Re: [xacml-comment] XACML Obligations - a concurrency model

[Erik Rissanen <erik@axiomatics.com>]

Hi,

Thanks for the very thorough analysis.

You can find the draft under "work in progress" on the XACML TC web 
page. It's called "obligation families".

The draft is very early work, carelessly written by me :-), and 
certainly not in any way ready for implementation. Just so everybody 
knows... :-)

I have not read your analysis very carefully yet, but I get the feeling 
that there has been a misunderstanding. The words "after", "before" and 
"with" are confusing. The draft does not attempt to define a 
temporal/causal order of enforcement. There was no intention to provide 
any guarantees with respect to side effects of enforcement.

I used those names because I couldn't think of anything better and 
because after and before are simple ways to implement the guarantees 
which I was describing.

What really was intended was to provide a mechanism which allows policy 
writers to have guarantees about the atomicity of the dual enforcement 
of the obligation and the access enforcement. To say things like "access 
must not be granted if obligations cannot be enforced", or "an 
obligation must not be enforced unless access enforcement is guaranteed 
to succeed", etc.

BTW, you are right, a simple method by which to implement the "After" 
guarantee is simply to never run the obligation. That is the intended 
meaning. It is intended for those cases where it is important that the 
obligation is _never_ enforced unless the access was successfully 
enforced. For instance, consider a case where a granted access means a 
billing event. If you are concerned about your customer relations you 
want to make sure that you never bill for an access which was not 
successful.

You should also note that in many cases some of these guarantees cannot 
be provided. If the guarantee cannot be provided the PEP should report 
failure. It might also be possible to detect this already at the time of 
policy deployment and the PDP could return an error saying that the 
policy contains requirements which cannot be enforced.

In any case, I am not pursuing this right now because I am focusing on 
getting the XACML 3.0 core done. I will look into this in detail once 
the core XACML stuff is done, and we revisit this work.

