's default service account which has
significantly more permissions.

Signed-off-by: Monis Khan <mok@vmware.com>

deploy/concierge/deployment.yaml

@@ -29,6 +29,18 @@ metadata:
   labels: #@ labels()
 ---
 apiVersion: v1
+kind: ServiceAccount
+metadata:
+  name: #@ defaultResourceNameWithSuffix("impersonation-proxy")
+  namespace: #@ namespace()
+  labels: #@ labels()
+  annotations:
+    #! we need to create this service account before we create the secret
+    kapp.k14s.io/change-group: "impersonation-proxy.concierge.pinniped.dev/serviceaccount"
+secrets: #! make sure the token controller does not create any other secrets
+- name: #@ defaultResourceNameWithSuffix("impersonation-proxy")
+---
+apiVersion: v1
 kind: ConfigMap
 metadata:
   name: #@ defaultResourceNameWithSuffix("config")
@@ -134,6 +146,8 @@ spec:
               mountPath: /etc/config
             - name: podinfo
               mountPath: /etc/podinfo
+            - name: impersonation-proxy
+              mountPath: /var/run/secrets/impersonation-proxy.concierge.pinniped.dev/serviceaccount
           livenessProbe:
             httpGet:
               path: /healthz
@@ -156,6 +170,12 @@ spec:
         - name: config-volume
           configMap:
             name: #@ defaultResourceNameWithSuffix("config")
+        - name: impersonation-proxy
+          secret:
+            secretName: #@ defaultResourceNameWithSuffix("impersonation-proxy")
+            items: #! make sure our pod does not start until the token controller has a chance to populate the secret
+              - key: token
+                path: token
         - name: podinfo
           downwardAPI:
             items:
@@ -265,3 +285,16 @@ spec:
       loadBalancerIP: #@ data.values.impersonation_proxy_spec.service.load_balancer_ip
       #@ end
       annotations: #@ data.values.impersonation_proxy_spec.service.annotations
+---
+apiVersion: v1
+kind: Secret
+metadata:
+  name: #@ defaultResourceNameWithSuffix("impersonation-proxy")
+  namespace: #@ namespace()
+  labels: #@ labels()
+  annotations:
+    #! wait until the SA exists to create this secret so that the token controller does not delete it
+    #! we have this secret at the end so that kubectl will create the service account first
+    kapp.k14s.io/change-rule: "upsert after upserting impersonation-proxy.concierge.pinniped.dev/serviceaccount"
+    kubernetes.io/service-account.name: #@ defaultResourceNameWithSuffix("impersonation-proxy")
+type: kubernetes.io/service-account-token

deploy/concierge/rbac.yaml

@@ -28,12 +28,6 @@ rules:
     resources: [ securitycontextconstraints ]
     verbs: [ use ]
     resourceNames: [ nonroot ]
-  - apiGroups: [ "" ]
-    resources: [ "users", "groups", "serviceaccounts" ]
-    verbs: [ "impersonate" ]
-  - apiGroups: [ "authentication.k8s.io" ]
-    resources: [ "*" ]  #! What we really want is userextras/* but the RBAC authorizer only supports */subresource, not resource/*
-    verbs: [ "impersonate" ]
   - apiGroups: [ "" ]
     resources: [ nodes ]
     verbs: [ list ]
@@ -64,6 +58,35 @@ roleRef:
   name: #@ defaultResourceNameWithSuffix("aggregated-api-server")
   apiGroup: rbac.authorization.k8s.io
 
+#! Give minimal permissions to impersonation proxy service account
+---
+apiVersion: rbac.authorization.k8s.io/v1
+kind: ClusterRole
+metadata:
+  name: #@ defaultResourceNameWithSuffix("impersonation-proxy")
+  labels: #@ labels()
+rules:
+  - apiGroups: [ "" ]
+    resources: [ "users", "groups", "serviceaccounts" ]
+    verbs: [ "impersonate" ]
+  - apiGroups: [ "authentication.k8s.io" ]
+    resources: [ "*" ]  #! What we really want is userextras/* but the RBAC authorizer only supports */subresource, not resource/*
+    verbs: [ "impersonate" ]
+---
+kind: ClusterRoleBinding
+apiVersion: rbac.authorization.k8s.io/v1
+metadata:
+  name: #@ defaultResourceNameWithSuffix("impersonation-proxy")
+  labels: #@ labels()
+subjects:
+  - kind: ServiceAccount
+    name: #@ defaultResourceNameWithSuffix("impersonation-proxy")
+    namespace: #@ namespace()
+roleRef:
+  kind: ClusterRole
+  name: #@ defaultResourceNameWithSuffix("impersonation-proxy")
+  apiGroup: rbac.authorization.k8s.io
+
 #! Give permission to the kube-cert-agent Pod to run privileged.
 ---
 apiVersion: rbac.authorization.k8s.io/v1