Open Stack

Hi all,

Inline.

From: Adam Young <ayoung at redhat.com<mailto:ayoung at redhat.com>>
Reply-To: "OpenStack Development Mailing List (not for usage questions)" <openstack-dev at lists.openstack.org<mailto:openstack-dev at lists.openstack.org>>
Date: Tuesday, May 5, 2015 at 8:34 PM
To: "openstack-dev at lists.openstack.org<mailto:openstack-dev at lists.openstack.org>" <openstack-dev at lists.openstack.org<mailto:openstack-dev at lists.openstack.org>>
Subject: Re: [openstack-dev] [keystone] On dynamic policy, role hierarchies/groups/sets etc.

On 05/05/2015 07:05 AM, Henry Nash wrote:
We’ve been discussing changes to these areas for a while - and although I think there is general agreement among the keystone cores that we need to change *something*, we’ve been struggling to get agreement on exactly how..  So to try and ground the discussion that will (I am sure) occur in Vancouver, here’s an attempt to take a step back, look at what we have now, as well as where, perhaps, we want to get to.

This is a great summary.  Thanks Henry.

Super helpful for sure!



The core functionality all this is related to is that of how does keystone & policy allow the checking of whether a given API call to an OpenStack service should be allowed to take place or not. Within OpenStack this is a two step process for an API caller….1) Get yourself a token by authentication and getting authorised for a particular scope (e.g. a given project), and then 2) Use that token as part of your API call to the service you are interested in. Assuming you do, indeed, have the rights to execute this API, somehow steps 1) and 2) give the policy engine enough info to say yes or no.

So first, how does this work today and (conceptually) how should we describe that?  Well first of all, in fact, strictly we don’t control access at the raw API level.  In fact, each service defines a series “capabilities” (which usually, but not always, map one-to-one with an API call).  These capabilities represent the finest grained access control we support via the policy engine. Now, in theory, the most transparent way we could have implemented steps 1) and 2) above would have been to say that users should be assigned capabilities to projects….and then those capabilities would be placed in the token….allowing the policy engine to check if they match what is needed for a given capability to be executed. We didn’t do that since, a) this would probably end up being very laborious for the administrator (there would be lots of capabilities any given user would need), and b) the tokens would get very big storing all those capabilities. Instead, it was recognised that, usually, there are sets of these capabilities that nearly always go together - so instead let’s allow the creation of such sets….and we’ll assign those to users instead. So far, so good. What is perhaps unusual is how this was implemented. These capability sets are, today, called Roles…but rather than having a role definition that describes the capabilities represented by that role….instead roles are just labels - which can be assigned to users/projects and get placed in a tokens.  The expansion to capabilities happens through the definition of a json policy file (one for each service) which must be processed by the policy engine in order to work out what whether the roles in a token and the role->capability mapping means that a given API can go ahead. This implementation leads to a number issues (these have all been raised by others, just pulling them together here):
As I understand how this works conceptually, a policy makes go/no-go decisions based on two kinds of properties: (1) properties about the user making the API call  (which are encoded in the token) and (2) the API call name and arguments.   Is that right?


i) The role->capability mapping is rather static. Until recently it had to be stored in service-specific files pushed out to the service nodes out-of-band. Keystone does now provide some REST APIs to store and retrieve whole policy files, but these are a) course-grained and b) not really used by services anyway yet.

ii) As more and more clouds become multi-customer (i.e. a cloud provider hosting multiple companies on a single OpenStack installation), cloud providers will want to allow those customers to administer “their bit of the cloud”. Keystone uses the Domains concept to allow a cloud provider to create a namespace for a customer to create their own projects, users and groups….and there is a version of the keystone policy file that allows a cloud provider to effectively delegate management of these items to an administrator of that customer (sometimes called a domain administrator).  However, Roles are not part of that namespace - they exists in a global namespace (within a keystone installation). Diverse customers may have different interpretations of what a “VM admin” or a “net admin” should be allowed to do for their bit of the cloud - but  right now that differentiation is hard to provide. We have no support for roles or policy that are domain specific.
I wondered if we could properly protect the API call for adding a new Role using the current mechanism.  So I came up with a simple example.

Suppose we want to write policy about the API call: addRole(user, role-name).  If we’re hosting both Pepsi and Coke, we want to write a policy that says that only someone in the Pepsi admin role can change roles for Pepsi users (likewise for Coke).  We’d want to write something like…

addRole(<user>, <role>) is permitted for <caller> if
    <caller> belongs to the Pepsi-admin role and
    <user> belongs to the Pepsi role

The policy engine knows if “<caller> belongs to the Pepsi-admin role” because that’s part of the token.  But the policy engine doesn’t know if “<user> belongs to the Pepsi role” because <user> is just an argument to the API call, so we don’t have role info about <user>.  This helps me understand *why* we can’t handle the multi-customer use case right now: the policy engine doesn’t have all the info it needs.

