Istio Soft Multi-Tenancy Support

Multi-tenancy is commonly used in many environments across many different applications, but the implementation details and functionality provided on a per tenant basis does not follow one model in all environments. The Kubernetes multi-tenancy working group is working to define the multi-tenant use cases and functionality that should be available within Kubernetes. However, from their work so far it is clear that only “soft multi-tenancy” is possible due to the inability to fully protect against malicious containers or workloads gaining access to other tenant’s pods or kernel resources.

Soft multi-tenancy

For this blog, “soft multi-tenancy” is defined as having a single Kubernetes control plane with multiple Istio control planes and multiple meshes, one control plane and one mesh per tenant. The cluster administrator gets control and visibility across all the Istio control planes, while the tenant administrator only gets control of a specific Istio instance. Separation between the tenants is provided by Kubernetes namespaces and RBAC.

One use case for this deployment model is a shared corporate infrastructure where malicious actions are not expected, but a clean separation of the tenants is still required.

Potential future Istio multi-tenant deployment models are described at the bottom of this blog.

Deployment

Multiple Istio control planes

Deploying multiple Istio control planes starts by replacing all namespace references in a manifest file with the desired namespace. Using istio.yaml as an example, if two tenant level Istio control planes are required; the first can use the istio.yaml default name of istio-system and a second control plane can be created by generating a new yaml file with a different namespace. As an example, the following command creates a yaml file with the Istio namespace of istio-system1.

$ cat istio.yaml | sed s/istio-system/istio-system1/g > istio-system1.yaml

The istio.yaml file contains the details of the Istio control plane deployment, including the pods that make up the control plane (Mixer, Pilot, Ingress, Galley, CA). Deploying the two Istio control plane yaml files:

$ kubectl apply -f install/kubernetes/istio.yaml
$ kubectl apply -f install/kubernetes/istio-system1.yaml

Results in two Istio control planes running in two namespaces.

$ kubectl get pods --all-namespaces
NAMESPACE       NAME                                       READY     STATUS    RESTARTS   AGE
istio-system    istio-ca-ffbb75c6f-98w6x                   1/1       Running   0          15d
istio-system    istio-ingress-68d65fc5c6-dnvfl             1/1       Running   0          15d
istio-system    istio-mixer-5b9f8dffb5-8875r               3/3       Running   0          15d
istio-system    istio-pilot-678fc976c8-b8tv6               2/2       Running   0          15d
istio-system1   istio-ca-5f496fdbcd-lqhlk                  1/1       Running   0          15d
istio-system1   istio-ingress-68d65fc5c6-2vldg             1/1       Running   0          15d
istio-system1   istio-mixer-7d4f7b9968-66z44               3/3       Running   0          15d
istio-system1   istio-pilot-5bb6b7669c-779vb               2/2       Running   0          15d

The Istio sidecar and addons, if required, manifests must also be deployed to match the configured namespace in use by the tenant’s Istio control plane.

The execution of these two yaml files is the responsibility of the cluster administrator, not the tenant level administrator. Additional RBAC restrictions will also need to be configured and applied by the cluster administrator, limiting the tenant administrator to only the assigned namespace.

Split common and namespace specific resources

The manifest files in the Istio repositories create both common resources that would be used by all Istio control planes as well as resources that are replicated per control plane. Although it is a simple matter to deploy multiple control planes by replacing the istio-system namespace references as described above, a better approach is to split the manifests into a common part that is deployed once for all tenants and a tenant specific part. For the Custom Resource Definitions, the roles and the role bindings should be separated out from the provided Istio manifests. Additionally, the roles and role bindings in the provided Istio manifests are probably unsuitable for a multi-tenant environment and should be modified or augmented as described in the next section.

Kubernetes RBAC for Istio control plane resources

To restrict a tenant administrator to a single Istio namespace, the cluster administrator would create a manifest containing, at a minimum, a Role and RoleBinding similar to the one below. In this example, a tenant administrator named sales-admin is limited to the namespace istio-system1. A completed manifest would contain many more apiGroups under the Role providing resource access to the tenant administrator.

