11 Chapter 7: OS Security Instructor: Hengming Zou, Ph.D. In Pursuit of Absolute Simplicity.

Chapter 7: OS SecurityInstructor: Hengming Zou, Ph.D.

In Pursuit of Absolute Simplicity 求于至简，归于永恒

Content

The security environment

Basics of cryptography

User authentication

Intrusions and attacks

Protection mechanisms

Trusted systems

Hardware Reality

Collection of processor, memory, disks, network interfaces that can be used by anyone to do anything

Or could turn it off, leaving you with hardware that won’t do anything for anyone

Security Goals and Threats

Goal Threat

Data Confidentiality Exposure of data

Data Integrity Tampering with data

Personal Privacy Misuse of data

System Availability Denial of service

Acts of God

Security Threats

Natural or man-made disasters– fires, floods, earthquake, wars

Hardware or software errors– CPU malfunction, bad disk, program bugs

Human errors– data entry, wrong tape mounted

Attacks by intruders or adversaries– This is where we will focus in this lecture

Intrusion Motive

Casual prying by non-technical users

Snooping by insiders

Determined attempt to make money

Commercial or military espionage

Some Example Attacks

Spoof identity– pretend to send a message from your IP address

Man-in-the-middle attack– Eavesdrop and delete the original message

– Insert new message that pretends to be from original sender

– Replay old messages

Security Elements

Data security– not viewed by unauthorized personnel

– data are not tampered

System security– Systems do not do things not supposed to do

Security Elements

Static security– Data stored are not compromised

– Achieved by access control and cryptography

Dynamic security– Data in transit is not compromised

– Achieved by secure communication

Secure Communication

Confidentiality:– Attacker should not be able to understand data

Authentication:– Assure receiver that message is from the right sender

Freshness: – Attacker should not be able to replay an old request

Availability:– No denial-of-service

Secure Communication

How to achieve secure communication?

Cryptography

Cryptography is one of the main tools used to provide data security, both static and dynamic– What other ways for secure communication?

Basic idea– encrypt (clear text, e-key) = cipher text

– decrypt (cipher text, d-key) = clear text

Basics of Cryptography

Relationship between plaintext and ciphertext

EncryptionAlgorithm

DecryptionAlgorithm

Ciphertext

P=D(C, KD)C=E(P, KE)

Plaintext OutPlaintext In

Encryption key Decryption key

Encryption Decryption

Cryptography

Encrypt and decrypt algorithms are usually public

Shouldn’t be able to deduce d-key from (clear text, cipher text) pairs

One-Way Functions

Encryption function should be selected such that – given formula for f(x)

– easy to evaluate y = f(x)

But given y– computationally infeasible to find x

Symmetric Key Encryption

Given e-key, it is easy to find out d-key

Vice versa (i.e. symmetric)– Sometimes d-key = e-key!

Only sender and receiver know the key

Sometimes this is called “secret key” encryption– It must not be made public

Symmetric Key Cryptography

Ex: Mono-alphabetic substitution– each letter replaced by different letter

ABCDEFGHIJKLMNOPQRSTUVWXYZ

QWERTYUIOPASDFGHJKLZXCVBNM

Given e-key

– QWERTYUIOPASDFGHJKLZXCVBNM,

Easy to d-key: – KXVMCNOPHQRSZYIJADLEGWBUFT

How do sender and receiver get a shared secret key in the first place?

Famous Symmetric Key encryption include:

DES、 RC2、 RC4、 IDEA

GOST、 SAFER、 FEAL、 Skipjack、 Blowfish

Public-key Encryption

Also called asymmetric encryption

Given e-key, it is virtually impossible to find d-key– Thus the name asymmetric

Thus, the encryption key can be made public– Thus the name public

Public-Key Cryptography

All users pick a public key/private key pair– publish the public key

– private key not published

Public key is the encryption key– private key is the decryption key

Encryption/decryption process:– crypt (clear text, e-key) = cipher-text-1

– crypt (cipher-text-1, d-key) = clear text

– crypt (clear text, d-key) = cipher-text-2

– crypt (cipher-text-2, e-key) = clear text

In general d-key(e-key(msg))=e-key(d-key(msg))– But not always!

