...

• Authoring XACML policies is an involved technical process, with behavior hinging upon the total policy set. For this reason policies/sets will be centralized, named and reused as much as possible. (Less is more)
• Administrators may choose to enforce a different set of XACML policies at any point within the repository tree.
• Metadata properties, such as ACLs or rights statements, can be used to avoid authoring more XACML.
  
  • Node Resource properties can determine the relevant policy within a set and the outcome from within that policy.
  • Policies may depend upon an access role attribute.
  • Policies may reference any value obtained via a SPARQL query, relative to the content resource, but the query must be mapped to a XACML attribute in configuration.
• Policies (and/or sets of them) must be stored in the repository.
• Policies must be enforced on externally managed content, i.e. projected nodes resources within a federated noderesource. (inc. filesystem connector)
• Must be able to authorize based on requesting I.P. address
• Must be able to authorize based on resource mixin types
• Must be able to authorize based on Hydra rightsMetadata datastream
• Must be able to authorize based on resource mimetype

...

• Must be possible to use same rules as defined in policies in the following contexts (except for #1, we only need to demonstrate/document the possibilities):
  1. calls to Fedora REST-API
  2. calls to Fedora Java classes
  3. calls to external Solr index
  4. calls to external triplestore

How to Map to XACML Policies

This includes how policies are stored in the repository and how they are linked with content objectsresources.

Proposed

Policy Persistence

Policy and Policy Set resources may be stored in any part of the repository tree at the discretion of the administrator.

Policies and Policy Sets are referenceable fcr:datastream resources that contain XACML XML.

Policy sets contain Fedora URI A top-level "fedora-policy" nt:folder node holds a flat list of referenceable policies and policy sets, as nt:file nodes with XACML XML. Some of the policy sets will contain internal ID references to other policy sets and policies in the folder.. Policy sets can define a tree structure of policies.

Policy URIs have the form info:fedora/path/to/PolicyResource. These URIs are the IDs of the Policies and PolicySets in the XACML datastream.

Finding the Effective Policy Set

One policy set in this the workspace will be configured (in the XACML Policy Finder Module) as the default policy , which for the workspace. This is the same as saying that this default policy set that is effective at the root of the Fedora /ModeShape node tree. This default policy must contain an empty target element, therefore it must be applicable to all access evaluationsresource tree and everywhere else unless overridden.

Any fcr:resource node may set a property "policy" which makes a strong reference to a single policy nodeor set resource. This overrides the effective XACML policy for itself and child nodesresources. This action requires administrator levels of access, as determined by the effective policy, or by use of a login with the fedoraAdmin role.

The Fedora XACML Policy Finder Module will implement XACML Policy Finder Module and retrieve the policy that is in scope implements the JBoss XACML PolicyFinderModule interface. It retrieves the policy or set that is effective for a given context node. It will search resource, and searches the tree for the closest parent with a policy property and return returns that XACML. It will also resolve resolves internal URI references between policies from policy sets at the request of the PDP, looking in the policy folder to find policies that are referenced in other policiesworkspace by policy URI.

Note: When combining XACML policies in sets, you specify a combining algorithm of either permit-override or deny-override. For this reason the policy property is single-valued.

Here is an example repository tree:

• ROOT
  • collection A
    • policy property (REFERENCE to --> policy A)
  • collection B (inherits default policy set from ROOT)
  • policies
    • default policy set (XACML policy set contains links to B and C)
    • policy set A (nt:file, referenceable) (XACML policy set XML contains links to default and D)
    • policy B
    • policy C
    • policy D

When combining XACML policies into sets, you have to also specify a combining algorithm of either permit-override or deny-override. For this reason we also want to resolve a single policy or policy set for a given node.

Issues

See JCR 3.8.2 Referential Integrity. It would be nice if referential integrity could be enforced for the default policy, in addition to the separately linked policies. However, my understanding is that the root node cannot have properties like this. Any ideas?

Any ideas for how to preserve referential integrity of the internal policy ID references within the XACML? Is this worth doing?

On a related note, is there any utility in doing policy-set definitions more formally as nodes linked by properties, i.e. without XML? The target section would always be empty, such that it can be used for all requests. This would add ref integrity to the graph of policies. It could be worth exploring. Such policy set nodes would use properties to link to their constituent policies and to specify the combining algorithm. (They could be converted to XACML by our policy finder module on their way to the PDP.)

How to Map XACML Attributes to Repository Data

Policies will need to refer to resource, subject and environment attributes to evaluate requests. How will the PDP resolve the referenced attributes? What is the most straightforward way to create an extensible mapping of XACML attribute to contextual repository data, given a node path?

