Domain Structure
Operating System Concepts
•
Access-right = <object-name, rights-set>
Rights-set is a subset of all valid operations that can be performed
on the object.
•
Domain = set of access-rights
•
Domain = user, process, or procedure
MODULE 10: PROTECTION & SECURITY
Andrzej Bednarski, Ph.D. student
Department of Computer and Information Science
Linköping University, Sweden
E-mail: andbe@ida.liu.se
URL: http://www.ida.liu.se/~andbe
The lecture notes are mainly based on Silberschatz’s, Galvin’s and Gagne’s book (“Operating System Concepts”, 6th ed.,
Addison-Wesley, 2002). No part of the lecture notes may be reproduced in any form, due to the copyrights reserved by
Addison-Wesley. These lecture notes should only be used for internal teaching purposes at the Linköping University.
10.1
TDDB63 – Operating System Concepts –
TDDB63 – Operating System Concepts –
A. Bednarski
10.4
Protection
Domain Implementation
•
•
•
•
•
•
•
•
System consists of 2 domains:
+ User
+ Supervisor
•
UNIX
+ Domain = user-id
+ Domain switch accomplished via file system.
Each file has associated with it a domain bit (setuid bit).
When file is executed and setuid = on, then user-id is set to
owner of the file being executed. When execution
completes user-id is reset.
Goals of Protection
Domain of Protection
Access Matrix
Implementation of Access Matrix
Revocation of Access Rights
Capability-Based Systems
Language-Based Protection
TDDB63 – Operating System Concepts –
A. Bednarski
10.2
Protection
A. Bednarski
10.5
Multics Rings
•
Operating system consists of a collection of objects, hardware or
software
•
Each object has a unique name and can be accessed through a
well-defined set of operations.
•
Protection problem - ensure that each object is accessed correctly
and only by those processes that are allowed to do so.
•
Need-to-know principle = at any time a process should only be
able to access only resources that it currently requires to
complete its task
TDDB63 – Operating System Concepts –
TDDB63 – Operating System Concepts –
A. Bednarski
•
•
Let Di and Dj be any two domain rings.
If j < i Ÿ Di  Dj
Violates the need-to-know principle
10.3
TDDB63 – Operating System Concepts –
A. Bednarski
10.6
1
Access Matrix
Implementation of Access Matrix
1. Global table
- Large, require I/O, grouping is limited
2. Each column = Access-control list for one object
Defines who can perform what operation.
Domain 1 = Read, Write
Domain 2 = Read
Domain 3 = Read
3. Each Row = Capability List (like a key)
Fore each domain, what operations allowed on what objects.
Object 1 – Read
Object 4 – Read, Write, Execute
Object 5 – Read, Write, Delete, Copy
Figure 1
TDDB63 – Operating System Concepts –
A. Bednarski
4. Lock mechanisms (lock and key)
10.7
TDDB63 – Operating System Concepts –
A. Bednarski
10.10
Matrix with Domains as Objects
Use of Access Matrix
•
If a process in Domain Di tries to do “op” on object Oj, then “op”
must be in the access matrix.
•
Can be expanded to dynamic protection.
+ Operations to add, delete access rights.
+ Special access rights:
owner of Oi
copy op from Oi to Oj
control – Di can modify Djs access rights
transfer – switch from domain Di to Dj
Figure 2
TDDB63 – Operating System Concepts –
A. Bednarski
10.8
Use of Access Matrix (Cont.)
•
TDDB63 – Operating System Concepts –
A. Bednarski
10.11
Access Matrix with Copyrights
Access matrix design separates mechanism from policy.
+ Mechanism
Operating system provides Access-matrix + rules.
OS ensures that the matrix is only manipulated by
authorized agents and that rules are strictly enforced.
+ Policy
User dictates policy.
Who can access what object and in what mode.
TDDB63 – Operating System Concepts –
A. Bednarski
10.9
TDDB63 – Operating System Concepts –
A. Bednarski
10.12
2
Access Matrix With Owner Rights
TDDB63 – Operating System Concepts –
A. Bednarski
Language-Based Protection
10.13
Modified Access Matrix of Figure 2
•
Specification of protection in a programming language allows the
high-level description of policies for the allocation and use of
resources.
•
Language implementation can provide software for protection
enforcement when automatic hardware-supported checking is
unavailable.
•
Interpret protection specifications to generate calls on whatever
protection system is provided by the hardware and the operating
system.