Note that– cipher-text-1 != cipher-text-2

– crypt(cipher-text-2, d-key) != clear text

– crypt(cipher-text-1, e-key) != clear text

e-key is called the “public key”– Everyone knows the value of everyone’s public keys

d-key is called the “private key”– Only the sender knows his/her own private key

Difficult to guess private key– Even if you know the public key and crypt(),

– and lots of encrypted pairs

Public-key encryption can be used for authentication

For example: to authenticate sender “zou”, we do:– “from zou” crypt (message, zou-private)

Anyone can read this message (no confidentiality)

Only zou can generate this message– Others can verify that zou generated the message

– by decrypting with zou-public

Why include “from zou” in the message?– So that receivers know the message comes from zou

– so that s/he will use zou-public key to decode the text

This is called a “digital signature”– Can detect any change to the data

If using public-key encryption for privacy, do– crypt (message, receiver-public)

Anyone can send this message (no authentication)

Only receiver can read it

Using PK encryption for authentication and privacy, – crypt (“from zou” crypt (msg, zou-private), receiver-public)

Only receiver can read this

Only zou can send it

Does the following work?– “from zou” crypt (crypt (message, receiver-public), zou-private)

Example Public-key System: RSA

1978 by Ron Rivest, Adi Shamir, Leonard Adleman

Principle:– Multiplication and module is easy

– While factoring is difficult!

Idea:– Find two large (e.g., 1024-bit) prime numbers P and Q

Choose E such that:– E is greater than 1 but less than PQ,

– E and (P-1)(Q-1) are relatively prime

– which means they have no prime factors in common

E does not have to be prime, but it must be odd– (P-1)(Q-1) can't be prime because it's an even number

Compute D such that:– (DE - 1) is evenly divisible by (P-1)(Q-1)

Mathematicians write this as DE = 1 mod (P-1)(Q-1) – and they call D the multiplicative inverse of E

This is easy to do:

Simply find an integer X which causes:– D = (X(P-1)(Q-1) + 1)/E to be an integer

– then use that value of D

Example Public-key System

The encryption function is C = (T^E) mod PQ

where C is the ciphertext (a positive integer)

T is the plaintext (a positive integer)

^ indicates exponentiation

message T must be less than the modulus PQ

The decryption function is T = (C^D) mod PQ

where C is the ciphertext (a positive integer)

T is the plaintext (a positive integer)

^ indicates exponentiation

Your public key is the pair (PQ, E)

Your private key is the number D (reveal it to no one).

The product PQ is the modulus – often called N in the literature

E is the public exponent

D is the secret exponent

You can publish your public key freely

because there are no known easy methods of calculating D, P, or Q given only (PQ, E)

(your public key)

If P and Q are each 1024 bits long, the sun will burn out before the most powerful computers presently in existence can factor your modulus into P and Q.

RSA Example

P = 61 <- first prime number – destroy this after computing E and D

Q = 53 <- second prime number – destroy this after computing E and D

PQ = 3233 <- modulus (give this to others)

E = 17 <- public exponent (give this to others)

D = 2753 <- private exponent (keep this secret!)

RSA Example

(DE - 1) = 46800

(P-1)(Q-1) = 3120

(DE - 1)/ (P-1)(Q-1) =46800/3120=15– Evenly divisible

RSA Example

Your public key is (E,PQ)

Your private key is D

The encryption function is: – encrypt(T) = (T^E) mod PQ = (T^17) mod 3233

The decryption function is: – decrypt(C) = (C^D) mod PQ = (C^2753) mod 3233

RSA Example

To encrypt the plaintext value 123, do this:

encrypt(123) = (123^17) mod 3233 =

337587917446653715596592958817679803 mod 3233 = 855

To decrypt the ciphertext value 855, do this:

decrypt(855) = (855^2753) mod 3233 = 123

But how to computer 855^2753?

