Proposal: separate yurt-manager clients (#2051)

Co-authored-by: luchen <[email protected]>

docs/proposals/20240517-separate-yurtmanager-clients.md

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+# Separate yurt-manager clients
+
+|              title              | authors   | reviewers | creation-date | last-updated | status |
+| :-----------------------------: | --------- | --------- | ------------- | ------------ | ------ |
+| Separate yurt-manager clients | @luc99hen |   @rambohe-ch        | 2024-05-17    |              |        |
+
+<!-- TOC -->
+* [Separate yurt-manager clients](#Separate-yurt-manager-clients)
+  * [Summary](#summary)
+  * [Motivation](#motivation)
+    * [Goals](#goals)
+    * [Non-Goals/Future Work](#non-goals)
+  * [Proposal](#proposal)
+  * [Implementation History](#implementation-history)
+<!-- TOC -->
+
+## Summary
+
+Yurt-manager is an important component in cloud environment for OpenYurt which contains multiple controllers and webhooks. Currently, those controllers and webhooks share one client and one set of RBAC (yurt-manager-role/yurt-manager-role-binding/yurt-manager-sa) which grows bigger as we add more function into yurt-manager. This mechanism makes a controller has access it shouldn't has. Furthermore, sharing one client and user agent makes it difficult to find out the request is from which controller from the audit logs.
+
+This proposal aims to address this issue by separating clients for different controllers and webhooks.
+
+## Motivation
+
+1. Sharing RBAC among controller/webhooks violates the principle of least authority.
+2. Sharing user agent is hard to trace the request from audit logs.
+
+### Goals
+
+1. Separate RBAC for different controllers and webhooks and make each has its own identity in Kubernetes.
+2. Separate User agents for different controllers and webhooks.
+3. Restrict each controller/webhook to only the permissions it may use.
+
+### Non-Goals
+
+1. Divide Yurt-manager into multiple components.
+
+## Proposal
+
+### Design principals
+
+1. Compatible with currently used controller-runtime framework. Developers of yurt-manager will not notice the changes underhood.
+2. Follow the principle of least authority. One controller/webhook should not be granted permissions it will not use.
+
+### Critical Questions
+
+1. What's the granularity of division?
+
+The structure of Yurt-manager is relatively complex. Yurt-manager is made up of controllers and webhooks. Some controllers may also have several sub-controllers. Also, for RBAC division, yurt-manager itself needs some base permissions to manage the whole component.
+
+```
+yurt-manager (base)
+- controllers
+  - yurt-app-set
+  - yurt-coordiator
+    - yurt-coordinator-cert
+	- pod-binding
+  - ...
+- webhooks
+  - yurt-app-set
+  - ...
+```
+
+In order to be compatible with the existing system, and also easy to manage permissions, we decide to divide the RBAC in this pattern
+
+```
+base: used in the entire life-cycle of yurt-manager, such as controller initialization and webhook server set up
+yurt-app-set(controller/webhook): used for yurt-app-set controller and webhook
+yurt-coordinator-cert(controller): used for yurt-coordinator-cert
+pod-binding(controller): used for pod-binding
+...
+```
+
+2. How to use different RBAC for different controller/webhooks?
+
+After different RBACs are prepared, we should make sure they are properly used by different controllers. We consider two feasible approaches here to achieve this goal.
+
+1) User impersonation
+
+Kubernets provides a [mechanism](https://kubernetes.io/docs/reference/access-authn-authz/authentication/#user-impersonation) that one user can act as another user through impersonation headers. These let requests manually override the user info a request authenticates as. 
+
+2) Token override
+
+We can override the client config' token with the prepared ServiceAccount token which has been bound to prescribed roles or cluster roles.
+
+Considering the additional complexity of the first approach, we choose the second one to override the token directly when building the client for each controller/webhook.
+
+### Solution Introduction
+
+The whole solution consists of two steps: 
+
+1. Generate RBAC
+
+First, prepare RBAC artifacts including role/clusterrole, rolebindidng/clusterrolebinding and serviceaccount for controllers and webhooks in yurt-manager. The specific steps are as following:
+
+1) Use bash scripts to search through the pkg/yurt-manager folder and find the components which need independent RBAC.
+2) Generate clusterrole and roles for those components one by one with `controller-gen`.
+3) Use bash scripts to complement the corresponding rolebinding/clusterrolebinding and serviceaccount artifacts.
+4) Gather all generated materials into one yaml in charts/yurt-manager/templates/yurt-manager-auto-generated.yaml with `kustomize`.
+
+Those steps above are performed automatically when you run `make manifests` command. Basically, yurt-manager developers don't need to know these implementations underhood.
+
+2. Prepare client
+
+Currently, all controllers use a client from `manager.GetClient()` which is provided by controller runtime framework directly. However the framework [doesn't provide any interface](https://github.com/kubernetes-sigs/controller-runtime/issues/2822) to provide different clients by controllers. Therefore, we have to implement a client wrapper to to construct a new client for every component based on the basic client.
