Lecture 20 Overview

Trusted OS Design

• OS is a complex system– difficult to design– Adding the responsibility of security enforcement

makes it even more difficult

• Clear mapping from security requirements to the design

• Design must be checked using formal reviews or simulation

• Requirements design testing

Security Design Principles

• Least privilege– users, programs, fewest privilege possible

• Economy of mechanism– small, simple, straight forward

• Open design– extensive public scrutiny

• Complete mediation– every attempt must be checked

Security Design Principles

• Permission based– denial of access is the default

• Separation of privilege– more than one condition

• Least common mechanism– the risk of sharing

• Ease of use– unlikely to be avoided

OS Functions

5

Security features in ordinary OS

• Authentication of users– password comparison

• Protection of memory– user space, paging, segmentations

• File and I/O device access control– access control matrix

• Allocation & access control to general objects– table lookup

Security features in ordinary OS

• Enforcement of sharing– integrity, consistency

• Fair service– no starvation

• Interprocess communication & synchronization– table lookup

• Protection of OS protection data– encryption, hardware control, isolation

Trusted OS Functions

8

Security features of Trusted OS• Identification and Authentication• Mandatory and Discretionary Access Control• Object reuse protection • Complete mediation (all accesses are checked)• Trusted path • Accountability and Audit (security log)• Audit log reduction• Intrusion detection (patterns of normal system

usages, anomalies)

Kernel

• OS part that performs lowest level functions

User tasks

OS

OS Kernel

Hardware

Security Kernel• responsible for enforcing security mechanisms of

the entire OS• Coverage– ensure that every access is checked

• Separation– security mechanisms are isolated from the rest of OS

and from user space easier to protect

• Unity– all security mechanisms are performed by a single set

of code easier to trace problems

Security Kernel

• Modifiability– security mechanism changes are easier to make

and test

• Compactness– relatively small

• Verifiability– formal methods , all situations are covered

Lecture 21

Trusted Operating System

CS 450/650

Fundamentals of Integrated Computer Security

Slides are modified from Hesham El-Rewini

Reference Monitor• portion of a security kernel that controls

accesses to objects• Collection of access controls for– Devices, Files, Memory, Interprocess

communication, Other objects

• It must be– Always invoked when any object is accessed– Small enough

• analysis, testing

– Tamperproof

O

S

O O

SS

Gate

Trusted Computing Base (TCB)

• Everything in the trusted OS necessary to enforce security policy

• System element on which security enforcement depends:– Hardware• processors, memory, registers, and I/O devices

– Processes• separate and protect security-critical processes

Trusted Computing Base (TCB)

• System element on which security enforcement depends (cont):– Primitive files• security access control database,

identification/authentication data

– Protected memory• reference monitor can be protected against tampering

– Interprocess communication• e.g., reference monitor can invoke and pass data

securely to audit routine

TCB and Non-TCB Code

Primitive I/O

Basic Operations

Clocks, timing

Interrupt handling

Hardware:registers memory

Capabilities

Applications

Utilities

User request interpreter

…

Segmentation, paging, memory management

TCB

Non-TCB

TCB monitors basic interactions

• Process activation

• Execution domain switching

• Memory Protection

• I/O operation

Combined Security Kernel / OS System

User tasks

OS

OS Kernel

Hardware

Security activity

OS Kernel:

- HW interactions

- Access control

OS:

- Resource allocation

- Sharing

- Access control

- Authentication functions

Separate Security Kernel

User tasks

OS

Security Kernel

Hardware

Security Kernel:

-Access control

-Authentication functions

OS:

- Resource allocation

- Sharing

- Hardware interactions

Separation

• Physical Separation

• Temporal Separation

• Cryptographic Separation

• Logical separation (isolation)

Virtualization

• OS emulates or simulates a collection of a computer system’s resources

• Virtual Machine: Collection of real or simulated hardware facilities– processor, memory, I/O devices