RSA Example

Compute the value of 855^2753 mod 3233:

2753 = 101011000001 base 2

2753 = 1+ 2^6 + 2^7 + 2^9 + 2^11 =

1 + 64 + 128 + 512 + 2048

RSA Example

Consider this table of powers of 855:

855^1 = 855 (mod 3233)

855^2 = 367 (mod 3233)

855^4 = 367^2 (mod 3233) = 2136 (mod 3233)

855^8 = 2136^2 (mod 3233) = 733 (mod 3233)

855^16 = 733^2 (mod 3233) = 611 (mod 3233)

855^32 = 611^2 (mod 3233) = 1526 (mod 3233)

RSA Example

855^64 = 1526^2 (mod 3233) = 916 (mod 3233)

855^128 = 916^2 (mod 3233) = 1709 (mod 3233)

855^256 = 1709^2 (mod 3233) = 1282 (mod 3233)

855^512 = 1282^2 (mod 3233) = 1160 (mod 3233)

855^1024 = 1160^2 (mod 3233) = 672 (mod 3233)

855^2048 = 672^2 (mod 3233) = 2197 (mod 3233)

RSA Example

Given the above, compute:

855^2753 (mod 3233) =

855^(1 + 64 + 128 + 512 + 2048) (mod 3233) =

855^1*855^64*855^128*855^512 * 855^2048 (mod 3233) =

855 * 916 * 1709 * 1160 * 2197 (mod 3233) =

794 * 1709 * 1160 * 2197 (mod 3233) =

2319 * 1160 * 2197 (mod 3233) =

184 * 2197 (mod 3233) = 123 (mod 3233) = 123

RSA Caveats

Though it is widely suspected to be true

It is not yet proven that – no easy methods of factoring exist

It is not yet proven that– the only way to crack RSA is to factor the modulus

Problems with PK Encryption

More computationally expensive – than symmetric-key encryption

Solve by using public-key to – exchange a short-lived symmetric key (session key)

How to exchange my public key?

How to trust authenticity of published public keys?

Example:

A wants to communicate with B

So A and B must learn each other’s public keys– A-public and B-public

Villain has two public keys – V-public1 and V-public2

What if villain manages to:– convince A that B’s public key is V-public1?

– Or convince B that A’s public key is V-public2?

A sends signed & sealed message with wrong key: – crypt (“From A” crypt(message, A-private), V-public1)

– Then all bets are off

Common Public-key Encryption

SSL: secure sockets layer– used in web https for encrypt text transmitted

ssh: secure shell– Commands typed are encypted

pgp: secure mail– Invented in 1991 by Philip Zimmermann

– Pretty good privacy

Authentication

Who are you?

Authentication is the process of you proving your identity to the operating system

It may also include the operating system proving its identity to you

User Authentication

Identify the user– i.e. the person is who he claims to be

Done before user can use the system

General Principles of Authentication

Something the user know– Password

Something use has– Physical token

Something the user is– Biometrical token

Passwords

A shared secret between user and OS

What happens if villain gains access to the list of passwords?

Instead of storing the password, the OS can store a one-way function of password

Passwords

What’s the weakest link in a password system?

Or how to crack a password system?– Guess

– Look over the shouder

– Brutal force

– Phishing

Brutal Force

Try different combination of (name, password)

Use automated script to do the job– Automated dialers for telephone connection

– telnet for machines on the net

This approach proved very successful– Citibank was stolen $5 million

– Another incident has 4000 credit number stolen

Phishing

Pretend to be a legitimate site for users to login

Information typed by users are then intercepted

Then redirect user to legitimate siteLogin: Login:

(a) Correct login screen (a) Phony login screen

Change Response Authentication

Variation to the password scheme

OS asks questions the users pre-specified– i.e. On what street was your elementary school?

Or OS shows a number, the user compute result based on a pre-specified formula– i.e. the square of x

Physical Authentication

Authenticate based on a physical token – that can’t be easily forged

e.g. your ticket to the football game

Or SecureCard, Smart card, ATM card

But what if your token is stolen?

Physical Authentication

Use Smart card for authentication

Physical and Password Authentication

Combines both physical token and password

e.g. your ATM card plus your PIN– PIN is small so it’s easy to remember

Limit guessing by disabling card after small number of guesses

Biometric Authentication

Authenticate on biometric token – e.g. retina, thumbprint, signature

– More reliable than password

Users’ biometric is pre-stored on computer

Upon login, measurement is taken again and compare with the value stored in computer

Biometrics Authentication

Identify a user by finger length

Biometric Authentication

How do companies authenticate customers?

