Generate polices ! Make Kubernetes life hassle free with Kyverno

24.07.2020 — kubernetes, kyverno — 1 min read

Briefly, Kyverno is a policy engine built for Kubernetes. It runs as an admission controller that can mutate and validate incoming resources. Also, Kyverno can generate any type of resource-based on various triggers from an admission request.

Kyverno generates resources based on a given set of conditions or triggers (read more about match/exclude). Kyverno generate a policy that support two types of rules.

Here is an example of a generated policy

Copy a resource from one namespace to another namespace. It's similar to copy command in a shell. The user has to define the clone block for providing the source resource path. In the below example source is config-template configmap in the default namespace

1apiVersion: kyverno.io/v1
2kind: ClusterPolicy
3metadata:
4  name: basic-policy
5spec:
6  rules:
7    - name: "Generate ConfigMap"
8      match:
9        resources:
10          kinds:
11          - Namespace
12      generate:
13        kind: ConfigMap # Kind of resource
14        name: default-config # Name of the new Resource
15        namespace: "{{request.object.metadata.name}}" # namespace that triggers this rule
16        synchronize : true
17        clone:
18          namespace: default
19          name: config-template

Generate a resource from inline data. In this type of generating policy users can define a defined data block directly in the policy.

1apiVersion: kyverno.io/v1
2kind: ClusterPolicy
3metadata:
4  name: basic-policy
5spec:
6  rules:
7    - name: "Generate Configmap"
8      match:
9        resources:
10          kinds:
11          - Namespace
12      generate:
13        kind: ConfigMap
14        name: config-app
15        namespace: "{{request.object.metadata.name}}" # namespace that triggers this rule
16        data:
17          data:
18            USERNAME: evalsocket=
19            DATABASE: blog
20        metadata:
21          labels:
22            purpose: config

Both examples above will trigger when a new namespace is created then Kyverno will automatically generate the configmaps in the newly created namespace.

1# Create a configmap in default namespace (We need this configmap for generating resources)
2➜  evalsocket ✗ kubectl create configmap config-template -n default --from-literal=special.how=very
3configmap/config-template created
4➜  evalsocket ✗ kubectl apply -f namespace-configmap.yaml
5clusterpolicy.kyverno.io/basic-policy created
6# Trigger the policy by creating a namespace
7➜  evalsocket ✗ kubectl create namespace kyverno-example
8namespace/kyverno-example created
9#Verify the resources
10➜  evalsocket ✗ kubectl get configmap -n kyverno-example
11NAME             DATA   AGE
12config-app       2      5s
13default-config   1      6s
14
15# We did it

Now let's discuss the behavior of the field synchronize: true. If you enable synchronization that means that Kyverno will manage your generated resources and administrators can only update/delete a resource from the policy. All direct actions on generated resources will be blocked by the kyverno. And in case of clone rule, the cluster-admin can update/delete generated resources from the source resources.

1# Let's try to delete configmap generated from default namespace. It should block your request basecause synchronize : true in case of generated configmap
2➜  evalsocket ✗  kubectl delete configmap default-config -n kyverno-example
3Error from server: admission webhook "nirmata.kyverno.resource.validating-webhook" denied the request: Resource is managed by a Kyverno policy and cannot be update manually. You can edit the generate policy to update this resource.
4
5#Now let's try to delete generated resource configmap that doesn't have synchronize : true and synchronize has default value false.
6➜  evalsocket ✗  kubectl delete configmap config-app -n kyverno-example
7configmap "config-app" deleted
8
9# Update Your configmap in default namespace  and check your update to see synchronize. Set special.how=why
10➜  evalsocket ✗ kubectl edit configmap config-template -n default
11configmap/config-template edited
12# Wait 5-6 sec and check
13➜  evalsocket ✗ kubectl get configmap default-config -n kyverno-example  -oyaml
14apiVersion: v1
15data:
16  special.how: why
17kind: ConfigMap
18metadata:
19  creationTimestamp: "2020-07-28T00:13:58Z"
20  labels:
21    app.kubernetes.io/managed-by: kyverno
22    app.kubernetes.io/synchronize: enable
23    kyverno.io/generated-by: Namespace--kyverno-example
24  name: default-config
25  namespace: kyverno-example
26  resourceVersion: "1682"
27  selfLink: /api/v1/namespaces/kyverno-example/configmaps/default-config
28  uid: b683b4c1-c498-4c25-b018-aa0c63cbcf7f

Let's try one more example. This time we will try to generate RBAC from generate policy. By default Kyverno service account doesn't have that permission, Admin has to manually provide access to Kyverno service account. Please read more about Kyverno role

Add admin privilege to Kyverno for creating the role

1apiVersion: rbac.authorization.k8s.io/v1
2kind: ClusterRole
3metadata:
4  name: kyverno:policycontroller
5rules:
6  - apiGroups:
7      - '*'
8    resources:
9      - '*'
10    verbs:
11      - create
12      - delete
13      - get
14      - list
15      - patch
16      - update
17      - watch
18---
19apiVersion: rbac.authorization.k8s.io/v1
20kind: ClusterRole
21metadata:
22  name: kyverno:userinfo
23rules:
24  - apiGroups:
25      - '*'
26    resources:
27      - roles
28      - clusterroles
29      - rolebindings
30      - clusterrolebindings
31      - configmaps
32    verbs:
33      - create
34      - delete
35      - get
36      - list
37      - patch
38      - update
39      - watch
40---
41kind: ClusterRoleBinding
42apiVersion: rbac.authorization.k8s.io/v1beta1
43metadata:
44  name: kyverno-admin-generate
45roleRef:
46  apiGroup: rbac.authorization.k8s.io
47  kind: ClusterRole
48  name: kyverno:generatecontroller # clusterRole defined above, to manage generated resources
49subjects:
50  - kind: ServiceAccount
51    name: kyverno-service-account # default Kyverno serviceAccount
52    namespace: kyverno

Now we are ready for generating role and cluster role. let's take an example policy for generating role

1apiVersion: kyverno.io/v1
2kind: ClusterPolicy
3metadata:
4  name: "gen-role-policy"
5spec:
6  background: false
7  rules:
8  - name: "gen-role"
9    match:
10        resources:
11          kinds:
12            - Namespace
13    generate:
14        kind: Role
15        name: "ns-role"
16        namespace: "{{request.object.metadata.name}}" # namespace that triggers this rule
17        synchronize: true
18        data:
19          rules:
20          - apiGroups: [""]
21            resources: ["pods"]
22            verbs: ["get", "watch", "list"]

Follow the steps to generate roles

1➜  evalsocket ✗  kubectl apply -f gen-role-policy.yaml
2clusterpolicy.kyverno.io/gen-role-policy created
3
4➜  evalsocket ✗ kubectl create ns new
5namespace/new created
6
7➜  evalsocket ✗ kubectl get Role -n new
8NAME     AGE
9ns-role   8s

via GIPHY

Some use cases for the generate policy are:

Create default Network policies for each namespace
Create default resource quotas for each namespace
For more example please check best practices

Check out the Kyverno GitHub page to learn more about the generate policy and several other features, like validating and deny rule. Find more about kyverno in the previous post Kubernetes Policy Management with Kyverno..