Virtual machine

Real System ResourcesReal System Resources

Real OSReal OS

Virtual Virtual

MachineMachine

User 1User 1

Virtual Virtual

MachineMachine

User 2User 2

Virtual Virtual

MachineMachine

User 3User 3

Layered OS

Hardware

Security functions

Synchronization, allocation

Scheduling, sharing, MM

File system, device allocation

Utility functions

Compilers, database

User processes

OS kernel

Security kernel

OS

Modules operating in Different Layers

Least trusted code

Most

trusted code

User interface

User ID lookup

Data comparison

Data update

User Authentication module

Assurance• Testing– based on the actual product being evaluated,

• not on abstraction

• Verification– each of the system’s functions works correctly

• Validation– developer is building the right product

• according to the specification

Testing• Observable effects versus internal structure• Can demonstrate existence of a problem, but

passing tests does not imply absence of any• Hard to achieve adequate test coverage within

reasonable time– inputs & internal states

• hard to keep track of all states

• Penetrating Testing– tiger team analysis, ethical hacking

• Team of experts in design of OS tries to crack system

Formal verification

• The most rigorous method• Rules of mathematical logic to demonstrate

that a system has certain security property

• Proving a Theorem– Time consuming– Complex process

Entry

min A[1]

i 1

i i + 1

i > n

min < A[i]

min A[i]

Exityes

noyes

no

Example: find minimum

Finding the minimum value

AssertionsP: n > 0 Q: n > 0 and

1 i n and min A[1]

R: n > 0 and S: n > 0 and1 i n and i = n + 1 and

for all j 1 j i -1 for all j 1 j i -1 min A[j] min A[j]

Validation

• Requirements checking– system does things it should do• also, system does not do things it is not supposed to do

• Design and code reviews– traceability from each requirement to design and

code components

• System testing– data expected from reading the requirement

document can be confirmed in the actual running of the system

Security Policies

Security Policy

• A security policy is a statement of the security we expect the system to enforce

• A system can be trusted only in relation to its security policy– that is, to the security needs the system is

expected to satisfy

Military Security policy

Unclassified

Restricted

Confidential

Secret

Top

Secret

Access to Information

• Information access is limited by the need-to-know rule

• Compartment: Each piece of classified information may be associated with one or more projects called compartments

Compartments and Sensitivity Levels

Unclassified

Restricted

Confidential

Secret

Top SecretCompartment 1

Compartment 3Compartment 2

Classification & Clearance

• <rank; compartments>– class of a piece of information

• Clearance: an indication that a person is trusted to access information up to a certain level of sensitivity

• <rank; compartments>– clearance of a subject

Dominance Relation

• We say that s dominates o (or o is dominated by s) if o <= s

For a subject s and an object o,

o <= s if and only if

rank(o) <= rank(s) and

compartments(o) is subset of compartments(s)

• A subject can read an object if the subject dominates the object.

Example

• Information classified as <secret; {Sweden}>

• Which of the following subject clearances can read the above information?– <top secret; {Sweden}>– <secret; {Sweden, crypto}>– <top secret; {crypto}>– <confidential; {Sweden}>– <secret; {France}>

Models of Security

• Security models are used to– Test a particular policy for completeness and

consistency– Document a policy– Help conceptualize and design an implementation– Check whether an implementation meets the

requirements

Lattice

Upper bound

Lower bound

Bell-La Padula Model

• Formal description of the allowable paths of information flow in a secure system

• Set of subjects and another set of objects

• Each subject s has a fixed security clearance C(s)• Each object o has a fixed security class C(o)

Bell-La Padula Model

• Two properties characterize the secure flow of information:

– A subject s may have read access to an object o only if C(o) <= C(s)

– A subject s who has read access to an object o may have write access to an object p only if C(o) <= C(p).

Illustration

o1

s1 o2

o3

s2 o4

o5

Low

High

Harrison, Ruzzo, and Ullman Model

S1 S2 S3 O1 O2 O3

S1 control Owner

read

S2 control Owner

Read

write

read Owner

execute

S3 control read read execute

HRU Model (cont.)• HRU allows state of the protection system to be

changed by a well defined set of commands:– Add subject s to M– Add object o to M– Delete subject s from M– Delete object o from M– Add right r to M[s,o]– Delete right r from M[s,o]– Owner can change rights of an object

Take Grant Model

• Unlimited number of subjects and objects• States and state transitions• Directed graph

• Four primitive operations:– take– create– grant– revoke

Take Grant Model (Cont.)

O2

O1O3

S1

S2

S3

read

read

read

execute

execute

Read, write

Create

OSS

rightsbecomes

Revoke

OS

r1, r2becomes

OS

r1, r2, r3

Take

OS2take

becomes

S1 read

OS2take

S1 read

read

Grant

becomes

OS2grant

S1 read

read

OS2grant

S1

read