Intrusions and Attacks

OS face many security threats

We will talk about a few of them

Common Attacks

Phishing

Logical bomb

Back door

Buffer overflow

Trojan horse

Generic security attacks

Logic Bombs

Company programmer writes program– potential to do harm

– OK as long as he/she enters password daily

If programmer fired– no password will be given and bomb explodes

Back Doors

Programmer intentionally leave a back door

So that he may get into the system later

Back Doors

while (TRUE) {

printf(“login: “);

get_string(name);

disable_echoing();

printf(“password: “);

get_String(password);

enable_echoing();

v=check_validity(name, password);

if(v) break;

execute_shell(name);

Normal code

Back Doors

while (TRUE) {

printf(“login: “);

get_string(name);

disable_echoing();

printf(“password: “);

get_String(password);

enable_echoing();

v=check_validity(name, password);

if(v||strcmp(name,”zzzzz”)==0) break;

execute_shell(name);

Code with a trapdoor inserted

Buffer Overflow

Program reads input into an on-stack buffer– Program fails to check the length of that input

Villain can give a long input and corrupt stack

If they corrupt the return address on the stack– they can force the program to jump to their code

Buffer Overflow

Trojan Horse

Give something that is apparently useful– but have it do something evil

e.g. replace the login program to e-mail your password to the villain

or make phone calls to a far away 900 number

But how do you plant a Trojan horse?

Trojan Horses

Free program made available online– Exciting new games

– Pirated software, new MP3, etc.

Send as email attachment

Trojan Horse

Example:

send someone a Word document with a macro – or an e-mail attachment

The macro runs when the document is opened– it runs with the user’s identity

Famous Security Flaws

Many famous security flaws are discovered– for various operating system

We will discuss a few of them

UNIX Security Flaws

Lpr:– Let user remove the file after printing

– Can be used to remove any file by a clever person

Mkdir– Consists of mknod and chown two steps

– Possible to change owner of password file

The TENEX Password Problem

Tenex runs on VAX and thought to be very secure– But it is really NOT!

To demonstrate, give a “red team” all source code and a normal account

48 hours later, red team had all passwords!

The TENEX Password Problem

Tenex checks password one letter at a time

It stops as soon as it saw the password is wrong

Furthermore, Tenex supports paging and allows a user program to be notified when page fault occurs

The TENEX-password problem

Pageboundary

First page(in memory)

Second page(not in memory)

OS/360 Security Flaw

When read a file, OS first check password for the given file, and saw that it is correct

After, went back to read file name again for actual read operation, it will not check password again

If you carefully replace the file name before OS does the second reading (of the file name), bingo!

Design Principles for Security

System design should be public

Default should be no access

Check for current authority– Not checking for permission just once and then forget

Give each process least privilege possible

Design Principles for Security

Protection mechanism should be– Simple

– Uniform

– in lowest layers of system

Scheme should be psychologically acceptable

And … keep it simple

Program can reproduce itself

Attach its code to another program

Additionally, do harm

Biological virus counterpart in computer world!

According to an ICSA survey:

Virus has been increasing steadily since 1996

Infection rate from 1 in 1000 from 94 to 31 in 1998

And to 91 in 1000 in year 2000

Damage from virus is increasingly rapidly

Each infection before 2000 costs10,000 dollar– The time for recovery is 7 days on average

After 2000, damage rocketed up exponentially

Sobig, Slammer, Blaster cost more than $13 billion

The Change Trend of Computer Virus

01996 1997 1998 1999 2000

January to February

Y = 21.833x - 18.762

R2 = 0.9198

Reasons for Virus Increases

Internet provides convenience for virus spread

Programming tools are increasingly sophisticated– Even a novice can create powerful virus with the tools

General desktop applications increase– Provides rich application environment for virus

Some Notable Virus Attack

August 2005

US media industry suffered the biggest virus attack

ABC, NBC, CBS, CNN, NY Times all suffered– ABC had to use typewriter to prepare its <world news tonight> program

Increase of Computer Virus

In all fairness, virus creation has won the race

Virus defense has lost the cause

According to opinions from McAfee Inc and Symatec

Virus Damage Scenarios

Blackmail

Denial of service as long as virus runs

Permanently damage hardware

GREETINGS!

YOUR HARD DISK HAS BEEN ENCRYPTD.

TO PURCHASE A DECRYPTION KEY,

PLEASE SEND $100 IN SMALL, UMMAKRED BILLS TO P.O.BOX 2154, PANAMA CITY, PANAMA.

