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:
- 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
- Move the Effect attribute up to the Policy level. Therefore, each
single Rule Policy now also preserves the Effect.
- 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.
- 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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