Re: [xacml] Combining algorithm combining orders

["Rich.Levinson" <rich.levinson@oracle.com>]

Hi All,

While there are probably some details in the previous email that need to 
be cleaned up, I think that it identifies a basic problem that needs to 
be addressed.

As indicated, Erik came up with a single rule policy where the rule had 
an effect of permit, and yet the policy that wrapped it effectively 
produced a deny at a higher level.

I believe the root cause of this problem is that the concept of 
"Effect", and its implementation in the current  XACML specs is 
fundamentally flawed and needs to be addressed.

Again, with Erik's use case, the flaw is obvious: a Rule with an Effect 
of Permit results in a Policy that causes a Deny. The flaw is that the 
concept of Effect is not handled properly. "Effect" effectively means 
that the block of logic contained within this element can ONLY produce 
either a Deny or a Permit, but not both. On the other hand, because a 
Policy does not have an Effect, it implicitly can produce either a Deny 
or a Permit. Therefore, to preserve the notion of "Effect", we have to 
include it also at the Policy level, with the following constraints: The 
Effect of a Policy must equal the "AND" of the Effects of all of its 
Rules. Therefore if a Policy has only Rules that are Permits, then the 
Effect of the Policy is Permit. If the Policy has Rules with both a 
Permit and Deny, then the Policy effectively has no "Effect" because the 
meaning of "Effect" is that only one type of decision is possible, and 
now the Policy can produce two types of decisions.

Therefore, I think my basic conclusions in prev email are correct about 
converging Policy and Rule into a single element - i.e. the only thing 
that distinguishes the Rule and single Rule Policy is the fact that the 
Rule has an Effect and the Policy doesn't, which has been shown above to 
be wrong and correctable by giving the Policy an Effect attribute, which 
effectively removes the distinction between Policy and Rule.

Comments and suggestions will be much appreciated.

