FSL:A Flow-based Security Language

Tim HinrichsNatasha GudeMartìn CasadoJohn MitchellScott Shenker

University of ChicagoNicira NetworksNicira NetworksStanford UniversityUC Berkeley

Local Area Networks

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.”

NOX: a Network Architecture(Ethane’s successor)

NetworkView

NetworkView

App 1App 1

App 2App 2

App 3App 3

OF Switch

OF SwitchWirelessOF Switch

NOX ControllerNOX Controller

PC

Off-the-shelfhosts

See [Gude2008]

NOX Operation

NOX Operation

SECURITYPOLICY

NOX Operation

FSL

FSL: Flow Security Language

FSL balances the desires to makeexpressing network policies natural and implementing policies efficient.

A Datalog Variant

Syntaxh :- 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.

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

•User source•Host source•Access point source

•User target•Host target•Access point target

•Protocol

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)

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

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)}

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.

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)

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.

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.

Questions

[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.

References

Related Work ComparisonLimitations• 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].

Backup

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:

Logic Data

Keywords

Conflicts

Prioritization

Conflicts

Conflicts are vital in collaborative settings because they allow administrators to express their true intentions.

Authorization systems cannot enforce conflicting security policies.

denyavoid

visitallow

ratelimit

denyavoidvisitallowratelimit

FSL Usage Overview

CombinedPolicy

AnalysisEngine

AuthorizationSystem

Policy1

Policyn

…

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.

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)

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.

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.

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)

Cascaded Policy Combination

Combined Policy

Policy1,1

Policy1,2

Policy1,m1…

Policyn,1

Policyn,2

Policyn,mn…

…

Cascaded Policy Combination

Combined Policy

Policy1

Policyn

…

1. Flatten cascades.2. Combine results.

Features

• Distributed policy authorship• External references• Conflict detection/resolution• Incremental policy authorship via priorities• Analyzability• High Performance: 104 queries/second

Layered language:

Logic Data

Keywords

Conflict Resolution

Prioritization

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.

10-5 seconds

true && false 2.7 x 10-9

function f (x y) (x && y)) f(true,false) 3.8 x 10-8

equalp (“mary had a little lamb”, “Mary Had A Little Lamb”)

2.1 x 10-6

samep (p(X,Y,X,a), p(Z,T,Z,a)) 6.7 x 10-6

matchp (p(X,Y,X,a), p(b,c,b,a)) 7.3 x 10-6

mgup (p(X,c,X,a), p(b,T,Z,a)) 1.3 x 10-5

unifyp (p(X,c,X,a), p(b,T,Z,a)) 2.7 x 10-5

Operation Avg. Seconds

Implementation Tests

Flows/s Mem (MB)

Rule Matches

0 rules 103,699 0 0

100 rules 100,942 1 2

500 rules 85,373 1 4

1,000 rules 76,336 2 10

5,000 rules 54,416 9 30

10,000 rules 46,956 38 52

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.