TDDB63 – Operating System Concepts –
A. Bednarski
10.17
A. Bednarski
10.18
Security
•
•
•
•
•
•
TDDB63 – Operating System Concepts –
A. Bednarski
10.14
The Security Problem
Authentication
Program Threats
System Threats
Threat Monitoring
Encryption
TDDB63 – Operating System Concepts –
Revocation of Access Rights
The Security Problem
•
•
•
Security must consider external environment of the system, and
protect it from:
+ unauthorized access.
+ malicious modification or destruction
+ accidental introduction of inconsistency.
•
Easier to protect against accidental than malicious misuse.
•
•
•
Immediate vs delayed
Selective vs general
+ For some users or for all users
Partial vs total
+ Subset of rights associated with an object
Temporary vs permanent
Capability List – Scheme required to locate capability in the
system before capability can be revoked.
+ Reacquisition: periodic revocation of capabilities
+ Back-pointers: Each object maintains a list of pointer to all
capabilities associated with that object
+ Indirection: indirect pointing to objects via a table (global)
+ Keys
TDDB63 – Operating System Concepts –
A. Bednarski
10.15
TDDB63 – Operating System Concepts –
A. Bednarski
10.19
3
Authentication
Threat Monitoring
•
User identity most often established through passwords, can be
considered a special case of either keys or capabilities.
•
Check for suspicious patterns of activity – i.e., several incorrect
password attempts may signal password guessing.
•
Passwords must be kept secret.
+ Frequent change of passwords.
+ Use of “non-guessable” passwords.
+ Log all invalid access attempts.
•
Audit log – records the time, user, and type of all accesses to an
object; useful for recovery from a violation and developing better
security measures.
•
Threat monitoring - Scan the system periodically for security
holes; done when the computer is relatively unused.
TDDB63 – Operating System Concepts –
A. Bednarski
10.20
TDDB63 – Operating System Concepts –
A. Bednarski
Program Threats
Threat Monitoring (Cont.)
•
Trojan Horse
+ Code segment that misuses its environment.
+ Exploits mechanisms for allowing programs written by users to
be executed by other users.
•
•
Trap Door
+ Specific user identifier or password that circumvents normal
security procedures.
+ Could be included in a compiler.
TDDB63 – Operating System Concepts –
A. Bednarski
10.21
Check for:
+ Short or easy-to-guess passwords
+ Unauthorized set-uid programs
+ Unauthorized programs in system directories
+ Unexpected long-running processes
+ Improper directory protections
+ Improper protections on system data files
+ Dangerous entries in the program search path (Trojan horse)
+ Changes to system programs: monitor checksum values
TDDB63 – Operating System Concepts –
A. Bednarski
10.25
Network Security Through Domain
Separation Via Firewall
System Threats
•
Worms – use spawn mechanism; standalone program
•
Internet worm
+ Exploited UNIX networking features (remote access) and bugs
in finger and sendmail programs.
+ Grappling hook program uploaded main worm program.
•
Viruses – fragment of code embedded in a legitimate program.
+ Mainly effect microcomputer systems.
+ Downloading viral programs from public bulletin boards or
exchanging floppy disks containing an infection.
+ Safe computing.
TDDB63 – Operating System Concepts –
10.24
A. Bednarski
10.22
TDDB63 – Operating System Concepts –
A. Bednarski
10.26
4
Encryption
•
Encrypt clear text into cipher text.
•
Properties of good encryption technique:
+ Relatively simple for authorized users to encrypt and decrypt
data.
+ Encryption scheme depends not on the secrecy of the
algorithm but on a parameter of the algorithm called the
encryption key.
+ Extremely difficult for an intruder to determine the encryption
key.
•
Data Encryption Standard substitutes characters and rearranges
their order on the basis of an encryption key provided to
authorized users via a secure mechanism. Scheme only as secure
as the mechanism.
TDDB63 – Operating System Concepts –
A. Bednarski
10.27
Encryption (Cont.)
•
Public-key encryption based on each user having two keys:
+ public key – published key used to encrypt data.
+ private key – key known only to individual user used to
decrypt data.
•
Must be an encryption scheme that can be made public without
making it easy to figure out the decryption scheme.
+ Efficient algorithm for testing whether or not a number is
prime.
+ No efficient algorithm is know for finding the prime factors of
a number.
TDDB63 – Operating System Concepts –
A. Bednarski
10.28
5