    Thanks,
    Rich


Rich.Levinson wrote:
> Hi Erik and TC,
>
> As indicated at today's meeting, I have concerns about the proposed 
> changes. I have read this email from Erik fairly carefully, as well as 
> the first part of the cited reference:
> http://lists.oasis-open.org/archives/xacml/200701/msg00020.html
> and I also started to follow the thread initiated by Daniel:
> http://lists.oasis-open.org/archives/xacml/200810/msg00027.html
>
> In the first cited reference, Erik states:
>    "P1 has a singe rule with a Permit effect."
> then goes on about how this Rule gets propagated up and effectively 
> turns into a Deny under the deny-overrides algorithm.
>
> I agree that this appears incorrect, and went to look for the source 
> of that problem.What I found was that it appears this problem results 
> from what appears to me to be an error in the deny-overrides *rule 
> *combining algorithm, which has what I believe is an incorrect piece 
> of logic on lines 5181-5184 of the XACML 2.0 core spec, p 133:
>
>    5181     if (potentialDeny)
>    5182     {
>    5183         return Indeterminate;
>    5184     }
>
> It is my opinion the line 5183 should say "return Deny", because it is 
> at this point that the "potentialDeny" condition that the algorithm 
> goes to the trouble to find, now throws away, which ends up creating 
> the situation where a rule with effect = Permit can produce a Deny at 
> the policy combining level.
>
> I also read Daniel's comment up to the point where he said:
>    "Just throwing an exception all the way to the client is what we 
> have tried to avoid.",
> with which I agree. I expect that many policy administrators will want 
> definitive results to come out of the PDP, and not any "Indeterminates".
>
> So, my first suggestion would be to fix what I consider to be an error 
> on line 5183 described above. The practical reason why I consider this 
> an error is that if there is insufficient information to evaluate a 
> rule that has an effect = Deny, then, I believe it is legitimate to 
> define a rule combining algorithm that says that rule should return 
> the effect = Deny so that one cannot remove input attributes in the 
> hopes of reversing a decision. One could also define a companion 
> combining algorithm the evaluated to Indeterminate in order to provide 
> that option for those who want it. This way a policy administrator 
> could choose on a case by case basis which behavior is preferable 
> based on specific situations.
>
> My second suggestion is more broad-based, and, in fact, I think we 
> should consider for 3.0 if there is some agreement. I am not expecting 
> what follows to be bought hook line and sinker at first glance, but I 
> do think it is worth consideration, especially since we are already 
> making a major paradigm shift with the category-based attributes that 
> are defined in 3.0, that effectively adds a new "dimension" to the 
> domain of attributes, which I believe is a desirable step forward from 
> the Subject,Resource,Action,Environment-based attributes of 2.0, which 
> based on previous discussions about the what appear to me as ambiguous 
> and unnecessary semantic restrictions on the Action attribute 
> "category". Personally, I would recommend in section B.6 of 3.0 that 
> the action attributes be defined within a given "resource" category, 
> as I believe that actions are attributes of resources and do not have 
> any meaningful definition outside of a resource context. However, that 
> is just a minor aspect of the overall suggestion I have related to the 
> combining algorithms.
>
> The main suggestion about combining-algorithms is that I believe, but 
> have not definitively proven yet, that there is an unnecessary layer 
> of combining with two types of rule and policy combining that exists 
> in 2.0. One easy way to see that this might be true is to do the 
> following thought experiment:
>
>   1. Wrap every "Rule" within a Policy element, which effectively turns
>      every policy into a single rule policy. However, if we do this,
>      then we lose the "Effect" behavior of the Rule, therefore
>   2. Move the Effect attribute up to the Policy level. Therefore, each
>      single Rule Policy now also preserves the Effect.
>   3. Clearly all multi-rule Policies must now become PolicySets, and
>      the policy-combining algorithm must be replaced by the logic of
>      the rule combining algorithm to take into account the Effect of
>      the Policies.
>   4. We are now left with a collection of single rule Policies (with an
>      effect) and PolicySets that have no effect since it was factored
>      out in step 3. This is where it gets "fuzzy", but I suspect that
>      we might be able to show that we can still use the rule combining
>      algorithms on a mix of entities some of which have an effect and
>      some which don't. I suspect that for example on lines 5174-5177
>      that we can just skip this block if the entity being tested is a
>      PolicySet, and execute it if the entity is a Policy. If so, then I
>      think we are done and have factored the distinction between
>      rule-combining and policy-combining out of the structure.
>
> If the above logic is correct, and we can reduce the structure to be 
> simply Policies with an effect and PolicySets with no effect, and 
> allow the combining algorithms to only use the "effect" segments on 
> the objects that contain an effect, then I think we reduce the 
