FSL: A Flow-based Security Language

1. FSL:A Flow-based Security Language University of Chicago Nicira Networks Nicira Networks Stanford University UC Berkeley Tim Hinrichs Natasha Gude Martìn Casado John Mitchell Scott Shenker

2. Local Area Networks

3. Network Policy Examples “Every wireless guest user must send HTTP requests through an HTTP proxy.” “No phone can communicate with any private computer.” “Superusers have no communication restrictions.” “Laptops cannot receive incoming connections.”

4. NOX: a Network Architecture(Ethane’s successor) App 1 NOX Controller Network View App 2 App 3 PC OF Switch Wireless OF Switch OF Switch See [Gude2008] Off-the-shelf hosts

5. NOX Operation

6. NOX Operation SECURITY POLICY

7. NOX Operation

8. FSL FSL: Flow Security Language FSL balances the desires to make expressing network policies natural and implementing policies efficient.

9. A Datalog Variant Syntax h :- b1,…,bn,c1,…,cm • h must exist. • Every variable in the body must appear in h. • Nonrecursive sentence sets. Semantics • Statement order is irrelevant. • Every sentence set is satisfied by exactly one model.

10. Network Flows Keywords for constraining flow route: • allow: allow the flow • deny: deny the flow • visit: force the flow to pass through an intermediary • avoid: forbid the flow from passing through an intermediary • ratelimit: limit on Mb/second • Protocol • User source • Host source • Access point source • User target • Host target • Access point target

11. Keyword: deny “No phone can communicate with any private computer.” deny(Usrc,Hsrc,Asrc,Utgt,Htgt,Atgt,Prot) :- phone(Hsrc) , private(Htgt) deny(Usrc,Hsrc,Asrc,Utgt,Htgt,Atgt,Prot) :- private(Hsrc) ,phone(Htgt) private(X) :-laptop(X) private(X) :-desktop(X)

12. Keyword: visit “Every wireless guest user must send HTTP requests through a proxy.” visit(Usrc,Hsrc,Asrc,Utgt,Htgt,Atgt,Prot,httpproxy) :- guest(Usrc) ,wireless(Asrc) , Prot=http

13. Operation Given FSL policy  and flow <us,hs,as,ut,ht,at,p>, ask  |= deny(us,hs,as,ut,ht,at,p)  |= allow(us,hs,as,ut,ht,at,p) {X |  |= visit(us,hs,as,ut,ht,at,p,X)} {X |  |= avoid(us,hs,as,ut,ht,at,p,X)} {X |  |= ratelimit(us,hs,as,ut,ht,at,p,X)}

14. FSL Complexity Query processing is PSPACE-complete in the size of the policy for an arbitrary query. When queries are restricted to keywords, query processing takes polynomial time in the size of the policy. If the tallest possible call stack (path through the dependency graph) is 1, then query processing takes linear time in the size of the policy.

15. Compilation Example “No phone can communicate with any private computer.” deny(Usrc,Hsrc,Asrc,Utgt,Htgt,Atgt,Prot) :- phone(Hsrc) , private(Htgt) deny(Usrc,Hsrc,Asrc,Utgt,Htgt,Atgt,Prot) :- private(Hsrc) ,phone(Htgt) private(X) :-laptop(X) private(X) :-desktop(X)

16. Compilation Example bool deny (Usrc,Hsrc,Asrc,Utgt,Htgt,Atgt,Prot) { return (phone(Hsrc) && private(Htgt)) || (private(Hsrc) && phone(Htgt)); } bool private(X) { return laptop(X) || desktop(X); } Assume the existence of functions for phone, laptop, desktop.

17. Deployment Experiences • On a small internal network (about 50 hosts), NOX has been in use over a year, and FSL has been in use for 10 months. • We are preparing for two larger deployments (of hundreds and thousands of hosts). • So far, policies are expressed over just a few classes of objects. Thus, we expect policies to grow slowly with the number of principals.

18. Questions

19. References [Gude2008] N. Gude, et. al. NOX: Towards an Operating System for Networks. Computer Communications Review 2008. [Hinrichs2009] T. Hinrichs, et. al. Design and Implementation of a Flow-based Security Language. Under review. Available upon request.