kind: Role
apiVersion: rbac.authorization.k8s.io/v1
metadata:
  namespace: istio-system1
  name: ns-access-for-sales-admin-istio-system1
rules:
- apiGroups: [""] # "" indicates the core API group
  resources: ["*"]
  verbs: ["*"]
---
kind: RoleBinding
apiVersion: rbac.authorization.k8s.io/v1
metadata:
  name: access-all-istio-system1
  namespace: istio-system1
subjects:
- kind: User
  name: sales-admin
  apiGroup: rbac.authorization.k8s.io
roleRef:
  kind: Role
  name: ns-access-for-sales-admin-istio-system1
  apiGroup: rbac.authorization.k8s.io

Watching specific namespaces for service discovery

In addition to creating RBAC rules limiting the tenant administrator’s access to a specific Istio control plane, the Istio manifest must be updated to specify the application namespace that Pilot should watch for creation of its xDS cache. This is done by starting the Pilot component with the additional command line arguments --appNamespace, ns-1. Where ns-1 is the namespace that the tenant’s application will be deployed in. An example snippet from the istio-system1.yaml file is shown below.

apiVersion: extensions/v1beta1
kind: Deployment
metadata:
  name: istio-pilot
  namespace: istio-system1
  annotations:
    sidecar.istio.io/inject: "false"
spec:
  replicas: 1
  template:
    metadata:
      labels:
        istio: pilot
    spec:
      serviceAccountName: istio-pilot-service-account
      containers:
      - name: discovery
        image: docker.io/<user ID>/pilot:<tag>
        imagePullPolicy: IfNotPresent
        args: ["discovery", "-v", "2", "--admission-service", "istio-pilot", "--appNamespace", "ns-1"]
        ports:
        - containerPort: 8080
        - containerPort: 443

Deploying the tenant application in a namespace

Now that the cluster administrator has created the tenant’s namespace (ex. istio-system1) and Pilot’s service discovery has been configured to watch for a specific application namespace (ex. ns-1), create the application manifests to deploy in that tenant’s specific namespace. For example:

apiVersion: v1
kind: Namespace
metadata:
  name: ns-1

And add the namespace reference to each resource type included in the application’s manifest file. For example:

apiVersion: v1
kind: Service
metadata:
  name: details
  labels:
    app: details
  namespace: ns-1

Although not shown, the application namespaces will also have RBAC settings limiting access to certain resources. These RBAC settings could be set by the cluster administrator and/or the tenant administrator.

Using kubectl in a multi-tenant environment

When defining route rules or destination policies, it is necessary to ensure that the kubectl command is scoped to the namespace the Istio control plane is running in to ensure the resource is created in the proper namespace. Additionally, the rule itself must be scoped to the tenant’s namespace so that it will be applied properly to that tenant’s mesh. The -i option is used to create (or get or describe) the rule in the namespace that the Istio control plane is deployed in. The -n option will scope the rule to the tenant’s mesh and should be set to the namespace that the tenant’s app is deployed in. Note that the -n option can be skipped on the command line if the .yaml file for the resource scopes it properly instead.

For example, the following command would be required to add a route rule to the istio-system1 namespace:

$ kubectl –i istio-system1 apply -n ns-1 -f route_rule_v2.yaml

And can be displayed using the command:

$ kubectl -i istio-system1 -n ns-1 get routerule
NAME                  KIND                                  NAMESPACE
details-Default       RouteRule.v1alpha2.config.istio.io    ns-1
productpage-default   RouteRule.v1alpha2.config.istio.io    ns-1
ratings-default       RouteRule.v1alpha2.config.istio.io    ns-1
reviews-default       RouteRule.v1alpha2.config.istio.io    ns-1

See the Multiple Istio control planes section of this document for more details on namespace requirements in a multi-tenant environment.

Test results

Following the instructions above, a cluster administrator can create an environment limiting, via RBAC and namespaces, what a tenant administrator can deploy.

After deployment, accessing the Istio control plane pods assigned to a specific tenant administrator is permitted:

$ kubectl get pods -n istio-system
NAME                                      READY     STATUS    RESTARTS   AGE
grafana-78d649479f-8pqk9                  1/1       Running   0          1d
istio-ca-ffbb75c6f-98w6x                  1/1       Running   0          1d
istio-ingress-68d65fc5c6-dnvfl            1/1       Running   0          1d
istio-mixer-5b9f8dffb5-8875r              3/3       Running   0          1d
istio-pilot-678fc976c8-b8tv6              2/2       Running   0          1d
istio-sidecar-injector-7587bd559d-5tgk6   1/1       Running   0          1d
prometheus-cf8456855-hdcq7                1/1       Running   0          1d

However, accessing all the cluster’s pods is not permitted:

