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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. Local Area Networks. Network Policy Examples.

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fsl a flow based security language

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

network policy examples
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
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

nox operation1
NOX Operation

SECURITY

POLICY

slide8
FSL

FSL: Flow Security Language

FSL balances the desires to make

expressing network policies natural and

implementing policies efficient.

a datalog variant
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.
network flows
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
keyword deny
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
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
Operation

Given FSL policy  and

flow , 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
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
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 example1
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
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.

references
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.

related work comparison
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].

fsl features
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

conflicts
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.

fsl usage overview
FSL Usage Overview

Policy

1

Policy

n

Combined

Policy

Analysis

Engine

Authorization

System

conflict resolution
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
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
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
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
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
Cascaded Policy Combination

Policy

1,m1

Policy

n,mn

Policy

1,2

Policy

n,2

Policy

1,1

Policy

n,1

Combined

Policy

cascaded policy combination1
Cascaded Policy Combination

Flatten cascades.

Combine results.

Policy

1

Policy

n

Combined

Policy

features
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

analysis algorithms
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
10-5 seconds

Operation

Avg. Seconds

ongoing work
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.

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