Automatic OIDC: Using Cloud Scheduler, Tasks, and PubSub to securely call Cloud Run, Cloud Functions

2019-05-20

This is a second in a series related to a versatile feature in Cloud Scheduler and Cloud Tasks and Cloud PubSub that automatically emits google OpenIDConnect and oauth2 access_token to outbound webhook calls.

When a Scheduled task fires and calls an HTTP endpoint, these services can automatically carry credentials of various types to authenticate itself:

This article focuses on (B) below, while the first part in the series is (A)


You can find the source here


Basically, this article shows how to get an id_token

  • from Any of: [ Cloud Scheduler, Cloud Tasks, Cloud PubSub* ]

and securely call

  • to Any of: [Cloud Run, Cloud Functions, Cloud Endpoints, Istio, {your application} ]

Specific services like Cloud Run, Cloud Functions and Cloud Endpoints can automatically validate the inbound id_token and check if it is properly signed by Google. Furthermore, you can apply IAM policies for the user or service account associated with said token. In other words, you can construct an IAM policy on Cloud Run that states “only allow this service account access where that account is one associated with a Cloud Scheduler job. For example, to secure Cloud Run, assign the IAM Role roles/run.invoker to the service behind the id_token (eg, Scheduler, Tasks or PubSub; as shown in the example in Scheduler --> Cloud Run below)

images/invoker_services.png

PubSub http_push targets are limited to domain verification

Create Service Account

First setup a specific service account you wish to scheduler to invoke the endpoint as. This is the service account that scheduler will run as and acquire the id_token for. The section after this, we will apply IAM policies for this service account for Cloud Scheduler

Note: Cloud Scheduler by default runs with a service account in the format service-$PROJECT_NUMBER@gcp-sa-cloudscheduler.iam.gserviceaccount.com

For other services:

I listed pubsub there for completeness but Pubsub only emits oidc tokens while Scheduler and Tasks can emit BOTH oidc and oauth2. The followup article about oidc tokens demonstrates how to setup Pubsub

  • Setup environment variables
export PROJECT_ID=`gcloud config get-value core/project`
export PROJECT_NUMBER=`gcloud projects describe $PROJECT_ID --format='value(projectNumber)'`
  • Create Scheduler Service Account
gcloud iam service-accounts create schedulerunner --display-name="Task Schedule Runner"

$ gcloud iam service-accounts list
NAME                                      EMAIL                                                                       DISABLED
Task Schedule Runner                      schedulerunner@$PROJECT_ID.iam.gserviceaccount.com                  False
  • Create an IAM Policy

This step will allow PubSub and Scheduler’s service accounts to acquire a token for schedulerunner@$PROJECT_ID.iam.gserviceaccount.com

(remember to substitute your PROJECT_NUMBER)

  • svc_policy.json:
{
  "bindings": [
    {
      "members": [
        "serviceAccount:service-$PROJECT_NUMBER@gcp-sa-pubsub.iam.gserviceaccount.com"
      ],
      "role": "roles/iam.serviceAccountTokenCreator"
    },
    {
      "members": [
        "serviceAccount:service-$PROJECT_NUMBER@gcp-sa-cloudscheduler.iam.gserviceaccount.com"
      ],
      "role": "roles/cloudscheduler.serviceAgent"
    }    
  ],
}

The iam.serviceAccountTokenCreator role includes concentric set of permission (see Concentric IAMCredentials Permissions: The secret life of signBlob. If you just need to generate an access_token or an id_token, consider generating a custom role with that specific capability.

  • Apply Policy file so Scheduler can impersonate
$ gcloud iam service-accounts set-iam-policy schedulerunner@$PROJECT_ID.iam.gserviceaccount.com  svc_policy.json  -q

images/token_creator_role.png

OIDC

The specific gcloud commands for Scheduler and PubSub to emit oidc tokens:

  • Cloud Scheduler
gcloud beta scheduler jobs create http
          | [--oidc-service-account-email=OIDC_SERVICE_ACCOUNT_EMAIL
          : --oidc-token-audience=OIDC_TOKEN_AUDIENCE]] [GCLOUD_WIDE_FLAG ...]
  • Cloud PubSub
gcloud alpha pubsub subscriptions create SUBSCRIPTION
        [--push-auth-service-account=SERVICE_ACCOUNT_EMAIL]
        [--push-auth-token-audience=OPTIONAL_AUDIENCE_OVERRIDE]

While on the validation, you can configure GCF to accept an oidc token:

  • Cloud Functions
gcloud functions add-iam-policy-binding $FUNCTION_NAME \
  --member serviceAccount:$SERVICE_ACCOUNT_A_EMAIL \
  --role roles/cloudfunctions.invoker

OIDC Audience

A bit about the audience: field in OIDC tokens.

