14.1 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Chapter 14: Protection Goals of Protection Principles of Protection Domain of Protection

14.1 Silberschatz, Galvin and Gagne ©2005Operating System Concepts

Chapter 14: ProtectionChapter 14: Protection

Goals of Protection

Principles of Protection

Domain of Protection

Access Matrix

Implementation of Access Matrix

Capability-Based Systems

Language-Based Protection


ObjectivesObjectives

Discuss the goals and principles of protection in a modern computer system

Explain how protection domains combined with an access matrix are used to specify the resources a process may access

Examine capability and language-based protection systems


Goals of ProtectionGoals of Protection

Operating system controls access to a collection of objects, hardware or software

Each object has a unique name and can be accessed through a well-defined set of operations.

The focus of this chapter is to describe the protection mechanisms the OS must provide so that the application programmers can use them in designing software that protects the resources created by an application subsystem

Protection problem : ensure that each object is accessed correctly and only by those processes that are allowed to do so.

Policies for accessing a resource can change over time


Principles of ProtectionPrinciples of Protection

Guiding principle – principle of least privilege

Programs, users and systems should be given just enough privileges to perform their tasks

For example, users should be given accounts with just the privileges the user needs


Domain of ProtectionDomain of Protection

Objects: mean both hardware (CPU, printers, memory, disks, …) and software objects (files, programs, semaphores, etc)

Each object has a unique name and can only be accessed though well defined operations, the operations depending on the object

For example, CPU can only be executed on, memory segments can be only read and written, files can be …..

Need to know principle: A process should be allowed to access only those resources that it currently requires to complete the task.


Domain StructureDomain Structure

Access-right = <object-name, rights-set>where rights-set is a subset of all valid operations that can be performed on the object.

Domain = set of access-rights A domain can be realized in many ways: For example,

Each user may be a domain. In this case the set of objects that can be accessed depends on the identity of the user.

Each process may be a domain. The set of objects that can be accessed depends on the id of the process.

Association between a process and a domain may be dynamic or static.


An Example:UNIXAn Example:UNIX

In UNIX, a domain is associated with a user. Switching domain corresponds to changing the user ID temporarily.

Domain switching is accomplished via file system. Each file has associated with it a owner identity and a domain bit

(setuid bit). When the setuid bit is on, and a user executes that file, the user

ID is set to that of the owner of the file; when the bit is off, however, the user ID does not change.

– For example, when a user process with user-id = A starts executing a file owned by B, whose associated domain bit is off, the user-id of the process remains A. When setuid bit is on, the user-id is set to that of the owner of the file, namely B. When execution completes, this temporary user ID change ends.

Domain switching is used when an otherwise privileged facility needs to be made available to a general user. For example, it might be desirable to allow users to access a network without letting them write their own networking programs.


Domain Implementation (Multics)Domain Implementation (Multics)

Protection domains are organized hierarchically in to a ring structure. Each ring corresponds to a single domain. Rings are numbered from 0 to 7.

Let Di and Dj be any two domain rings.

j < i Di Dj

i.e., a process executing in domain Dj has more privileges than a process executing in domain Di .

Multics Rings


Access MatrixAccess Matrix

View protection as a matrix (access matrix)

Rows represent domains

Columns represent objects

Access(i, j) is the set of operations that a process executing in Domaini can invoke on Objectj


Access MatrixAccess Matrix

Figure A


Use of Access MatrixUse 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 Dj’s access rights

– transfer – switch from domain Di to Dj


Use of Access Matrix (Cont.)Use of Access Matrix (Cont.)

Access matrix design separates mechanism from policy.

Mechanism

Operating system provides access-matrix + rules.

It 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.


Implementation of Access MatrixImplementation of Access Matrix

Implementing access Matrix as a matrix is very expensive in terms of space consumed because it is sparse, i.e., most of the entries in the matrix are null. So the following two approaches are used for implementing Access Matrix Each column = Access-control list for one object

Defines who can perform what operation.

Domain 1 = Read, WriteDomain 2 = ReadDomain 3 = Read

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


Access Matrix of Figure A With Domains as ObjectsAccess Matrix of Figure A With Domains as Objects

Figure B


Capability-Based Systems Capability-Based Systems

Hydra

Fixed set of access rights known to and interpreted by the system.

Interpretation of user-defined rights performed solely by user's program; system provides access protection for use of these rights as well as for the use of system-defined rights.

Cambridge CAP System provides support for two kinds of capabilities

Data capability – It can be used to provide access rights to objects, but the only rights provided are standard read, write, execute of individual storage segments associated with the object.

Software capability -interpretation left to the subsystem, through its protected procedures.


Language-Based ProtectionLanguage-Based Protection

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.


Protection in Java 2Protection in Java 2

Protection is handled by the Java Virtual Machine (JVM)

A class is assigned a protection domain when it is loaded by the JVM. (probably from an URL)

The protection domain indicates what operations the class can (and cannot) perform.

If a library method is invoked that performs a privileged operation, the stack is inspected to ensure the operation can be performed by the class.

Documents

14.1 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Chapter 14: Protection Goals of Protection Principles of Protection Domain of Protection