This document discusses protection in operating systems. It covers goals of protection including ensuring only authorized access to objects. Key concepts covered include domains of protection, access matrices, implementation of access matrices, revocation of access rights, capability-based systems, and language-based protection. The document provides examples and explanations of these protection concepts.

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O P E R A T I N G S Y S T E M S
Module 19 : Protection
• Goals of Protection
• Domain of Protection
• Access Matrix
• Implementation of Access Matrix
• Revocation of Access Rights
• Capability-Based Systems
• Language-Based Protection

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Module 19: Protection
• Goals of Protection
• Domain of Protection
• Access Matrix
• Implementation of Access Matrix
• Revocation of Access Rights
• Capability-Based Systems
• Language-Based Protection

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Protection
• