gke-metadata-server

A GKE Metadata Server emulator for making it easier to use GCP Workload Identity Federation inside non-GKE Kubernetes clusters, e.g. KinD, on-prem, managed Kubernetes from other clouds, etc. This implementation tries to mimic the gke-metadata-server DaemonSet deployed automatically by Google in the kube-system namespace of GKE clusters that have the feature Workload Identity Federation for GKE enabled. See how the GKE Metadata Server works.

Usage

Steps:

1. Configure GCP Workload Identity Federation for Kubernetes.
2. Deploy gke-metadata-server in the cluster using the Workload Identity Provider full name, obtained after step 1.
3. Add the label iam.gke.io/gke-metadata-server-enabled: "true" to the Nodes you want to have the emulator Pods running on. Only Pods running on these Nodes will be able to use the emulator. The most recommended way to do this is by using Node pooling/grouping features from your Kubernetes provider.
4. See ./testdata/pod.yaml for an example of how to configure your Pods and their ServiceAccounts.
5. (Optional but highly recommended) Verify gke-metadata-server's artifact signatures to make sure you are deploying authentic artifacts distributed by this project.

Configure GCP Workload Identity Federation for Kubernetes

Steps:

1. Create a Workload Identity Pool and Provider pair for the cluster.
2. Grant Kubernetes ServiceAccounts permission to impersonate Google Service Accounts.

Official docs and examples are here.

More examples for all the configuration described in this section are available here: ./terraform/test.tf. This is where we provision the infrastructure required for testing this project in CI and development.

Create a Workload Identity Pool and Provider pair for the cluster

Before granting ServiceAccounts of a Kubernetes cluster permission to impersonate Google Service Accounts, a Workload Identity Pool and Provider pair must be created for the cluster in Google Cloud Platform.

A Pool is a namespace for Subjects and Providers, where Subjects represent the identities to whom permission to impersonate Google Service Accounts are granted, i.e. the Kubernetes ServiceAccounts in this case.

A Provider stores the OpenID Connect configuration parameters retrieved from the cluster for allowing Google to verify ServiceAccount Tokens issued by the cluster. Specifically for the creation of a Provider, see an example in the ./Makefile, target create-or-update-provider.

The ServiceAccount Tokens issued by Kubernetes are JWT tokens. Each such token is exchanged for an OAuth 2.0 Access Token that impersonates a Google Service Account. During a token exchange, the Subject is mapped from the sub claim of the JWT (achieved in the Makefile example by --attribute-mapping=google.subject=assertion.sub). In this case the Subject has the following format:

system:serviceaccount:{k8s_namespace}:{k8s_sa_name}

The Subject total length cannot exceed 127 characters (docs).

Attention 1: If you update the private keys of the ServiceAccount Issuer of the cluster you must update the Provider with the new OpenID Connect configuration, otherwise service will be disrupted.

Attention 2: Please make sure not to specify any audiences when creating the Provider. The emulator uses the default audience when issuing the ServiceAccount Tokens. The default audience contains the full name of the Provider, which is a strong restriction:

//iam.googleapis.com/{provider_full_name}

where {provider_full_name} has the form:

{pool_full_name}/providers/{provider_short_name}

and {pool_full_name} has the form:

projects/{gcp_project_number}/locations/global/workloadIdentityPools/{pool_short_name}

Grant Kubernetes ServiceAccounts permission to impersonate Google Service Accounts

For getting access to GCP resources

For allowing the Kubernetes ServiceAccount {k8s_sa_name} from the namespace {k8s_namespace} to impersonate the Google Service Account {gcp_service_account}@{gcp_project_id}.iam.gserviceaccount.com, grant the IAM Role roles/iam.workloadIdentityUser on this Service Account to the following principal:

principal://iam.googleapis.com/{pool_full_name}/subject/system:serviceaccount:{k8s_namespace}:{k8s_sa_name}

This principal will be reflected as a Subject in the Google Cloud Console webpage of the Pool.

For issuing Google OpenID Connect Identity Tokens (and getting access on external systems)

If you plan to use the GET /computeMetadata/v1/instance/service-accounts/default/identity API for issuing Google OpenID Connect Identity Tokens to use in external systems, you must also grant the IAM Role roles/iam.serviceAccountOpenIdTokenCreator on the target Google Service Accounts to the Kubernetes ServiceAccount of the emulator, i.e. the following principal:

principal://iam.googleapis.com/{pool_full_name}/subject/system:serviceaccount:kube-system:gke-metadata-server

If the target Google Service Accounts are all contained in the same project, you can grant this role at the project level. If the target Google Service Accounts are in different projects, the recommended way is to grant this role on each target Google Service Account individually. Regardless of the approach you choose for granting this role, the principal above must be granted the role on the target Google Service Accounts, either at the resource level (directly on the target Google Service Accounts), or at the project level, or at the folder level, or at the organization level. The lower the level, the better in terms of security.