20. Related Work Comparison Limitations • Not using FOL, Modal logic, Linear logic • No existential variables • No recursion • Fixed conflict resolution scheme • No delegation • No history/future-dependent policies • Centralized enforcement • Limited metalevel operations Novel language features • Access control decisions are constraints. • Conflict resolution produces constraint set For citations, see [Hinrichs2009].

21. Backup

22. FSL Features • Logical language: Distributed policy authorship • External references • Conflicts, conflict detection, conflict resolution • Incremental policy authorship via priorities • Analyzability • High Performance: 104-105 queries/second Layered language: Prioritization Conflicts Keywords Logic Data

23. Conflicts deny avoid visit allow ratelimit deny avoid visit allow ratelimit Conflicts are vital in collaborative settings because they allow administrators to express their true intentions. Authorization systems cannot enforce conflicting security policies.

24. FSL Usage Overview Policy 1 Policy n … Combined Policy Analysis Engine Authorization System

25. Conflict Resolution • No conflicts: conflicts are errors. • Most restrictive: choose instructions that give users the least rights. • Most permissive: choose policy instructions that give users the most rights. • Cancellation: a flow with conflicting constraints has no constraints.

26. Conflict Resolution as a Tool Fixing the conflict resolution mechanism allows certain policies to be expressed very simply. Example (Open Policy): allow everything not explicitly denied. allow(Usrc,Hsrc,Asrc,Utgt,Htgt,Atgt,Prot) deny(Usrc,Hsrc,Asrc,Utgt,Htgt,Atgt,Prot) :- phone(Hsrc) ,private(Htgt) deny(Usrc,Hsrc,Asrc,Utgt,Htgt,Atgt,Prot) :- private(Hsrc) ,phone(Htgt)

27. Incremental Policy Authoring To tighten a FSL policy, one needs only to add statements to it. The conflict resolution strategy ensures that the most restrictive constraints are used. To relax a FSL policy, it is therefore insufficient to simply add statements.

28. Prioritized Policies Borrow a mechanism from Cascading Style Sheets (CSS). To relax security incrementally, FSL allows one policy to be overridden by another policy. P1 < P2 A request constrained by P2 is only constrained by P2.

29. Example P1 P2 allow(Usrc,Hsrc,Asrc,Utgt,Htgt,Atgt,Prot)  Usrc=ceo allow(Usrc,Hsrc,Asrc,Utgt,Htgt,Atgt,Prot) :- superuser(Usrc) superuser(bob) superuser(alice) deny(Usrc,Hsrc,Asrc,Utgt,Htgt,Atgt,Prot) :-phone(Hsrc) , private(Htgt) deny(Usrc,Hsrc,Asrc,Utgt,Htgt,Atgt,Prot) :-private(Hsrc) ,phone(Htgt) private(X) :- laptop(X) private(X) :- desktop(X) visit(Usrc,Hsrc,Asrc,Utgt,Htgt,Atgt,Prot,httpproxy) :-guest(Usrc) ,wireless(Asrc) , Prot=http allow(Usrc,Hsrc,Asrc,Utgt,Htgt,Atgt,ssh) :- guest(Usrc) ,server(Htgt)

30. Cascaded Policy Combination Policy 1,m1 Policy n,mn … … Policy 1,2 Policy n,2 … Policy 1,1 Policy n,1 Combined Policy

31. Cascaded Policy Combination Flatten cascades. Combine results. Policy 1 Policy n … Combined Policy

32. Features • Distributed policy authorship • External references • Conflict detection/resolution • Incremental policy authorship via priorities • Analyzability • High Performance: 104 queries/second Layered language: Prioritization Conflict Resolution Keywords Logic Data

33. Analysis Algorithms Flattened Cascade: a policy cascade expressed as a flat policy. Group Normal Form: every rule body consists only of external references (and =). Conflict Conditions: conditions on external references under which there will be a conflict. Conflict-free Normal Form: equivalent policy (under conflict resolution) without conflicts.

34. 10-5 seconds Operation Avg. Seconds

35. Implementation Tests

36. Ongoing Work Currently, each flow initiation requires contacting a central controller. The route for that flow is cached at the router. Working to generalize this caching scheme. Each trip to the central controller caches more than just the route for one flow.