Scoped Kubelet API Access

contributors/design-proposals/kubelet-authorizer.md

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+# Scoped Kubelet API Access
+
+Author: Jordan Liggitt (jliggitt@redhat.com)
+
+## Overview
+
+Kubelets are primarily responsible for:
+* creating and updating status of their Node API object
+* running and updating status of Pod API objects bound to their node
+* creating/deleting "mirror pod" API objects for statically-defined pods running on their node
+
+To run a pod, a kubelet must have read access to the following objects referenced by the pod spec:
+* Secrets
+* ConfigMaps
+* PersistentVolumeClaims (and any bound PersistentVolume or referenced StorageClass object)
+
+As of 1.6, kubelets have read/write access to all Node and Pod objects, and 
+read access to all Secret, ConfigMap, PersistentVolumeClaim, and PersistentVolume objects.
+This means that compromising a node gives access to credentials that allow modifying other nodes,
+pods belonging to other nodes, and accessing confidential data unrelated to the node's pods.
+
+This document proposes limiting a kubelet's API access using a new node authorizer, admission plugin, and additional API validation:
+* Node authorizer
+  * Authorizes requests from nodes using a fixed policy identical to the default RBAC `system:node` cluster role
+  * Further restricts secret and configmap access to only allow reading objects referenced by pods bound to the node making the request
+* Node admission
+  * Limit nodes to only be able to mutate their own Node API object
+  * Limit nodes to only be able to create mirror pods bound to themselves
+  * Limit nodes to only be able to mutate mirror pods bound to themselves
+  * Limit nodes to not be able to create mirror pods that reference API objects (secrets, configmaps, service accounts, persistent volume claims)
+* Additional API validation
+  * Reject mirror pods that are not bound to a node
+  * Reject pod updates that remove mirror pod annotations
+
+## Alternatives considered
+
+**Can this just be enforced by authorization?**
+
+Authorization does not have access to request bodies (or the existing object, for update requests),
+so it could not restrict access based on fields in the incoming or existing object.
+
+**Can this just be enforced by admission?**
+
+Admission is only called for mutating requests, so it could not restrict read access.
+
+**Can an existing authorizer be used?**
+
+Only one authorizer (RBAC) has in-tree support for dynamically programmable policy.
+
+Manifesting RBAC policy rules to give each node access to individual objects within namespaces
+would require large numbers of frequently-modified roles and rolebindings, resulting in
+significant write-multiplication.
+
+Additionally, not all clusters will use RBAC, but all useful clusters will have nodes.
+A node-specific authorizer allows cluster admins to continue to use their authorization mode of choice.
+
+## Node identification
+
+The first step is to identify whether a particular API request is being made by 
+a node, and if so, from which node.
+
+The proposed node authorizer and admission plugin will take a `NodeIdentifier` interface:
+
+```go
+type NodeIdentifier interface {
+  // IdentifyNode determines node information from the given user.Info.
+  // nodeName is the name of the Node API object associated with the user.Info,
+  // and may be empty if a specific node cannot be determined.
+  // isNode is true if the user.Info represents an identity issued to a node.
+  IdentifyNode(user.Info) (nodeName string, isNode bool)
+}
+```
+
+The default `NodeIdentifier` implementation:
+* `isNode` - true if the user groups contain the `system:nodes` group
+* `nodeName` - populated if `isNode` is true, and the user name is in the format `system:node:<nodeName>`
+
+This group and user name format match the identity created for each kubelet as part of [kubelet TLS bootstrapping](https://kubernetes.io/docs/admin/kubelet-tls-bootstrapping/).
+
+## Node authorizer
+
+A new node authorizer will be inserted into the authorization chain:
+* API server authorizer (existing, authorizes "loopback" API clients used by components within the API server)
+* Node authorizer (new)
+* User-configured authorizers... (e.g. ABAC, RBAC, Webhook)
+
+The node authorizer does the following:
+1. If a request is not from a node (`IdentifyNode()` returns isNode=false), reject
+2. If a request is not allowed by the rules in the default `system:node` cluster rule, reject
+3. If a specific node cannot be identified (`IdentifyNode()` returns nodeName=""):
+    * If in compatibility-mode (default), allow. This lets nodes that don't use node-specific identities continue to work with the broad authorization rules in step 2.
+    * If in strict-mode, reject. This lets deployments that provision all nodes with individual identities to indicate that only identifiable nodes should be allowed.