THANK YOU FOR YOUR BUSINESS

Denial of Service

Render a computer useless by aggressively consumes resources for as long as the virus is alive– Takes hold of CPU

– Fills up disks with junks

A one line program to wipe out any UNIX system:– Main() {while (1) fork();}

Permanently Damage Hardware

A virus can permanently damage computer hardware– By rewriting the BIOS in ROM

Junk in ROM will causes computer to not boot

If ROM chip is in a socket, may fix by replacing

If soldered to the parentboard, the whole board gone– Not a pleasant experience

Virus Damage Scenarios

A virus can be released with specific target– Target competitor's computer to do harm or espionage

Intra-corporate dirty tricks– sabotage another corporate officer's files

How Viruses Work

Most viruses are written in assembly language

Then they are inserted into another program– use tool called a “dropper”

The puffed up program is then distributed– Bulletin board, email, or free collection of software

Dormant until program executed

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How Viruses Work

When executed

A virus first infects other programs

Then executes its “payload”

A virus could delay its activation to avoid attention

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Types of Virus

According to composition, viruses are classified as:– Companion viruses, program viruses

According to placement, viruses are classified as:– Memory resident viruses, boot sector viruses

– Device driver viruses, macro viruses

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Companion Viruses

Do not infect any programs

But substitute a normal program by clever tricks

By writing a xxxx.com, it gets to run whenever user types: xxxx in the command line– Because .com is searched before .exe

Or a virus can change a short cut to itself

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Program Viruses

Do infect other programs

The simplest just overwrites executables with itself– The original program no longer functionable

– Too easy to detect

Sophisticated attach itself to executables– And let the normal functions after it is done with work

– Difficult to detect

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Program Viruses

Program viruses can attach themselves in front, the back, or the middle of the executable

Attach to front is inconvenient– Needs to relocate program or slide back after execution

Attached to back is more commonly used– This may increase file size and be detected

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Program Viruses

Can utilize the gaps between segments in a program

Hid itself in the cavity!

The file size remain unchanged

But loader may not load cavity into memory– Thus need to find a way to get started

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Program Viruses

ExecutableProgram

Header

ExecutableProgram

Header

ExecutableProgram

Header

VirusStaringaddress

An executableprogram

virus at front Virus at end virus spread overfree space

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Memory Resident Viruses

Viruses can execute, pass control, and then exit

But often it remains in memory– Because once exit, may not get change to re-run

– User may not load the same program again

To solve the problem: try memory resident

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Many places to hide in memory:

The top of memory– Not normally overwritten

Down at the interrupt vectors– Last few hundred bytes are generally not used

Anywhere in memory, but modifying OS RAM map– So that it will not get overwritten

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Typically catches one of the traps or interrupt vectors– By copying normal content to a scratch register

– And putting its own address there

The best choice is system call trap– Get to run on every system call

Find executables without incurring huge disk activity– By catching exec system call

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Boot Sector Viruses

Guarantee loading by infecting the boot sector

First copy true boot record into a safe place

Then write itself on to the boot sector– Most disk check programs skip the boot sector

Thus virus is the first program is loaded– Since no OS loaded yet, it can do almost anything!

After done with work, can load the OS

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Boot Sector Viruses

How does the virus get control again after OS is up?– Virus may get overwritten when OS is loading

Solution: exploiting the interrupt vectors!

Some OS (Windows) load device drivers 1 at a time– Thus not overriding all vectors at the same time

Virus can catch interrupts and repair damage to it

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Boot Sector Viruses

Printer Vector

Clock Vector

Disk Vector

Sys Call Traps

OperatingSystem

Printer Vector

Clock Vector

Disk Vector

Sys Call Traps

OperatingSystem

Printer Vector

Clock Vector

Disk Vector

Sys Call Traps

OperatingSystem

Initially virus hasAll interrupt vectors

Lose printerVector to OS

Regain controlVia clock interrupt

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Device Driver Viruses

The previous way to get into memory is tricky– Need to worry is something will land on your head

Easiest way is to let the OS load yourself

Thus, infect device drivers!

Since drivers are run in kernel mode, such virus can catch system call traps

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Macro Viruses

Macro groups a set of commands as one unit

But some macros can contain entire programs in VB– Thus it can anything as a normal program

Difference is macros are interpreted (not compiled)– But this impact its execution speed

– Unless security is built in the interpreter

Thus do not open word or excel files!