Alternatively, grant direct resource access to the Kubernetes ServiceAccount

Workload Identity Federation for Kubernetes allows you to directly grant Kubernetes ServiceAccounts access to Google resources, without the need to impersonate a Google Service Account. This is done by granting the given IAM Roles directly to principals of the form described above for impersonation. See docs.

Some specific GCP services do not support this method. See docs.

The GET /computeMetadata/v1/instance/service-accounts/default/identity API is not supported by this method. If you plan to use this API, you must use the impersonation method described above.

Deploy gke-metadata-server in your cluster

Using the Helm Chart

A Helm Chart is available in the following Helm OCI Repository:

ghcr.io/matheuscscp/gke-metadata-server-helm:{helm_version} (GitHub Container Registry)

Here {helm_version} is a Helm Chart version, i.e. the field .helm in the file ./versions.yaml. Check available releases in the GitHub Releases Page.

See the Helm Values API in the file ./helm/gke-metadata-server/values.yaml. Make sure to specify at least the full name of the Workload Identity Provider.

Using the Timoni Module

If you prefer something newer and strongly typed, a Timoni Module is available in the following Timoni OCI Repository:

ghcr.io/matheuscscp/gke-metadata-server-timoni:{timoni_version} (GitHub Container Registry)

Here {timoni_version} is a Timoni Module version, i.e. the field .timoni in the file ./versions.yaml. Check available releases in the GitHub Releases Page.

See the Timoni Values API in the files ./timoni/gke-metadata-server/templates/config.tpl.cue and ./timoni/gke-metadata-server/templates/settings.cue. Make sure to specify at least the full name of the Workload Identity Provider.

Using only the container image (with your own Kubernetes manifests)

Alternatively, you can write your own Kubernetes manifests and consume only the container image:

ghcr.io/matheuscscp/gke-metadata-server:{container_version} (GitHub Container Registry)

Here {container_version} is an app container version, i.e. the field .container in the file ./versions.yaml. Check available releases in the GitHub Releases Page.

Verify the image signatures

For verifying the images above use the cosign CLI tool.

Verify the container image

For verifying the image of a given Container GitHub Release (tags v{container_version}), download the digest file container-digest.txt attached to the Github Release and use it with cosign:

cosign verify ghcr.io/matheuscscp/gke-metadata-server@$(cat container-digest.txt) \
  --certificate-oidc-issuer=https://token.actions.githubusercontent.com \
  --certificate-identity=https://github.com/matheuscscp/gke-metadata-server/.github/workflows/release.yml@refs/heads/main

Automatic verification

If you are using Sigstore's Policy Controller for enforcing policies you can automate the image verification using Keyless Authorities.

If you are using Kyverno for enforcing policies you can automate the image verification using Keyless Verification.

Verify the Helm Chart image

For verifying the image of a given Helm Chart GitHub Release (tags helm-v{helm_version}), download the digest file helm-digest.txt attached to the Github Release and use it with cosign:

cosign verify ghcr.io/matheuscscp/gke-metadata-server-helm@$(cat helm-digest.txt) \
  --certificate-oidc-issuer=https://token.actions.githubusercontent.com \
  --certificate-identity=https://github.com/matheuscscp/gke-metadata-server/.github/workflows/release.yml@refs/heads/main

Automatic verification

If you are using Flux for deploying Helm Charts you can automate the image verification using Keyless Verification.

Verify the Timoni Module image

For verifying the image of a given Timoni Module GitHub Release (tags timoni-v{timoni_version}), download the digest file timoni-digest.txt attached to the Github Release and use it with cosign:

cosign verify ghcr.io/matheuscscp/gke-metadata-server-timoni@$(cat timoni-digest.txt) \
  --certificate-oidc-issuer=https://token.actions.githubusercontent.com \
  --certificate-identity=https://github.com/matheuscscp/gke-metadata-server/.github/workflows/release.yml@refs/heads/main

Automatic verification

If you are using the Timoni CLI to deploy Modules/Bundles you can automate the image verification using Keyless Verification.

Limitations and Security Risks

Pod identification by IP address

The emulator uses the client IP address reported in the HTTP request to uniquely identify the requesting Pod in the Kubernetes API (just like in the native GKE implementation).

If an attacker can easily perform IP address impersonation attacks in your cluster, e.g. ARP spoofing, then they will most likely exploit this design choice to steal your credentials. Please evaluate the risk of such attacks in your cluster before choosing this tool.

(Please also note that the attack explained in the link above requires Pods configured with very high privileges, which should normally not be allowed in sensitive/production clusters. If the security of your cluster is really important, then you should be enforcing restrictions for preventing Pods from being created with such high privileges in the majority of cases.)