+4. If a request is for a secret, configmap, persistent volume or persistent volume claim, reject unless the verb is `get`, and the requested object is related to the requesting node:
+
+    * node -> pod
+    * node -> pod -> secret
+    * node -> pod -> configmap
+    * node -> pod -> pvc
+    * node -> pod -> pvc -> pv
+    * node -> pod -> pvc -> pv -> secret
+5. For other resources, allow
+
+Subsequent authorizers in the chain can run and choose to allow requests rejected by the node authorizer.
+
+## Node admission
+
+A new node admission plugin is made available that does the following:
+
+1. If a request is not from a node (`IdentifyNode()` returns isNode=false), allow the request
+2. If a specific node cannot be identified (`IdentifyNode()` returns nodeName=""):
+  * If in compatibility-mode (default), allow. This lets nodes that don't use node-specific identities continue to work.
+  * If in strict-mode, reject. This lets deployments that provision all nodes with individual identities to indicate that only identifiable nodes should be allowed.
+3. For requests made by identifiable nodes:
+  * Limits `create` of node resources:
+    * only allow the node object corresponding to the node making the API request
+  * Limits `create` of pod resources:
+    * only allow pods with mirror pod annotations
+    * only allow pods with nodeName set to the node making the API request
+    * do not allow pods that reference any API objects (secrets, serviceaccounts, configmaps, or persistentvolumeclaims)
+  * Limits `update` of node and nodes/status resources:
+    * only allow updating the node object corresponding to the node making the API request
+  * Limits `update` of pods/status resources:
+    * only allow reporting status for pods with nodeName set to the node making the API request
+  * Limits `delete` of node resources:
+    * only allow deleting the node object corresponding to the node making the API request
+  * Limits `delete` of pod resources:
+    * only allow deleting pods with nodeName set to the node making the API request
+
+## API Changes
+
+Change Pod validation for mirror pods:
+  * Reject `create` of pod resources with mirror pod annotations that do not specify a nodeName
+  * Reject `update` of pod resources with mirror pod annotations that modify or remove the mirror pod annotation
+
+## RBAC Changes
+
+As of 1.6, the `system:node` cluster role is automatically bound to the `system:nodes` group when using RBAC.
+
+Because the node authorizer accomplishes the same purpose, with the benefit of additional restrictions
+on secret and configmap access, this binding is no longer needed, and will no longer be set up automatically.
+
+The `system:node` cluster role will continue to be created when using RBAC,
+for compatibility with deployment methods that bind other users or groups to that role.
+
+## Migration considerations
+
+### Kubelets outside the `system:nodes` group
+
+Kubelets outside the `system:nodes` group would not be authorized by the node authorizer,
+and would need to continue to be authorized via whatever mechanism currently authorizes them.
+The node admission plugin would not restrict requests from these kubelets.
+
+### Kubelets with undifferentiated usernames
+
+In some deployments, kubelets have credentials that place them in the `system:nodes` group,
+but do not identify the particular node they are associated with.
+Those kubelets would be broadly authorized by the node authorizer,
+but would not have secret and configmap requests restricted.
+The node admission plugin would not restrict requests from these kubelets.
+
+### Upgrades from previous versions
+
+Versions prior to 1.7 that have the `system:node` cluster role bound to the `system:nodes` group would need to
+remove that binding in order for the node authorizer restrictions on secret and configmap access to be effective.
+
+## Future work
+
+Node and pod mutation, and secret and configmap read access are the most critical permissions to restrict.
+Future work could further limit a kubelet's API access:
+* only get persistent volume claims and persistent volumes referenced by a bound pod
+* only write events with the kubelet set as the event source
+* only get/list/watch pods bound to the kubelet's node (requires additional list/watch authorization capabilities)
+* only get/list/watch it's own node object (requires additional list/watch authorization capabilities)
+
+Features that expand or modify the APIs or objects accessed by the kubelet will need to involve the node authorizer.
+Known features in the design or development stages that might modify kubelet API access are:
+* [Dynamic kubelet configuration](https://github.com/kubernetes/features/issues/281)
+* [Local storage management](https://github.com/kubernetes/community/blob/master/contributors/design-proposals/local-storage-overview.md)
+* [Bulk watch of secrets/configmaps](https://github.com/kubernetes/community/pull/443)