When a property is not set, then the attribute finder needs to return an empty-set. (This is true for all attribute finding.)

Resource Attributes

Uses cases mention enforcement scenarios based on resource attributes, namely mime-type and mix-in types.

Ways we might map a resource attribute to a property value (preference for those friendly to Fedora developers):

• Predicate URI (in the RDF sense)
• Property name (effectively same as above, but more tied to modeshape API)
• Mix-in Type (another case of a predicate)
• SPARQL query
• JCR query
• XPath-like JCR expression

It might be nice if Fedora developers can define the attributes used in policies by referencing URIs they already see in the object graph representation.

Given a particular ModeShape property has been found, the data-type can be tested vs. the requested data-type and they should match so that literal values in a policy can be compared with returned values.

Proposed

We build a Resource Attribute Finder Module that is configured with a map of the resource attributes supported in policies. Each attribute has an ID, a data type, an expression, and expression type/grammar. The expressions are interpreted relative to the context node in these grammars:

• JCR 1.0 XPath
• RDF Predicate URI (returns the objects of any triples where context nodes is the subject)

Issues

Do we need SPARQL? How do we inject the context node path or URI? (help)

Subject Attributes

Policies will want to reference attributes of the subject.

One idea is to map as much as we can through the servlet request and it's headers, i.e. a pass-through of request headers related to subjects.

We can add the effective access roles to the XACML request, and/or make them available via the attribute finder module.

Proposed

We build a Subject Attribute Finder Module that is configured with a map of the subject attributes supported in policies. Each attribute has an ID, a data type, an expression and expression type/grammar:

• group, string, header: X-forward-group-name, request-context
• IP, string, IP, request-context
• role, string, role, fedora-context (maps to the effective fedora roles for a node)

Issues

There has to be a cleaner way to express these mappings, right? A dotted expression language? (help)

Shall we use the JBoss PicketBox XACML Engine?

• •  collection X
    • policy set Z(XACML contains links to Y and W)
    • policy Y
    • policy W
    • My Documents
      • policy property --> policy set Z

No Applicable Policies

This situation can arise when the only policy set (or policy) for some content contains a target element or any other XACML construct that restricts it's applicability to authz requests. If the PDP can find no policy that targets the request, it returns a NOT_APPLICABLE result to the XACML AuthZ Delegate. The delegate will then return false to ModeShape, indicating that the action is not permitted.

Implementation

The delegate uses a JBoss XACML engine. PicketLink and PicketBox projects use the same XACML PDP, which is the Sun XACML implementation repackaged by JBoss. (PicketLink is a larger umbrella project of security services.)

The JBoss XACML engine has no significant runtime dependencies, outside of a PicketLink utilities jar. The other dependencies are the Java Servlet API and XML APIs.

Relevant APIs to look at:

• org.jboss.security.xacml.sunxacml.finder.PolicyFinderModule is used to find a policy (or policy set) that matches the request evaluation context. Also used to lookup a policy that is referenced within a policy set by ID.
• org.jboss.security.xacml.sunxacml.finder.AttributeFinderModule is used to find attribute values when evaluating a policy.
• Constructing a policy set for the JBOSS engine:
  • see JCR 2.0 16.3 and JBossLDAPPolicyLocator as an example.

Propose Yes

The Sun XACML engine has been around for a while and seen plenty of production use. The JBoss project is also relied upon by a larger security framework.

Issues

It seems configurable enough, but that would be the main reservation and is to be determined.

Sun XACML Javadoc:  http://sunxacml.sourceforge.net/javadoc/

Local PDP

...

Is this better implemented as a remote or a local PDP service. The PDP can be used as a bean without the webapp runtime, or it can be configured as a separate service (SOAP). The trade-offs are identified in the table below.

Internal PDP (within ModeShape JVM)	External PDP (remote XACML service)
Minimal administrative overhead through dependency injection, etc..	Flexible, can be any XACML implementation
ModeShape cache will keep frequently used ACL metadata in memory. Removes the need for any additional cache.	Decent performance may require custom metadata caches.
No network overhead making connections or marshaling data.	Network latency, etc..
Decision and policy cache invalidation may be based on events.	Cache invalidation requires wiring JCR or Fedora JMS specifics into the chosen XACML service. Cache invalidation would be asynchronous.
Adds complexity to the runtime webapp – moving closer to a monolithic, coupled application.

Proposed

...

Cascading Deletes

When Modeshape checks for permission to remove a resource and the authz delegate returns true, there are no followup checks for removal of the child resources. The children (and their children, etc) are deleted along with the parent, but the AuthZ Delegate gets no permission callback for them.