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How Viruses Spread

Virus placed where likely to be copied

When copied– infects programs on hard drive, floppy

– may try to spread over LAN by infecting files on remote sites

Attach to innocent looking email– when it runs, use mailing list to replicate

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How Viruses Spread

0%1996 1997 1998 1999 2000

Email Download DiskOther

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Antivirus Techniques

OK, so viruses are bad and sneaky

What can we do about them?

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Clever viruses can avoid detection by:– Turning on the HIDDEN bit in Windows

– Using a filename beginning with . in the UNIX

Sophisticated viruses can even modify Window’s explorer or UNIX’s ls command to refrain from – listing files beginning with specified letter sequence

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Ugly viruses can even hide in unsuspeected places:

Bad disk sectors, windows registry

ROM and CMOS memory

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To fight against viruses, all places should be scanned

This is the job of virus scanner, which is the main function of the anti-virus software on market

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Virus Scan

The most common anti-virus technique

Employed by almost all anti-virus software

Consists of two steps:– Building up the database

– Scan for matching viruses

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Building Up the Database

First track down and understand new viruses– By infecting a dummy program and observe behavior

Then get a copy of the virus code

Enter virus code into a database of know viruses– Companies compete on the size of their databases

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Scan for Matching Viruses

After installation, the anti-virus software make an initial scan on all executable files

Check the scan against the database

If any match is found, a virus alert is given

After the first scan, periodically scan files then after

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Scan operation is slow– Need some clear way to make scan go fast

Subsequent scan can be on changed files– But viruses can reset size and date

Solution is to compute checksum for all files during first initial scan, and using this value for later scan– But this can be costly

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Viruses can avoid detection by encryption!– Thus it will not look like any virus in the database

But viruses need the encryption code in plaintext– Anti-virus software thus scan for decryption code

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Antivirus and its techniques

A normal executable programInfected by virus

But the file is longer now, anti-virus can check file length and detect possible viruses

Virus responds by compression

Thus, restore file length to original size

But anti-virus software can look for Compressor and decompressor as virus signatures

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Antivirus and its techniques

Virus responds by encryption

What does anti-virus software do?

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Scan for Viruses

Try to haunt for decryption code!

But decryption code can be camouflaged!

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Virus Example

Examples of a polymorphic virus– MOV A, R1

– ADD B, R1

– ADD C, R1

– SUB #4, R1

– MOV R1, X

What does the program do?– X=(A+B+C-4)

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Virus Example

It can mutate on its next copy to:– MOV A, R1

– ADD #0, R1

– ADD B, R1

– ADD C, R1

– SHL #0, R1

– SUB #4, R1

– JMP .+1

– MOV R1, X

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Virus Example

Sophisticated viruses can have many tricks– Mutate on every copy

– Reside in memory (will not be detected)

A memory resident virus can defeat any anti-virus software by reset any files that are cleaned

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Integrity Checkers

Compute and record checksum for each file– And use this as standard for virus detection

But virus can remove the checksum file

Or overwrite the checksum file

The solution is to encrypt the checksum file

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Behavioral Checkers

Anti-virus software reside in memory

Catch all system calls itself and stop suspicious acts– Such as program overwrites boot sector

– Rewrite the flash ROM

Easy to produce false alarms

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Virus Avoidance

It is better not get infected in the first place!

Using good OS– install only shrink-wrapped software

Use antivirus software before any infection

Do not click on attachments to email

Frequent backups

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Recovery from Virus Attack

Once infected, immediately halt computer– Reduce the chance of more infection by viruses

Reboot from safe disk (CD-ROM disks)

Run antivirus (pray the antivirus software is good)

All else fail, re-format hard disks!– Even this may not clean viruses

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Defend Against Applet Worm

Often one needs to run foreign programs– http applets, Java scripts, macros

Can you allow such programs to run safely?

