port serviceaccount token volume design proposal to KEP

keps/sig-storage/20180515-svcacct-token-volumes.md

@@ -0,0 +1,179 @@
+---
+title: Service Account Token Volumes
+authors:
+  - "@smarterclayton"
+  - "@liggitt"
+  - "@mikedanese"
+owning-sig: sig-storage
+participating-sigs:
+  - sig-auth
+reviewers:
+  - "@jsafrane"
+  - "@tallclair"
+approvers:
+  - "@saad-ali"
+editor: "@zshihang"
+creation-date: 2018-05-15
+last-updated: 2020-03-09
+status: implemented
+see-also:
+  - "https://github.com/kubernetes/community/blob/e3a6b8172fec9506b15520549e52be683e30cbfb/contributors/design-proposals/storage/svcacct-token-volume-source.md"
+---
+
+# Service Account Token Volumes
+
+## Table of Contents
+
+<!-- toc -->
+- [Summary](#summary)
+- [Motivation](#motivation)
+- [Proposal](#proposal)
+  - [Token Volume Projection](#token-volume-projection)
+  - [Alternatives](#alternatives)
+- [Graduation Criteria](#graduation-criteria)
+<!-- /toc -->
+
+## Summary
+
+Kubernetes is able to provide pods with unique identity tokens that can prove
+the caller is a particular pod to a Kubernetes API server. These tokens are
+injected into pods as secrets. This proposal proposes a new mechanism of
+distribution with support for improved service account tokens and explores how
+to migrate from the existing mechanism backwards compatibly.
+
+## Motivation
+
+Many workloads running on Kubernetes need to prove to external parties who they
+are in order to participate in a larger application environment. This identity
+must be attested to by the orchestration system in a way that allows a third
+party to trust that an arbitrary container on the cluster is who it says it is.
+In addition, infrastructure running on top of Kubernetes needs a simple
+mechanism to communicate with the Kubernetes APIs and to provide more complex
+tooling. Finally, a significant set of security challenges are associated with
+storing service account tokens as secrets in Kubernetes and limiting the methods
+whereby malicious parties can get access to these tokens will reduce the risk of
+platform compromise.
+
+As a platform, Kubernetes should evolve to allow identity management systems to
+provide more powerful workload identity without breaking existing use cases, and
+provide a simple out of the box workload identity that is sufficient to cover
+the requirements of bootstrapping low-level infrastructure running on
+Kubernetes. We expect that other systems to cover the more advanced scenarios,
+and see this effort as necessary glue to allow more powerful systems to succeed.
+
+With this feature, we hope to provide a backwards compatible replacement for
+service account tokens that strengthens the security and improves the
+scalability of the platform.
+
+
+## Proposal
+
+Kubernetes should implement a ServiceAccountToken volume projection that
+maintains a service account token requested by the node from the TokenRequest
+API.
+
+### Token Volume Projection
+
+A new volume projection will be implemented with an API that closely matches the
+TokenRequest API.
+
+```go
+type ProjectedVolumeSource struct {
+  Sources []VolumeProjection
+  DefaultMode *int32
+}
+
+type VolumeProjection struct {
+  Secret *SecretProjection
+  DownwardAPI *DownwardAPIProjection
+  ConfigMap *ConfigMapProjection
+  ServiceAccountToken *ServiceAccountTokenProjection
+}
+
+// ServiceAccountTokenProjection represents a projected service account token
+// volume. This projection can be used to insert a service account token into
+// the pods runtime filesystem for use against APIs (Kubernetes API Server or
+// otherwise).
+type ServiceAccountTokenProjection struct {
+  // Audience is the intended audience of the token. A recipient of a token
+  // must identify itself with an identifier specified in the audience of the
+  // token, and otherwise should reject the token. The audience defaults to the
+  // identifier of the apiserver.
+  Audience string
+  // ExpirationSeconds is the requested duration of validity of the service
+  // account token. As the token approaches expiration, the kubelet volume
+  // plugin will proactively rotate the service account token. The kubelet will
+  // start trying to rotate the token if the token is older than 80 percent of
+  // its time to live or if the token is older than 24 hours.Defaults to 1 hour
+  // and must be at least 10 minutes.
+  ExpirationSeconds int64
+  // Path is the relative path of the file to project the token into.
+  Path string
+}
+```
+
+A volume plugin implemented in the kubelet will project a service account token
+sourced from the TokenRequest API into volumes created from
+ProjectedVolumeSources. As the token approaches expiration, the kubelet volume
+plugin will proactively rotate the service account token. The kubelet will start
+trying to rotate the token if the token is older than 80 percent of its time to
+live or if the token is older than 24 hours.
+
+To replace the current service account token secrets, we also need to inject the
+clusters CA certificate bundle. Initially we will deploy to data in a configmap
+per-namespace and reference it using a ConfigMapProjection.
+
+A projected volume source that is equivalent to the current service account
+secret:
+
+```yaml
+sources:
+- serviceAccountToken:
+    expirationSeconds: 3153600000 # 100 years
+    path: token
+- configMap:
+    name: kube-cacrt
+    items:
+    - key: ca.crt
+      path: ca.crt
+- downwardAPI:
+    items:
+    - path: namespace
+      fieldRef: metadata.namespace
+```
+
+
+This fixes one scalability issue with the current service account token
+deployment model where secret GETs are a large portion of overall apiserver
+traffic.
+
+A projected volume source that requests a token for vault and Istio CA:
+
+```yaml
+sources:
+- serviceAccountToken:
+    path: vault-token
+    audience: vault
+- serviceAccountToken:
+    path: istio-token
+    audience: ca.istio.io
+```
+
+### Alternatives
+
+1.  Instead of implementing a service account token volume projection, we could
+    implement all injection as a flex volume or CSI plugin.
+    1.  Both flex volume and CSI are alpha and are unlikely to graduate soon.
+    1.  Virtual kubelets (like Fargate or ACS) may not be able to run flex
+        volumes.
+    1.  Service account tokens are a fundamental part of our API.
+1.  Remove service accounts and service account tokens completely from core, use
+    an alternate mechanism that sits outside the platform.
+    1.  Other core features need service account integration, leading to all
+        users needing to install this extension.
+    1.  Complicates installation for the majority of users.
+
+
+## Graduation Criteria
+
+TBD