Open Stack

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.


> 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):
>
> 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.
>
> 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). 
> 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.
>
> 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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