Hello Erik and Oleksandr,
I would like to throw 2 cents in here, as well. As Erik states, we have
not been actively reviewing this spec since it is not planned as part
of the core release of 3.0. The issue caught my attention, because I
hadn't realized this stateful type of info had been added to
Obligations WD3:
http://www.oasis-open.org/committees/download.php/27230/xacml-3.0-obligation-v1-wd-03.zip
which, it might be useful to note, was actually released on 20-Feb-08,
as opposed to the date in the spec which was not updated and is the
same as WD2, which is 28-Dec-07.
In any event, I had done some thinking about Obligations earlier wrt to
XACML 2.0 and my sense was that it would make sense to think of 2
classes of Obligations:
- pre-Condition Obligations, which need to be in effect before
access is granted, for example, the subject guarantees not to disclose
the data being requested to 3rd parties.
- post-Condition Obligations, which generally will take effect
after access is granted, for example, the subject guarantees to delete
the data after 30 days, although, this may also be viewed as a
pre-Condition Obligation; possibly a better example would be that the
PEP will send an email to the party about whom the data being returned
is about.
Using 2.0, the way I would probably go about implementing case 1, the
pre-condition, would be to issue a Deny with an accompanying Obligation
which the PEP could inform the user that in order to access the data,
that the user would have to agree to some conditions, which the PEP
could obtain, and then the PEP would re-submit the request with an
additional Attribute indicating that this Obligation had been met
before the request was being made.
With this approach, I see little need to introduce the potential
complexity that the WD3 proposal includes with the "before, after,
with, any" constructs, which as the previous emails indicate are not
particularly clearly definable without probably the necessity of
introducing some kind of workflow at the PEP which will guarantee
ordering of tasks or require some kind of 2-phase commit to ensure
that Obligations and access decisions are not in violation of some yet
to be defined rules for some yet to be defined use cases.
If there are such use cases, I think it would be advisable to introduce
them as a basis for requiring these capabilities, before we consume a
number of cycles analyzing the technology.
Thanks,
Rich
Oleksandr Otenko wrote:
4921A860.5000102@oracle.com" type="cite">Thanks
for your response.
I notice there are obligations such as Attribute Assignment. I regard
it is possible to use the values of the attributes during concurrent
decision processes, therefore it is important to understand the
concurrency model.
BTW, as far as I am aware, the ATMs first withdraw the money, then
dispense, and only if the dispense phase fails, they execute a recovery
or rollback step. There is no guarantee when the rollback step
completes, and in some implementations you can see the account debited
for significant periods of time. Account update will be a common case
of obligation, and often it will be important to lock some amount even
upon failure than overspend.
Alex
Erik Rissanen wrote:
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
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