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Reference Monitors

Enforcement of Access Control

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Overview of Today’s Lecture:

• Introduction

• Operating System Integrity

• Hardware Security Features

• Protecting Memory

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Introduction:

Fundamental Concepts:

• Reference Monitor – an abstract concept

• Security Kernel – its implementation

• Trusted Computing Base (TCB) – kernel + other

protection mechanisms

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Reference Monitor (RM):

“An access control concept that refers to an abstract machine that mediates all access to objects by subjects.”

• Must be tamper proof/resistant

• Must always be invoked when access to object required

• Must be small enough to be verifiable / subject to analysis to ensure its correctness

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Security Kernel:

“The hardware, firmware, and software elements of a TCB that implement the reference monitor.”

• Must mediate all access

• Must be protected from modification

• Must be verifiable for correctness

• Ideally in the bottom layers of a system

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Trusted Computing Base (TCB):

“The totality of protection mechanisms within a computer system responsible for enforcing a security policy”

• One or more components

• Enforce a unified security policy over a product or system

• Correct enforcement depends on components within

• and input by system administrators

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Reference Monitor Placement:

Can be placed anywhere

Hardware

Operating System Kernel

Operating System

Services Layer

Application

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Reference Monitor Placement:

In relation to application it should control:

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program

RM

program RM

RM programapplication

kernel

RM in kernel Interpreter In-line RM

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Execution Monitors:

Decision of a RM depends on:

• Information about a request

• Information about the target

RMs differentiated based on the above:

• History of execution - Execution monitor

• Future of execution - Static type checking

• Rewriting

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Operating System Integrity:

• OS is not only the arbitrator of access requests

• OS is itself an object of access control

“Users must not be able to modify the operating system”

• Users should be able to use the OS

• Users should not be able to misuse the OS

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Modes of Operation:

Distinguish computations done “on behalf of”:

• the OS

• the user

A Status flag allows the OS to operate in different modes.

e.g. In Unix – supervisor (root) and user modes

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Controlled Invocation:

• User requiring supervisor mode for an operation

• Processor switches between modes

• Only predefined set of operations performed in

supervisor mode

• System returns to user mode

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Hardware Security Features:

Reasons for placing security in lower system levels:

• Possibility to evaluate security to a higher degree

reasonably simple structures

security mechanism compromised if layer below attacked

• Performance overheads reduced

• Access control decisions far removed from decisions made by applications

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Input/Output:

• How to ensure secure I/O operations?

e.g. user inputs username and password (input)

e.g. user signs documents (output)

• A trusted path between I/O device and the TCB required

• example – secure attention sequence (Windows)

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Memory Structures:

Security characteristics of memory structures:

1. RAM – (R/W) - Cannot guarantee integrity or confidentiality

2. ROM – built-in integrity guarantee, good for storing parts of an OS

3. EPROM – useful for storing parts of OS or crypto keys, advanced attacks may pose a threat

4. WROM – good for storing crypto keys, disks used for audit trail logs

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continued…

Volatile memory

• loses its contents on power off

• neither instantaneous nor complete

• reconstructable using special electronics

• defence – repeated overwrites

Non-volatile (permanent) memory

• if attacker has access by bypassing CPU

• further measures required (e.g. cryptography)

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continued…

Memory

• main memory

• cache

• buffers

• etc..

Data object may exist simultaneously in more than one location!

Copy held in an unprotected memory = risk

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Processes and Threads:

Process – program in execution, important unit of control in an OS and for security

• Works in its own address space

• Communicates with other processes with help of OS

• Separation useful for security

Thread – a strand of execution within a process

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Controlled Invocation - Interrupts:

Exceptions/Interrupts/Traps

Interruptions of executions due to errors, user request, hardware failure, etc…

• Handled by CPU

• Improper handling leads to security flaws

CTRL-C during supervisor mode operations

Interrupt table entry change

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Processing Interrupt:

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TRAP #n

interrupt vector

interrupt handler

Interrupt Interrupt vector table Memory

n

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Processing Interrupt:

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TRAP #n

interrupt vector

interrupt handler

Interrupt Interrupt vector table Memory

n

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viral code

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Intel 80x86:

• 2-bit field in status register

• Defines four privilege levels (protection rings)

• Only one instruction can change this (POPF)

• Instruction can only be executed at level 0

• Procedure -> object – in own or outer rings

• Procedure -> subroutine - only within own ring

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Intel 80x86:

• How to manage access to operations requiring higher privileges?

Gates

• System object pointing to a procedure

• In the same ring as the calling procedure

• Has different privilege level than code it points to

• Allow execute-only access to procedure in inner ring

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Intel 80x86:

Confused Deputy Problem:

• Outer ring -> Gate to copy an object from inner ring to outer ring

• This will not be prevented

• Doesn’t violate security policy

• Security policy needs to be extended – caller privilege

• 80x86 contains prevention mechanism

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Protecting Memory:

• OS integrity – preserved by separation of user & kernel space

Separation of users:

• File management – logical memory object

• Memory management – physical memory objects

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continued…

Segmentation – divides data into logical units

• Good basis for enforcing security policy

• Variable length – difficult memory management

Paging – divides memory into pages of equal size

• Popular – efficient memory management

• Not good for access control

• A page might contain objects requiring different protection

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continued…

• Possibility of a covert channel

• Logical objects stored across boundaries

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Pa$$w0RD

Pa$$w0RD

Pa$$w0RD

pageboundary

step 1 step 2 step 3

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Secure Addressing:

• Confinement of processes to separate address spaces

• Control access to data objects in memory

1. OS modifies addresses received from user

(address sandboxing)

2. OS constructs effective addresses from relative ones

(relative addressing)

3. OS checks whether address within given bounds

(base register addressing)

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Summary:

• How Access Control is enforced

• Why OS integrity is important

• Security features of existing hardware

• How to control access to memory

Next Lecture

Hands-on Unix Security

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End

3007/02/08