You must set this field for the invoking service and specify the fully qualified URL of the receiving service. For example, if you are invoking Cloud Run or Cloud Functions, the id_token must include the URL/path of the service.

{
  "aud": "https://uri-of-intended-service",
  "azp": "107145139691231222712",
  "email": "service-$PROJECT_NUMBER@gcp-sa-cloudscheduler.iam.gserviceaccount.com",
  "email_verified": true,
  "exp": 1556665461,
  "iat": 1556661861,
  "iss": "https://accounts.google.com",
  "sub": "107145139691231222712"
}

If the target is your own application endpoint, you must validate the inbound id_token. The appendix section shows token validation in python and envoy proxy.


Ok, now to the tests,

PubSub –> Cloud Run

In the first set of samples, we will integrate PubSub and Cloud Scheduler with Cloud Run

setup a sample Cloud Run Application with authentication enabled:

gcloud beta run deploy hello --image gcr.io/cloudrun/hello  --region=us-central1  -q

Service [hello] revision [hello-00001] has been deployed and is serving traffic at https://hello-6w42z6vi3q-uc.a.run.app

Which means we need to add the originating serivce accounts (for PubSub and Scheduler) into roles/run.Invoker:

images/run_invoker.png

Now deploy a PubSub push notificatoin target. Remember to set the audience to the URL of the Run instance

gcloud alpha pubsub topics create runtopic

gcloud alpha pubsub subscriptions create psub --topic runtopic \
  --push-endpoint https://hello-6w42z6vi3q-uc.a.run.app \
  --push-auth-service-account=schedulerunner@$PROJECT_ID.iam.gserviceaccount.com \
  --push-auth-token-audience=https://hello-6w42z6vi3q-uc.a.run.app

Publish a message

gcloud alpha pubsub  topics publish runtopic --message='hello world'

See invocation on Cloud Run:

images/pubsub_run.png

Scheduler –> Cloud Run

Similar to the above, invoke Cloud Run using a Scheduled Job

gcloud beta scheduler jobs create http oidctest  --schedule "5 * * * *" --http-method=GET \
  --uri=https://hello-6w42z6vi3q-uc.a.run.app \
  --oidc-service-account-email=schedulerunner@$PROJECT_ID.iam.gserviceaccount.com    \
  --oidc-token-audience=https://hello-6w42z6vi3q-uc.a.run.app

For the impatient, invoke the scheduled job now

gcloud beta scheduler jobs run oidctest

Check logs In Cloud Run:

images/scheduler_run.png

And corresponding in Cloud Scheduler:

images/scheduler_run_log.png

Scheduler –> Cloud Functions

First lets setup a receiving cloud function with authentication

  • main.py:
def hello_http(request):
    request_json = request.get_json(silent=True)
    request_args = request.args
    return 'Hello World!'
  • Deploy
gcloud functions deploy  authfn --entry-point=hello_http --trigger-http --runtime=python37

    entryPoint: hello_http
    name: authfn
    httpsTrigger:
      url: https://us-central1-$PROJECT_ID.cloudfunctions.net/authfn
    name: projects/$PROJECT_ID/locations/us-central1/functions/authfn

Note the url, we will use this later while defining the endpoints

gcloud alpha functions add-iam-policy-binding authfn \
  --member serviceAccount:schedulerunner@$PROJECT_ID.iam.gserviceaccount.com \
  --role roles/cloudfunctions.invoker

images/gcf_iam.png

  • Deploy Scheduled Task
gcloud beta scheduler jobs create http gcftest  --schedule "5 * * * *" --http-method=GET \
  --uri=https://us-central1-$PROJECT_ID.cloudfunctions.net/authfn \
  --oidc-service-account-email=schedulerunner@$PROJECT_ID.iam.gserviceaccount.com    \
  --oidc-token-audience=https://us-central1-$PROJECT_ID.cloudfunctions.net/authfn
gcloud beta scheduler jobs run gcftest

Then on the Scheduler logs:

images/scheduler_gcf_log.png

On GCF Logs

images/gcf_scheduler_log.png

Cloud PubSub –> Cloud Functions

As of 5/18/19, PubSub does NOT allow registration of push endpoints to arbitrary http targets

gcloud alpha pubsub subscriptions create psubgcf --topic runtopic \
  --push-endpoint https://us-central1-$PROJECT_ID.cloudfunctions.net/authfn \
  --push-auth-service-account=schedulerunner@$PROJECT_ID.iam.gserviceaccount.com \
  --push-auth-token-audience=https://us-central1-$PROJECT_ID.cloudfunctions.net/authfn

gives

ERROR: Failed to create subscription [projects/@$PROJECT_ID/subscriptions/psubgcf]: The supplied HTTP URL is not registered in the subscription's parent project (url="https://us-central1-@$PROJECT_ID.cloudfunctions.net/authfn", project_id="....").