But otherwise, it seems, we could handle the multi-customer use-case using mechanism that already exists.  Are there other examples where they can’t write policy because the engine doesn’t have enough info?



iii) Although as stated in ii) above, you can write a policy file that differentiates between various levels of admin, or fine-tunes access to certain capabilities, the reality is that doing this is pretty un-intuative. The structure of a policy.json file that tries to do this is, indeed, complex (see Keystone’s as an example: https://github.com/openstack/keystone/blob/master/etc/policy.v3cloudsample.json<https://urldefense.proofpoint.com/v2/url?u=https-3A__github.com_openstack_keystone_blob_master_etc_policy.v3cloudsample.json&d=AwMF-g&c=Sqcl0Ez6M0X8aeM67LKIiDJAXVeAw-YihVMNtXt-uEs&r=B6BWd4kFfgOzAREgThxkmTZKy7dDXE2-eBAmL0PBK7s&m=oTioE6xzyoS2OLqOV-NzZvrwGjo6BhuvBscZJpeOePA&s=JldcdJTa6nt_bI_LQD1BxnCB0eHHaQZkD7fJhjU5chQ&e=>). Adding more capability to this will likely only make the situation worse.

We have a number of specs taking shape to try and address the above (a number of them competing), so I wanted to propose with a set of guidelines for these:

a) Making the policy centrally sourced (i.e. in keystone) and more dynamic seems eminently sensible. We’ll need to work on notifications etc. for how services know the policy has changed, of course. Such a centralised capability allows us to not just use a json file to store policy, but perhaps a database - allowing more fine-grained access to policy rules via an API. See: https://review.openstack.org/#/c/147651/ and https://review.openstack.org/#/c/133814/ as examples.

b) One of the core disagreements has been around whether any additional structure we add to roles is processed at token generation time or at token analysis time by the policy engine. To be honest, I don’t think our deployers care - as long as we don’t break something like making tokens even bigger.  What they will care about is whether they can hold in the heads the concepts for what it is they need to set up to achieve the policy framework that want. Let’s concentrate on making this easy for them, and under the hood we’ll solve the bits and bites.
Just so I’m following… At one end of the design spectrum, the token contains nothing but an unforgeable ID giving the identity of the user, and at the other end of the spectrum, the token encodes all information necessary for the policy-engine to make access control decisions.  If we choose to make it contain just the unforgeable ID, each service would be responsible for mapping a user ID to all the policy-relevant info about that user.  If the token contains all policy-relevant data, it gets huge, but there’s no burden on the service.

Currently we’re somewhere in between where the token contains the unforgeable ID and the names of a few things (roles), and each service needs to know how to map those roles into policy-relevant data.  That sound about right?

Tim

c) We have had competing suggestions for role sets/group/hierarchies (see: https://review.openstack.org/#/c/125704/ and https://review.openstack.org/#/c/133855/ ). I would suggest that we go for a base functionality of role sets (where a role set can contain roles or other role sets)….where these can either be global in scope or
While I agree with the basic approach, I would argue instead that a Role is a set of capabilities, and so we don't need role sets, we need capability sets (which we have) and then we say a role can contain other roles.  The set of capabilites is then defined as the union of the capabilities assigned to it directly and the capabilites assigned to subordinate roles.

The set can be easily defined in the policy.json file.  So the requirment then is to keep the Keystone view of these nested roles in sync.  The database driven approach makes this simpler, but this can be done today by hand with the existing policy file.  Demonstrating this is part of my dynamic policy presentation.


domain specific.  Both need to be supported and it must be possible for a cloud provider to delegate to a domain admin the ability to create their own role sets. Whether roles sets are processed by the policy engine or at token generation time (see b) above) is something we need to hash out. I’m actually Ok with either…as long as one development route is not inordinately longer than the other - and, at least for me, domain specific role sets must be in any first implementation (this is the customer need I see most). I wouldn’t rule out a development plan where we 1) get the API right, 2) implement it so that the tokens and policy doesn’t have to change (i.e. we expand role sets at token generation time), and then 3) push this capability into the policy engine itself.  If we can skip 2) and get to 3) quickly, more the better.
I think do step 3 first;  we can make the policy engine handle the rules inferences for roles as sets of  capabilities.  Policy generation from the database happens second, and the API for more fine grained control happens third.


d) I’d like to keep in mind an eventual destination where services could “register their capabilities” via an API, policy rules and roles/sets can then be created via APIs that then allow assignments to be made in terms that make sense to a domain administrator (i.e. in terms that are meaningful to them), that make a customer hosted on a shared cloud feel that this really is "their cloud”.
That should work. In order for a user to get access to those new capabilites we'd have three choices:

1.  Add them to an existing role
2.  Add them to a new role and assign that new role as a subset of an existing role
3.  Add them to a new role and assign them to the user directly.


Henry







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