Pods running on the host network

In a cluster there may also be Pods running on the host network, i.e. Pods with the field spec.hostNetwork set to true. Such Pods share the IP address of the Node where they are running on, i.e their IP is not unique, and therefore they cannot be uniquely identified by the emulator through the client IP address reported in the HTTP request (like mentioned in the previous section).

If your use case requires, Pods running on the host network are allowed to use a shared Kubernetes ServiceAccount configured through the following pair of annotations or labels on the Node:

node.gke-metadata-server.matheuscscp.io/serviceAccountName: <name>
node.gke-metadata-server.matheuscscp.io/serviceAccountNamespace: <namespace>

Annotations are preferred over labels because they are less impactful to etcd since they are not indexed. Labels are also supported because not all Kubernetes providers support configuring annotations on Node pools/groups. KinD, for example, does not (see testdata/kind.yaml).

Remember to add appropriate Node selectors/tolerations to your Pods so they are scheduled on the Nodes where the shared ServiceAccount is configured. There are many different ways to achieve this depending on your use case, so please refer to the Kubernetes documentation for more information.

Be careful when sharing identities between too many workloads! Please assess your security requirements and the risks of sharing identities between workloads in your cluster.

Routing Modes

The Google libraries attempt to retrieve tokens from the hard-coded endpoint 169.254.169.254:80. In order to make this work, the emulator must intercept the traffic from the client Pods to this endpoint. To solve this problem, the emulator supports two routing modes: eBPF and Loopback. See details of each mode below. In both modes the unencrypted communication between the emulator Pod and a client Pod never leaves the Node where they are both running on. This is exactly how the native GKE implementation works.

The annotation or label node.gke-metadata-server.matheuscscp.io/routingMode can be added to a Node in order to configure the routing mode of the emulator in that particular Node. The default is eBPF.

If you are using similar tools or workload identity features of managed Kubernetes services from other clouds, this configuration may have a direct conflict with other such tools or features. It's a recurrent practice among cloud providers using this hard-coded IP address and port to implement workload identity. If your use case requires, the recommended solution is to use Node pooling/grouping features from your Kubernetes provider to isolate the workloads that need to use the emulator as mentioned in the Usage section. Remember to add the label iam.gke.io/gke-metadata-server-enabled: "true" to such Nodes and appropriate Node selectors/tolerations to your Pods.

eBPF

In this routing mode the emulator hooks an eBPF program to the connect() syscall. The eBPF program looks at the destination address and, if it matches the hard-coded address mentioned above, modifies the address to the actual address of the emulator.

This mode may have conflicts with other eBPF-based tools running in the cluster. It works well with a default installation of Cilium, but if you are using Cilium to replace kube-proxy (docs) then this mode will have a direct conflict with your setup, since for this Cilium feature to work it needs to hook to many additional points, including the connect() syscall.

The advantage of this mode is that the emulator can bind to any port, not just port 80. This is because we modify the destination address of connections targeting our endpoint of interest, so we have the freedom to choose any port.

Loopback

In this routing mode the emulator adds the hard-coded address mentioned above to the lo interface of the Node with the label lo:gke-md-sv and the host scope. This way the emulator can bind to the hard-coded address and port 80, and the traffic from the client Pods to this address will be routed to the emulator.

This mode does not clash with other eBPF-based tools running in the cluster, but it has the disadvantage of not being able to bind to any port, only port 80. And for this to work properly the emulator Pods need to run on the host network, so they occupy port 80 in the network namespace of the Node.

Token Cache

When the emulator is configured to cache tokens, the issued Google OAuth 2.0 Access and OpenID Connect Identity Tokens are cached and returned to client Pods on every request until their expiration.

This means that even if you revoke the required permissions for the Kubernetes ServiceAccount to issue those tokens, the client Pods will still get those tokens from the emulator until they expire. This is a limitation of how the permissions are evaluated: they are evaluated only when the tokens are issued, which is what caching tries to avoid. If your use case requires immediate revocation of permissions, then you should not use token caching. The Helm Chart and Timoni Module distributed here have options to disable token caching, look at their respective values APIs.

Tokens usually expire in at most one hour.

Disclaimer

This project was not created by Google. Enterprise support from Google is not available. Use this tool at your own risk. (But please do feel free to report bugs and CVEs, request help, new features and contribute.)

Furthermore, this tool is not necessary for using GCP Workload Identity Federation inside non-GKE Kubernetes clusters. This is just a facilitator. Kubernetes and GCP Workload Identity Federation work together by themselves. This tool just makes your Pods need much less configuration to use GCP Workload Identity Federation for Kubernetes, by making the configuration as close as possible to how Workload Identity Federation for GKE is configured.