cinder/doc/source/configuration/block-storage/service-token.rst

Using service tokens

Warning

For all OpenStack releases after 2023-05-10, it is required that Nova be configured to send a service token to Cinder and Cinder to receive it. This is required by the fix for CVE-2023-2088. See OSSA-2023-003 for details.

When a user initiates a request whose processing involves multiple services (for example, a boot-from-volume request to the Compute Service will require processing by the Block Storage Service, and may require processing by the Image Service), the user's token is handed from service to service. This ensures that the requestor is tracked correctly for audit purposes and also guarantees that the requestor has the appropriate permissions to do what needs to be done by the other services.

There are several instances where we want to differentiate between a request coming from the user to one coming from another OpenStack service on behalf of the user:

• For security reasons There are some operations in the Block Storage service, required for normal operations, that could be exploited by a malicious user to gain access to resources belonging to other users. By differentiating when the request comes directly from a user and when from another OpenStack service the Cinder service can protect the deployment.

• To prevent long-running job failures: If the chain of operations takes a long time, the user's token may expire before the action is completed, leading to the failure of the user's original request.

  One way to deal with this is to set a long token life in Keystone, and this may be what you are currently doing. But this can be problematic for installations whose security policies prefer short user token lives. Beginning with the Queens release, an alternative solution is available. You have the ability to configure some services (particularly Nova and Cinder) to send a "service token" along with the user's token. When properly configured, the Identity Service will validate an expired user token when it is accompanied by a valid service token. Thus if the user's token expires somewhere during a long running chain of operations among various OpenStack services, the operations can continue.

Note

There's nothing special about a service token. It's a regular token that has been requested by a service user. And there's nothing special about a service user, it's just a user that has been configured in the Identity Service to have specific roles that identify that user as a service.

The key point here is that the "service token" doesn't need to have an extra long life -- it can have the same short life as all the other tokens because it will be a fresh (and hence valid) token accompanying the (possibly expired) user's token.

Configuration

To configure an OpenStack service that supports Service Tokens, like Nova and Cinder, to send a "service token" along with the user's token when it makes a request to another service, you must do the following:

1. Configure the "sender" services to send the token when calling other OpenStack services.
2. Configure each service's user to have a service role in Keystone.
3. Configure the "receiver" services to expect the token and validate it appropriately on reception.

Send service token

To send the token we need to add to our configuration file the [service_user] section and fill it in with the appropriate configuration for your service user (username, project_name, etc.) and set the send_service_user_token option to true to tell the service to send the token.

The configuration for the service user is basically the normal keystone user configuration like we would have in the [keystone_authtoken] section, but without the 2 configuration options we'll see in one of the next subsection to configure the reception of service tokens.

In most cases we would use the same user we do in [keystone_authtoken], for example for the nova configuration we would have something like this:

[service_user]
send_service_user_token = True

# Copy following options from [keystone_authtoken] section
project_domain_name = Default
project_name = service
user_domain_name = Default
password = abc123
username = nova
auth_url = http://192.168.121.66/identity
auth_type = password

Service role

A service role is nothing more than a Keystone role that allows a deployment to identify a service without the need to make them admins, that way there is no change in the privileges but we are able to identify that the request is coming from another service and not a user.

The default service role is service, but we can use a different name or even have multiple service roles. For simplicity's sake we recommend having just one, service.

We need to make sure that the user configured in the [service_user] section for a project has a service role.

Assuming our users are nova and cinder from the service project and the service role is going to be the default service, we first check if the role exists or not:

$ openstack role show service

If it doesn't, we need to create it

$ openstack role create service

Check if the users have the roles assigned or not:

$ openstack role assignment list --user cinder --project service --names
$ openstack role assignment list --user nova --project service --names

And if they are not we assign the role to those users

$ openstack role add --user cinder --project service service
$ openstack role add --user nova --project service service

More information on creating service users can be found in the Keystone documentation

Receive service token

Now we need to make the services validate the service token on reception, this part is crucial.

The 2 configuration options in [keystone_authoken] related to receiving service tokens are service_token_roles and service_token_roles_required.

The service_token_roles contains a list of roles that we consider to belong to services. The service user must belong to at least one of them to be considered a valid service token. The value defaults to service, so we don't need to set it if that's the value we are using.

Now we need to tell the keystone middleware to actually validate the service token and confirm that it's not only a valid token, but that it has one of the roles set in service_token_roles. We do this by setting service_token_roles_required to true.

So we would have something like this in our [keystone_authtoken] section:

[keystone_authtoken]
service_token_roles = service
service_token_roles_required = true

Troubleshooting

If you've configured this feature and are still having long-running job failures, there are basically three degrees of freedom to take into account: (1) each source service, (2) each receiving service, and (3) the Identity Service (Keystone).

1. Each source service (basically, Nova and Cinder) must have the [service_user] section in the source service configuration file filled in as described in the service-token-configuration section above.

   Note

   As of the 2023.1 release, Glance does not have the ability to pass service tokens. It can receive them, though. The place where you may still see a long running failure is when Glance is using a backend that requires Keystone validation (for example, the Swift backend) and the user token has expired.

2. There are several things to pay attention to in Keystone:

   • When service_token_roles_required is enabled you must make sure that any service user who will be contacting that receiving service (and for whom you want to enable "service token" usage) has one of the roles specified in the receiving services's service_token_roles setting. (This is a matter of creating and assigning roles using the Identity Service API, it's not a configuration file issue.)
   • Even with a service token, an expired user token cannot be used indefinitely. There's a Keystone configuration setting that controls this: [token]/allow_expired_window in the Keystone configuration file. The default setting is 2 days, so some security teams may want to lower this just on general principles. You need to make sure it's not set too low to be completely ineffective.
   • If you are using Fernet tokens, you need to be careful with your Fernet key rotation period. Whoever sets up the key rotation has to pay attention to the [token]/allow_expired_window setting as well as the obvious [token]/expiration setting. If keys get rotated faster than expiration + allow_expired_window seconds, an expired user token might not be decryptable, even though the request using it is being made within allow_expired_window seconds.

To summarize, you need to be aware of:

• Keystone: must allow a decent sized allow_expired_window (default is 2 days)
• Each source service: must be configured to be able to create and send service tokens (default is OFF)
• Each receiving service: has to be configured to accept service tokens (default is ON) and require role verification (default is OFF)