1. CS703 - Advanced Operating Systems
By Mr. Farhan Zaidi

2. Lecture No. 39

3. Overview of today’s lecture
Introduction to security and protection
Security issues
Policy vs Mechanism
Design principles for security
Security requirements
Security related terminology
Introduction to user authentication

4. Security & Protection
The purpose of a protection system is to prevent accidental or intentional misuse of a system
Accidents
A program mistakenly deletes the root directory. No one can login.
This sort of problem (relatively) easy to solve: just make the likelihood small.
Malicious abuse:
A high school hacker breaks the password for user B of accounting system A and transfers $3 million to his account.
This kind of problem very hard to completely eliminate (no loopholes, can’t play on probabilities)

5. Security issues Separate processes execute in separate memory space
Isolation
Separate processes execute in separate memory space
Process can only manipulate allocated pages
Authentication
Who can access the system. Involves proving identities to the system
Access control
When can process create or access a file?
Create or read/write to socket?
Make a specific system call?
Protection problem
Ensure that each object is accessed correctly and only by those processes that are allowed to do so
Comparison between different operating systems
Compare protection models: which model supports least privilege most effectively?
Which system best enforces its protection model?

6. Policy versus mechanism
A good way to look at the problem is to separate policy (what) from mechanism (how)
A protection system is the mechanism to enforce a security policy
roughly the same set of choices, no matter what policy
A security policy delineates what acceptable behavior and unacceptable behavior.
Example security policies:
that each user can only allocate 40MB of disk
that no one but root can write to the password file
that you can’t read my mail.

7. There is no perfect protection system
Very simple point, very easy to miss:
Protection can only increase the effort (“work factor”) needed to do something bad. It cannot prevent it.
Even assuming a technically perfect system, there are always the four Bs:
Burglary: if you can’t break into my system, you can always steal it (called “physical security”)
Bribery: find whoever has access to what you want and bribe them.
Blackmail.
Bludgeoning. Or just beat them until they tell you.

8. Design Principles for Security
System design should be public
Default should be no access
Check for current authority
Give each process least privilege possible
Protection mechanism should be
simple
uniform
in lowest layers of system
Scheme should be psychologically acceptable

10. Terminology I: the entities
Principals – who is acting?
User / Process Creator
Code Author
Objects – what is that principal acting on?
File
Network connection
Rights – what actions might you take?
Read
Write
Familiar UNIX file system example:
owner / group / world
read / write / execute

11. Terminology II: the activities
Authentication – who are you?
identifying principals (users / programs)
Authorization – what are you allowed to do?
determining what access users and programs have to specific objects
Auditing – what happened
record what users and programs are doing for later analysis / prosecution

12. User Authentication Basic Principles. Authentication must identify:
Something the user knows
Something the user has
Something the user is
This is done before user can use the system