Understanding Effective Actions
In PolicyGlass we express ARPs (Action
Resource
policyglass.principal.Principal
) as though they are potentially
infinite sets.
In reality they are finite sets because there are only a finite number of allowed actions, resources, or principals. However because actions are being constantly updated by AWS, and new resources and princiapls are being created all the time, we here treat them as infinite sets because their extent is unknowable by us when we are parsing the policy.
Difference
The difference between set x and set y is the elements that are contained in set x that are not contained in set y. In essence it’s a subtraction. Remove the elements in set y from set x and you have the difference.
Simple
Let’s say we calculate the difference between two effective actions like so.
>>> from policyglass import EffectiveAction, Action
>>> x = EffectiveAction(inclusion=Action("S3:*"))
>>> y = EffectiveAction(inclusion=Action("S3:Get*"))
>>> x.difference(y)
[EffectiveAction(inclusion=Action('S3:*'), exclusions=frozenset({Action('S3:Get*')}))]
The result is that the inclusion from y is added to the exclusions of x.
A is the
inclusion
fromx
B is the
inclusion
fromy
The inclusion from x is added as an exclusion of y is because our Actions are essentially infinite sets. The wildcard at the end of S3:*
could extend to an infinitely long string for all we know, so we can’t create an Action
that
expresses S3:* but not S3:Get* so we must add it as an exclusion in an EffectiveAction
.
This is the reason EffectiveAction
s exist, so we can express the
intersection of the complement of ininite set B with inifite set A.
Complex
Let’s say we have two effective actions like so:
>>> from policyglass import EffectiveAction, Action
>>> x = EffectiveAction(inclusion=Action("S3:*"))
>>> y = EffectiveAction(inclusion=Action("S3:Get*"), exclusions=frozenset({Action("S3:GetObject")}))
>>> print(x.difference(y))
[EffectiveAction(inclusion=Action('S3:*'), exclusions=frozenset({Action('S3:Get*')})),
EffectiveAction(inclusion=Action('S3:GetObject'), exclusions=frozenset())]
Let’s unpack what happened here.
We added the inclusion (
S3:get*
) from y to the exclusions of xWe returned y unchanged.
Why? What’s happening here?
A is our
inclusion
fromx
B is our
exclusion
fromx
(which we added in order to accurately refect the difference)C is our
inclusion
fromy
D is our
exclusion
fromy
In the simple example we added the inclusion from y to the exclusions of x because that’s what difference means, but why did we return y unchanged in the list?
Remember that the exclusions in an EffectiveAction are negations, they are holes punched in what’s allowed. As a result, what is in the exclusion of y should not be removed from x because it’s explicitly not part of y.
Because we can’t express the fact that we want to exclude B and C but include A in our result, we have to return
two separate EffectiveAction
s, one which includes A but the entirety of B, and another that just includes D.