> complexity of the syntax (either no longer have "Rule"s or no longer 
> have "Policy"s) at the cost of only what appears to me to be a tiny 
> increase in the complexity of the rule-combining syntax (which is to 
> effectively allow for rules that have no effect and simply bypass the 
> effect logic).
>
> Bottom line is that I am offering this as a potential suggestion. 
> Because the logic to analyze it is fairly complex, I allow for the 
> fact that maybe I have missed something essential. However, since we 
> are making some fairly broad changes in 3.0, I am offering this as a 
> potential "simplification". At the very least, I am looking to find 
> what I might be missing that would prevent this "simplification".
>
>    Thanks,
>    Rich
>
>
>
>
>
>
>
> Erik Rissanen wrote:
>> All,
>>
>> In my opinion the both the deny overrides and permit overrides policy 
>> combining algorithms are wrong. They treat errors in an incorrect and 
>> unsecure manner. The corresponding rule combining algorithms are 
>> correct, and the policy combining algorithms should work in a similar 
>> manner.
>>
>> The Indeterminate result in XACML is used in those places where the 
>> result of some part of the policy cannot be determined because of 
>> some form of error. It is important that if such an indeterminate 
>> result affects the final decision of the PDP, that the indeterminate 
>> is returned to the PEP so that the PEP knows that there is an error. 
>> If so is not done, errors in policies might not be noticed or an 
>> attacker might be able to force the PEP into some bad action by 
>> causing errors in the PDP (for instance by interrupting attribute 
>> provisioning services).
>>
>> If we want to determine whether an Indeterminate should affect a 
>> result, we can consider the indeterminate to be a result which 
>> potentially could be either Permit, Deny or NotApplicable and for 
>> each of them see what the overall result would be. If the overall 
>> result depends on which choice we make for the Indeterminate, then 
>> the Indeterminate should be propagated "further up" in the policy 
>> evaluation. (If you don't understand what I mean, just read on, I 
>> will provide examples.)
>>
>> Currently the permit overrides policy combining algorithm works like 
>> this (using the updated textual description):
>>
>> 1.    If any policy evaluates to "Permit", the result is "Permit".
>> 2.    Otherwise, if any policy evaluates to "Deny", the result is 
>> "Deny".
>> 3.    Otherwise, if any policy evaluates to "Indeterminate", the 
>> result is "Indeterminate".
>> 4.    Otherwise, the result is "NotApplicable".
>>
>> Notice how a deny goes before indeterminate. That is a bad priority. 
>> Consider a policy set with two policies A and B. Assume that A is 
>> deny and B is indeterminate. Consider what the overall result could 
>> be if policy B would not have an error in it:
>>
>> A: Deny
>> B: could be Deny
>> -> overall would be Deny
>>
>> A: Deny
>> B: could be NotApplicable
>> -> overall would be Deny
>>
>> A: Deny
>> B: could be Permit
>> -> overall would be Permit
>>
>> Notice how the result depends on what the indeterminate could 
>> potentially be. However the current definition gives a definite Deny 
>> in all cases. This breaks the error propagation safety of the 
>> combining algorithm. In some applications availability could be 
>> critical, and a PEP might have a bias that indeterminate means that 
>> access is allowed. In such a case harm could be caused by this behavior.
>>
>>
>> Now consider the deny overrides instead. Currently the deny overrides 
>> policy combining algorithm works like this (using the updated textual 
>> description):
>>
>> 1.    If any policy evaluates to "Deny", the result is "Deny".
>> 2.    Otherwise, if any policy evaluates to "Indeterminate", the 
>> result is "Deny".
>> 3.    Otherwise, if any policy evaluates to "Permit", the result is 
>> "Permit".
>> 4.    Otherwise, the result is "NotApplicable".
>>
>> Note how an indeterminate is converted to a Deny. This gives the 
>> following behavior.
>>
>> A: Permit
>> B: could be Deny
>> -> overall would be Deny
>>
>> A: Permit
>> B: could be NotApplicable
>> -> overall would be Permit
>>
>> A: Permit
>> B: could be Permit
>> -> overall would be Permit
>>
>> Again, there is a definite result from the combining algorithm 
>> although the underlaying logic is ambiguous.
>>
>> As I said we have discussed this previously. The threads are here:
>>
>> http://lists.oasis-open.org/archives/xacml/200701/msg00020.html
>> http://lists.oasis-open.org/archives/xacml/200702/msg00019.html
>>
>> I propose that we define new identifiers for new combining algorithms 
>> for 3.0 which work like this:
>>
>> Deny overrides:
>> 1.    If any policy evaluates to "Deny", the result is "Deny".
>> 2.    Otherwise, if any policy evaluates to "Indeterminate", the 
>> result is "Indeterminate".
>> 3.    Otherwise, if any policy evaluates to "Permit", the result is 
>> "Permit".
>> 4.    Otherwise, the result is "NotApplicable".
>>
>> Permit overrides:
>> 1.    If any policy evaluates to "Permit", the result is "Permit".
>> 2.    Otherwise, if any policy evaluates to "Indeterminate", the 
>> result is "Indeterminate".
>> 3.    Otherwise, if any policy evaluates to "Deny", the result is 
>> "Deny".
>> 4.    Otherwise, the result is "NotApplicable".
>>
>> Best regards,
>> Erik
>>
>>
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