The JBoss/Sun PDP has a notion of scope for an evaluation that can be EvaluationCtx.SCOPE_IMMEDIATE, EvaluationCtx.SCOPE_CHILDREN or EvaluationCtx.SCOPE_DESCENDANTS. If we are to use the scope feature, we need to implement another interface to retrieve children/descendants. This interface is a org.jboss.security.xacml.sunxacml.finder.ResourceFinder. When descendants are in the evaluation scope, then each is evaluated in turn by the PDP. The PDP traverses the descendants first, resolving applicable policies for each resource in turn. So this strategy should honor policy overrides with the scope of a tree delete action.

XACML AuthZ Delegate

This is the implementation of the AuthZ Delegate interface. The delegate functions as a Policy Enforcement Point (PEP) in the XACML orchestration. The delegate formulates requests for decisions and dispatch them to the JBossPDP. It interprets the results and return an appropriate response to the hasPermission() method. Most of the work for this component is in building a request context for the PDP.

PolicyFinderModule (w/PolicyLocator for JBossPDP configuration)

Fedora will need an implementation of this interface, originally part of Sun XACML. This finder module will deliver the correct policy set for the resource ID in a XACML request context according the rules for "Finding the Effective Policy Set" above. A similar kind of lookup, based on resource path, happens in the access roles provider.

This is admittedly a little abstruse, but the policy finder module becomes part of the PDP via an implementation of the PolicyLocator interface. An implementation must have a no argument constructor and received configuration via a setOptions() callback method. The PolicyLocator has a map where it will store its PolicyFinderModule for use by the JBossPDP. A simple example is the JBossPolicyLocator, which puts a list of pre-configured policies into a wrapper policy finder module. This wrapper policy finder module is added to the locator's internal map under the policy finder module key. To see how the PDP builds the list of policy finder modules at initialization time, see JBossPDP.boostrapPDP(), especially the createPolicyFinderModules() method. There you can see it build a list of finder modules from each configured policy locator.

The policy lookup operation involves the following steps:

1. finding the nearest real resource for a given the path, since access checks are sometimes performed on new resources before they are added.
2. Then you find the nearest parent with a policy property.
3. Then you read in and return the policy XACML. (There is a PolicyReader class you can use here)
4. It will also need to support requests for policies by resource URI, i.e. resolving policies that are linked from the first policy.

ModeShapeResourceFinderModule

Implements callback methods for finding child and descendant resources. This is used for delete and possibly move operations, which have cascading effect on the resource tree.

AttributeFinderModule(s)

Attribute finder modules do not work the same as policy finder modules. The AttributeLocator is an implementation of the AttributeFinderModule interface.

ResourceAttributeFinderModule(s)

These finder modules retrieve information about the resource which is being accessed. They should be implemented as several finder modules for clarity.

A triple finder module resolves attribute IDs (URIs) to RDF properties on fedora resources. Attributes are not configured in advance. XACML authors may reference any URI and if there is one or more triple with the correct subject resource and predicate, then the resource will be returned. It should match the data type expected in XACML, which is also part of the arguments passed to the finder module.

A SPARQL finder module retrieves data indirectly linked to the subject resource. This finder module will resolve attributes via a configured map of attribute IDs to SPARQL queries.

A common fedora finder module retrieves the standard attributes noted in the Fedora XACML Attributes page.

JBoss has an abstract class called StorageAttributeLocator which may be useful for formulating SPARQL queries that function much like prepared statements against a DB, where there is a replacement value. That pattern may be useful for a ResourceAttributeFinderModule.

SubjectAttributeFinderModule

EnvironmentAttributeFinderModule

XACML Role-Based Access Control

Fedora implements the XACML 2.0 Role-Based Access Control profile out-of-the-box.  Under the profile, policies are divided into two types:  Roles and Permissions.  Role policies only define Subjects, then link to Permission policies.  Permission policies only define permission sets:  Rules that apply to Resources and Actions.  Policies are grouped into PolicySets.  Permission policy sets are never referenced directly, but only through Role policy sets.

This model makes it simple to assign multiple roles to intersecting sets of permissions, and to allow for hierarchical roles:  more powerful roles can inherit the permissions of lesser roles, then extend them in their own permission policy.  Thus, an admin role inherits the permissions of both reader and writer roles.

The Fedora default policies implement the roles admin, writer, and reader.  See Fedora Basic Roles - ModeShape Permission for a mapping of roles to ModeShape actions and permissions..

Fedora XACML RBAC policies on github

Some notes on identifier conventions:

• PolicySetId identifiers will be URIs of the form info:fedora/policies/PolicyResource.  This identifier will be resolvable to the path to the policy resource in the repository.

• Internal policy element identifiers for the XACML elements PolicyId and RuleId will have the urn prefix fcrepo-xacml.