Best regards,
Erik
Oleksandr Otenko wrote:
> Hi,
>
> We were discussing XACML draft with David with regards to the
> Obligations definitions. Can you help me understand the reasoning 
> behind them? I cannot find the draft myself, but I have issues with 
> what David quoted; I trust it is a complete copy:
>
>> *Obligation before access*
>> This mode guarantees that if the access control decision is 
>> successfully enforced, then the obligation is also successfully 
>> enforced. (This mode does NOT guarantee that access decision is 
>> successfully enforced even if the obligation is enforced.)
>>
>> *Obligation after access*
>> This mode guarantees that if the obligation is successfully enforced, 
>> then the access control decision is also successfully enforced. (This 
>> mode does NOT guarantee that the obligation is successfully enforced 
>> even if the access decision is enforced.)
>>
>> *Obligation with access*
>> This mode guarantees that the obligation is successfully enforced if 
>> and only if the access control decision is successfully enforced.
>> *Any timing*
>> In this mode the PEP makes a best effort attempt to enforce both the 
>> obligation and the access decision and no guarantee is made about 
>> success.
>
>
> I have no problem with the definitions as they are, but want to clarify
> it means that:
>
>    * *With* is the same as *After* in this interpretation
>    * *Any timing* can be the same as *After*
>    * *After* is not obligatory for enforcement
>    * (Thus only *Before* is a true obligation)
>
> This becomes evident if you consider a concurrency model with
> *happens-before* predicate (e.g. as used in Java memory model, 3-rd
> edition). Please, follow me on this:
>
> Since *After* starts after the access decision is enforced, there is no
> guarantee when the effects of the obligation are observed. This is fine
> with me, as this is the definition of the "best effort", and there is
> the place for that in the security policies. However, the cheapest way
> to implement "best effort" is to /never/ execute such obligation. This
> is made clear in the statement about *After*, but: a) this is just like
> *Any timing*, so the latter can be deprecated; b) marking obligation
> *After* makes it not obligatory for enforcement (kind of, it becomes a
> non-Obligation).
>
> The definition of *With* in the form quoted also lacks the definition of
> the timing the effects of *With* are observed. All that it states, is
> that it can be observed after a successful access control decision
> enforcement: it doesn't state /how soon/. This allows for an
> implementation that performs the "best effort" enforcement. This also is
> fine with me, but then *With* becomes the same as *After*.
>
> Please, correct me, but it seems the definition of the obligations in
> the current form can be rewritten using the *happens-before* predicate
> as follows:
>
>    * The last step of *Before* Obligation *happens-before* the first
>      step of access control decision enforcement process
>    * The last step of the access control decision enforcement process
>      *happens-before* the /first/ step of *After* Obligation
>    * The last step of the access control decision enforcement process
>      *happens-before* the /last/ step of *With* Obligation
>    * *Any timing* has no *happens-before* relationship with access
>      control decision enforcement process (unsynchronized; hmmmm, maybe
>      there is some value in having it, after all)
>
> You can see why I think *With* is the same as *After*: *After*'s /last/
> step is executed in the same way as *With*'s, and since there is no
> requirement about the timing of the /first/ step of *With*, the least
> effort implementation is to implement it as *After*.
>
> I propose to modify the definition of *With* using the *happens-before*
> predicate, which allows to express the timing of the enforcement more
> precisely, and makes the purpose of the obligations more clear:
>
>    * The last step of *Before* Obligation *happens-before* the first
>      step of access control decision enforcement process
>          o This makes sure the effects of the Obligation can be
>            observed /during the access control decision enforcement
>            process/
>          o *Caveat*: the effects of the Obligation may not be observed,
>            if there was an intervening Obligation enforced concurrently
>    * The last step of the access control decision enforcement process
>      *happens-before* the first step of *After* Obligation
>          o This makes sure the effects of the access control decision
>            enforcement process can be observed /during the Obligation
>            enforcement/
>          o *Caveat*: the effects of the decision enforcement may not be
>            observed, if there was an intervening decision enforced
>            concurrently
>    * The last step of *With* Obligation *happens-before* the last step
>      of the access control decision enforcement process
>          o There is no guarantee that either of the processes will
>            observe the effects of the other
>          o This makes sure the effects of the Obligation can be
>            observed at the same time the effects of the access control
>            decision enforcement are
>          o *Caveat*: the effects of the Obligation may not be observed,
>            if there was an intervening Obligation enforced concurrently
>          o *Caveat*: this does not explain what happens to the
>            Obligation, if the access control decision is /not enforced/
>            (e.g. fails)
>
> If it is critical to observe all effects (reads: it is critical to beat
> all caveats), use the synchronization techniques between the monitor,
> the Obligation enforcer, and the decision enforcer.
>
> Notice that now *With* can be implemented as *Before*, but that is good
> as far as the word Obligation is concerned. There is a significant
> difference in the wording of the two, which allows the implementers to
> optimize *With* differently to *Before*.
>
>
> *Discussion of With Obligation*
>
> Access control decision enforcement and Obligation enforcement are
> independent processes. This means that without explicit synchronization
> the effects of the processes can be observed in an arbitrary order. The
> existing wording of the draft allows for the effects of *With* to not be
> observed at all. My concern is that it will be implemented as such, and
> the only Obligation mandatory for enforcement is *Before*. The
> modification I propose is necessary, because there is no way to
> /guarantee/ when the process /ends/.
>
> The modified definitions are based on a concurrency model, and they
> clearly state the observed effects of each Obligation in a concurrent 
> world.
>
> In my opinion it is not possible to implement the "if and only if" part
> of the *With* Obligation without a concurrency issue. This is because
> either it is enforced *After* the access control decision enforcement is
> known to have succeeded (no point to have *With* as a separate type), or
> allow the Obligation's effects to be observed upon access control
> decision enforcement failure. My proposal is to allow the latter, as in
> that case it makes sense to have *With* as a separate type of the
> Obligation.
>
> It is possible to reduce the implementation options by placing further
> constraints, e.g.:
>
>    * The first step of the access control decision enforcement
>      *happens-before* the first step of *With* Obligation
>          o This makes sure that the implementation is not the same as
>            that of *Before*
>          o The only reason you may observe the effects of this
>            Obligation is that the decision enforcement was /attempted/
>    * The effects of the *With* Obligation should not be observed, if
>      the first step of the *With* Obligation *happens-before* the
>      access control decision enforcement failure
>          o This is a lenient implementation of the "if and only if"
>            requirement of the *With* Obligation
>          o There is a possibility that the effect of the *With*
>            Obligation is observed for indefinite time, at least if its
>            last step /also/ *happened-before* the failure to enforce
>            the access control decision
>          o There is a point for the implementors to spend the effort
>            rolling the Obligation's effects back upon the failure of
>            decision enforcement, to create a /fairer/ system of
>            obligations; but if that is critical, a new constraint needs
>            to be introduced to clarify before when the last step of the
>            rollback should happen
>
>
>
> Regards,
>
> Dr. Alex Otenko
>
>