+
+```go 
+func GetClientByControllerNameOrDie(mgr manager.Manager, controllerName, namespace string) client.Client {
+	// if controllerName is empty, return the base client of manager
+	if controllerName == "" {
+		return mgr.GetClient()
+	}
+
+	clientStore.lock.Lock()
+	defer clientStore.lock.Unlock()
+
+	if cli, ok := clientStore.clientsByName[controllerName]; ok {
+		return cli
+	}
+
+	// check if controller-specific ServiceAccount exist
+	_, err := getOrCreateServiceAccount(mgr.GetClient(), namespace, controllerName)
+	if err != nil {
+		return nil
+	}
+
+	// get base config
+	baseCfg := mgr.GetConfig()
+
+	// rename cfg user-agent
+	cfg := rest.CopyConfig(baseCfg)
+	rest.AddUserAgent(cfg, controllerName)
+
+	// add controller-specific token wrapper to cfg
+	cachedTokenSource := transport.NewCachedTokenSource(&tokenSourceImpl{
+		namespace:          namespace,
+		serviceAccountName: controllerName,
+		cli:                mgr.GetClient(),
+		expirationSeconds:  defaultExpirationSeconds,
+		leewayPercent:      defaultLeewayPercent,
+	})
+	cfg.Wrap(transport.ResettableTokenSourceWrapTransport(cachedTokenSource))
+
+	// construct client from cfg
+	clientOptions := client.Options{
+		Scheme: mgr.GetScheme(),
+		Mapper: mgr.GetRESTMapper(),
+		// todo: this is just a default option, we should use mgr's cache options
+		Cache: &client.CacheOptions{
+			Unstructured: false,
+			Reader:       mgr.GetCache(),
+		},
+	}
+
+	cli, err := client.New(cfg, clientOptions)
+	if err != nil {
+		panic(err)
+	}
+	clientStore.clientsByName[controllerName] = cli
+
+	return cli
+}
+
+var (
+	// defaultExpirationSeconds defines the duration of a TokenRequest in seconds.
+	defaultExpirationSeconds = int64(3600)
+	// defaultLeewayPercent defines the percentage of expiration left before the client trigger a token rotation.
+	// range[0, 100]
+	defaultLeewayPercent = 20
+)
+
+// migrate from kubernetes/staging/src/k8s.io/controller-manager/pkg/clientbuilder/client_builder_dynamic.go
+// change client to controller-runtime client
+type tokenSourceImpl struct {
+	namespace          string
+	serviceAccountName string
+	cli                client.Client
+	expirationSeconds  int64
+	leewayPercent      int
+}
+
+func (ts *tokenSourceImpl) Token() (*oauth2.Token, error) {
+	retTokenRequest := &v1authenticationapi.TokenRequest{
+		Spec: v1authenticationapi.TokenRequestSpec{
+			ExpirationSeconds: utilpointer.Int64Ptr(ts.expirationSeconds),
+		},
+	}
+
+	backoff := wait.Backoff{
+		Duration: 500 * time.Millisecond,
+		Factor:   2, // double the timeout for every failure
+		Steps:    4,
+	}
+	if err := wait.ExponentialBackoff(backoff, func() (bool, error) {
+		sa, inErr := getOrCreateServiceAccount(ts.cli, ts.namespace, ts.serviceAccountName)
+		if inErr != nil {
+			klog.Warningf("get or create service account failed: %v", inErr)
+			return false, nil
+		}
+
+		if inErr = ts.cli.SubResource("token").Create(context.Background(), sa, retTokenRequest); inErr != nil {
+			klog.Warningf("get token failed: %v", inErr)
+			return false, nil
+		}
+
+		return true, nil
+	}); err != nil {
+		return nil, fmt.Errorf("failed to get token for %s/%s: %v", ts.namespace, ts.serviceAccountName, err)
+	}
+
+	if retTokenRequest.Spec.ExpirationSeconds == nil {
+		return nil, fmt.Errorf("nil pointer of expiration in token request")
+	}
+
+	lifetime := retTokenRequest.Status.ExpirationTimestamp.Time.Sub(time.Now())
+	if lifetime < time.Minute*10 {
+		// possible clock skew issue, pin to minimum token lifetime
+		lifetime = time.Minute * 10
+	}
+
+	leeway := time.Duration(int64(lifetime) * int64(ts.leewayPercent) / 100)
+	expiry := time.Now().Add(lifetime).Add(-1 * leeway)
+
+	return &oauth2.Token{
+		AccessToken: retTokenRequest.Status.Token,
+		TokenType:   "Bearer",
+		Expiry:      expiry,
+	}, nil
+}
+
+func getOrCreateServiceAccount(cli client.Client, namespace, name string) (*v1.ServiceAccount, error) {
+	sa := &v1.ServiceAccount{
+		ObjectMeta: metav1.ObjectMeta{
+			Name:      name,
+			Namespace: namespace,
+		},
+	}
+	err := cli.Get(context.TODO(), client.ObjectKey{Namespace: namespace, Name: name}, sa)
+	if err == nil {
+		return sa, nil
+	}
+	if !apierrors.IsNotFound(err) {
+		return nil, err
+	}
+
+	// Create the namespace if we can't verify it exists.
+	// Tolerate errors, since we don't know whether this component has namespace creation permissions.
+	ns := &v1.Namespace{
+		ObjectMeta: metav1.ObjectMeta{
+			Name: namespace,
+		},
+	}
+	if err := cli.Get(context.TODO(), client.ObjectKey{}, ns); apierrors.IsNotFound(err) {
+		if err = cli.Create(context.TODO(), ns); err != nil && !apierrors.IsAlreadyExists(err) {
+			klog.Warningf("create non-exist namespace %s failed:%v", namespace, err)
+		}
+	}
+
+	// Create the service account
+	err = cli.Create(context.TODO(), sa)
+	if apierrors.IsAlreadyExists(err) {
+		// If we're racing to init and someone else already created it, re-fetch
+		err = cli.Get(context.TODO(), client.ObjectKey{Namespace: namespace, Name: name}, sa)
+		return sa, err
+	}
+	return sa, err
+}
+
+```
+
+If you are developing a new controller/webhook in yurt-manager, you should use `GetClientByControllerNameOrDie(mgr, controllerName)` instead of `mgr.GetClient()` to apply your own RBAC.
+
+## Implementation History
+
+* [ ]  05/17/2024: Draft proposal created;