$ kubectl get pods --all-namespaces
Error from server (Forbidden): pods is forbidden: User "dev-admin" cannot list pods at the cluster scope

And neither is accessing another tenant’s namespace:

$ kubectl get pods -n istio-system1
Error from server (Forbidden): pods is forbidden: User "dev-admin" cannot list pods in the namespace "istio-system1"

The tenant administrator can deploy applications in the application namespace configured for that tenant. As an example, updating the Bookinfo manifests and then deploying under the tenant’s application namespace of ns-0, listing the pods in use by this tenant’s namespace is permitted:

$ kubectl get pods -n ns-0
NAME                              READY     STATUS    RESTARTS   AGE
details-v1-64b86cd49-b7rkr        2/2       Running   0          1d
productpage-v1-84f77f8747-rf2mt   2/2       Running   0          1d
ratings-v1-5f46655b57-5b4c5       2/2       Running   0          1d
reviews-v1-ff6bdb95b-pm5lb        2/2       Running   0          1d
reviews-v2-5799558d68-b989t       2/2       Running   0          1d
reviews-v3-58ff7d665b-lw5j9       2/2       Running   0          1d

But accessing another tenant’s application namespace is not:

$ kubectl get pods -n ns-1
Error from server (Forbidden): pods is forbidden: User "dev-admin" cannot list pods in the namespace "ns-1"

If the add-on tools, example Prometheus, are deployed (also limited by an Istio namespace) the statistical results returned would represent only that traffic seen from that tenant’s application namespace.

Conclusion

The evaluation performed indicates Istio has sufficient capabilities and security to meet a small number of multi-tenant use cases. It also shows that Istio and Kubernetes cannot provide sufficient capabilities and security for other use cases, especially those use cases that require complete security and isolation between untrusted tenants. The improvements required to reach a more secure model of security and isolation require work in container technology, ex. Kubernetes, rather than improvements in Istio capabilities.

Issues

• The CA (Certificate Authority) and Mixer pod logs from one tenant’s Istio control plane (e.g. istio-system namespace) contained ‘info’ messages from a second tenant’s Istio control plane (e.g. istio-system1 namespace).

Challenges with other multi-tenancy models

Other multi-tenancy deployment models were considered:

1. A single mesh with multiple applications, one for each tenant on the mesh. The cluster administrator gets control and visibility mesh wide and across all applications, while the tenant administrator only gets control of a specific application.

2. A single Istio control plane with multiple meshes, one mesh per tenant. The cluster administrator gets control and visibility across the entire Istio control plane and all meshes, while the tenant administrator only gets control of a specific mesh.

3. A single cloud environment (cluster controlled), but multiple Kubernetes control planes (tenant controlled).

These options either can’t be properly supported without code changes or don’t fully address the use cases.

Current Istio capabilities are poorly suited to support the first model as it lacks sufficient RBAC capabilities to support cluster versus tenant operations. Additionally, having multiple tenants under one mesh is too insecure with the current mesh model and the way Istio drives configuration to the Envoy proxies.

Regarding the second option, the current Istio paradigm assumes a single mesh per Istio control plane. The needed changes to support this model are substantial. They would require finer grained scoping of resources and security domains based on namespaces, as well as, additional Istio RBAC changes. This model will likely be addressed by future work, but not currently possible.

The third model doesn’t satisfy most use cases, as most cluster administrators prefer a common Kubernetes control plane which they provide as a PaaS to their tenants.

Future work

Allowing a single Istio control plane to control multiple meshes would be an obvious next feature. An additional improvement is to provide a single mesh that can host different tenants with some level of isolation and security between the tenants. This could be done by partitioning within a single control plane using the same logical notion of namespace as Kubernetes. A document has been started within the Istio community to define additional use cases and the Istio functionality required to support those use cases.

References

• Video on Kubernetes multi-tenancy support, Multi-Tenancy Support & Security Modeling with RBAC and Namespaces, and the supporting slide deck.
• KubeCon talk on security that discusses Kubernetes support for “Cooperative soft multi-tenancy”, Building for Trust: How to Secure Your Kubernetes.
• Kubernetes documentation on RBAC and namespaces.
• KubeCon slide deck on Multi-tenancy Deep Dive.
• Google document on Multi-tenancy models for Kubernetes. (Requires permission)
• Cloud Foundry WIP document, Multi-cloud and Multi-tenancy
• Istio Auto Multi-Tenancy 101