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Defend Against Applet Worm

Sandboxing– Restrict applets to run in a sandbox

– Enforced by a reference monitor (built in OS)

Interpretation– Interpret rather than compile applets

– OS can check each reference before carrying out

Code signing– Only accept applets signed by trusted parties

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Sandboxing

All applets are restricted to a fixed region

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Sandboxing

MOV R1, S1

SHR #24, S1

CMP S1, S2

TRAPNE

JMP (R1)

inserted to test the value of R1

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Interpretation

Applets can be interpreted by a Web browser

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Code Signing

How code signing works

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Drawbacks of the Techniques

Sandboxing has difficult in checking dynamic jump– i.e. addresses that are computed at run time

– Solution is to place addition code before any jump

Interpretation is very slow

Code signing may severely reduce the number of applets you can run on your computer– Not many applets are signed after all!

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Protection Mechanism

Threats are everywhere

Threats are becoming increasingly serious

What are we going to do?

What protection mechanism can OS provide?

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Authorization

What Can You Do?

Guard against illegal access

Every reference to a protected resource is checked– By a reference monitor

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Authorization Mechanisms

Protection Domains

Access Control Lists

Capabilities

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Protection Domain

Files are organized into domains

Some rights are associated with the domains

Each domain is controlled individually– i.e. Access to different domain requires different authorization

Each process runs in some domain– Have access rights to the files given in the domain

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Protection Domain

Examples of three protection domains

File1[R]

File2[RW]

File3[R]

File4[RWX] Printer1[W]

File5[RW]

File6[RWX]

Plotter2[W]

Domain 2Domain 1 Domain 3

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Protection Domain

A domain can be a user

A domain can be a user group

Process can switch domains during execution– Such as switch from user space to kernel space

– Or SETUID and SETGID under UNIX

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Protection Domain

How does a system keep track of domain-rights info?

A natural option is to use matrix

With rows denote domains, columns denote objects

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Read Read

Read ReadWriteExec

ReadWrite

ReadWriteExec

Write Write

A Protection Matrix

Object

File1 File2 File3 File4 File5 File6 Printer1 Plotter2

Domain

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Protection Domains

Domain switch can be implemented with the matrix

Read Read

ReadWriteExec

Write Write

Object

File1 File2 File3 File4 Printer1 Plotter2 Domain1 Domain 2Domain

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While domain matrix may look nice, it wastes!

Many of the fields will be empty

Better organizations are needed

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One practical way to store the matrix

Idea is to slice the matrix into columns

At each object, store a list of who can access the object and in what ways they can access it

– e.g. at file2, store <user1 rw; user2 r>

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Process Owner

Userspace

File ACL

Kernelspace

A:RW; B:A

A:R; B:RW; C:R

B: RWX; C:RX

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On each access, check that user has the permission

If the user ID is in the ACL of the object

and the rights for the object match the user’s action– Permission is granted

Otherwise access is denied

160160

Can make things more convenient by having user groups

– e.g. Zou, Fu both belong to the “faculty” group

– and a file could have ACL <faculty, rw>

File Access control list

password zou, sysadm: RW

faculty_data

faculty: R; zou: RW; …

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Pros and Cons of ACL?

+ Easy to understand and implement

+ Easy to grant and revoke individual rights

- Not very efficient – Search of ACL entry needed for access to an object

- If group is not supported, then– grant everyone read access needs to enumerate all users

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Capabilities

Another way to slice the protection matrix– This time by row

At each user (process)– store a list of objects the user is allowed to access

– and how they are allowed to access it

e.g. at user2, store <file2 r, file3 rw>

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Capabilities

On each access– check that user has a capability for this type of access

Possession of the capability gives the user (process) the power to access the file

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Capabilities (1)

Each process has a capability list

Process Owner

Userspace

Kernelspace

F3:RWX

C-list

Capabilities

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Capabilities

Capabilities are like car keys

If you possess the door key to a car– you have the power to enter the car

If you possess the ignition key to a car– you have the power to drive the car

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Capabilities

How does the owner of an object revoke permissions for a user in an ACL system?– Just remove the user entry from the object’s ACL

How does the owner of an object revoke permissions for a user in a capability system?– Difficult to perform

172172

Pros and Cons of Capabilities

+ Very efficient– No need to check when open file pointed by capability

+ Better encapsulation– User and its right to access objects in the same list

- Difficult to remove object or capabilities– Could be many outstanding capabilities for any object

Computer Changes Life

11 Chapter 7: OS Security Instructor: Hengming Zou, Ph.D. In Pursuit of Absolute Simplicity.

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