Using service tokens¶
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.
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.
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:
Configure the “sender” services to send the token when calling other OpenStack services.
Configure each service’s user to have a service role in Keystone.
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 (
project_name, etc.) and set the
send_service_user_token option to
true to tell the service to send the
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
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
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
We need to make sure that the user configured in the
for a project has a service role.
Assuming our users are
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 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
So we would have something like this in our
[keystone_authtoken] service_token_roles = service service_token_roles_required = true
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).
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 Configuration section above.
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.
There are several things to pay attention to in Keystone:
service_token_roles_requiredis 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_rolessetting. (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_windowin 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_windowsetting as well as the obvious
[token]/expirationsetting. If keys get rotated faster than
allow_expired_windowseconds, an expired user token might not be decryptable, even though the request using it is being made within
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)