The domain .coudfunctions.net is not under a whitelist that allowed the earlier PubSub -> CLoud Run integration (i.,e an http target ofrun.net is whitelisted in PubSub).

  • This will be addressed on GCP where you can whitelist cloudfunctions.net domain.
  • Wokraround would be to setup custom domains for GCF (via Firebase Functions/hosting)

Scheduler –> { your application }

Emitting an OIDC token from any of the above to your application is pretty much the same. Specify the URL, audience during configuration and make sure the remote service you are running accepts https (and is not self-signed).

You can use the Python sample or Envoy to validate the token (see appendix below)

gcloud beta scheduler jobs create http apptest  --schedule "5 * * * *" --http-method=GET \
  --uri=https://your_app_url \
  --oidc-service-account-email=schedulerunner@$PROJECT_ID.iam.gserviceaccount.com    \
  --oidc-token-audience=https://your_app_url

NOTE: your remote server mush have a valid SSL certificate and cannot be self-signed.

Cloud Task –> Cloud Run

I don’t have a sample handy but here is golang app that enqueues a Task that carries an oidc token:

	location := "us-central1"
	parent := "projects/" + projectID + "/locations/" + location + "/queues/" + queueName
	taskName := parent + "/tasks/" + randTaskID

	req := &taskspb.CreateTaskRequest{
		Parent: parent,
		Task: &taskspb.Task{
			Name: taskName,
			PayloadType: &taskspb.Task_HttpRequest{
				HttpRequest: &taskspb.HttpRequest{
					Url:        "https://hello-6w42z6vi3q-uc.a.run.app",
					HttpMethod: taskspb.HttpMethod_GET,
					AuthorizationHeader: &taskspb.HttpRequest_OidcToken{
						OidcToken: &taskspb.OidcToken{
							ServiceAccountEmail: "schedulerunner@$PROJECT_ID.iam.gserviceaccount.com",
							Audience:            "https://hello-6w42z6vi3q-uc.a.run.app",
						},
					},
				},
			},
		},
	}
	resp, err := c.CreateTask(ctx, req)
	if err != nil {
		log.Fatalf("ERROR: ", err.Error())
	}

OIDC Verification

from google.oauth2 import id_token
import google.auth
import google.auth.transport.requests

target_audience = 'https://myapp-6w42z6vi3q-uc.a.run.app'
certs_url='https://www.googleapis.com/oauth2/v1/certs'

idt='<YOURIDTOKEN>'

request = google.auth.transport.requests.Request()
print id_token.verify_token(idt,request,certs_url=certs_url)

OIDC Creation

  • Service Account
from google.oauth2 import id_token
from google.oauth2 import service_account
import google.auth
import google.auth.transport.requests
from google.auth.transport.requests import AuthorizedSession

target_audience = 'https://myapp-6w42z6vi3q-uc.a.run.app'
url = 'https://myapp-6w42z6vi3q-uc.a.run.app'
certs_url='https://www.googleapis.com/oauth2/v1/certs'

additional_claims = { }

creds = service_account.IDTokenCredentials.from_service_account_file(
        '/path/to/svc_account.json',
        target_audience= target_audience, additional_claims=additional_claims)

authed_session = AuthorizedSession(creds)

# make authenticated request
r = authed_session.get(url)
print r.status_code
print r.text
  • GCE|GKE|GCF|Run
from google.oauth2 import id_token
from google.auth import compute_engine
import google.auth
import google.auth.transport.requests
from google.auth.transport.requests import AuthorizedSession
from google.oauth2.credentials import Credentials
import requests

target_audience = 'https://myapp-6w42z6vi3q-uc.a.run.app'
url = 'https://myapp-6w42z6vi3q-uc.a.run.app'
certs_url='https://www.googleapis.com/oauth2/v1/certs'
metadata_url = "http://metadata/computeMetadata/v1/instance/service-accounts/default/identity?audience=" + target_audience

r = requests.get(metadata_url, headers={"Metadata-Flavor":"Google"})

idt=''
creds = google.oauth2.credentials.Credentials(idt)
authed_session = AuthorizedSession(creds)

# make authenticated request
r = authed_session.get(url)

print r.status_code
print r.text

Envoy

Sample usage using gcloud’s own client_id. Use this just for testing!

export ID_TOKEN=$(gcloud config config-helper --format 'value(credential.id_token)')

curl -H "Authorization: Bearer $ID_TOKEN" http://localhost:10000/```

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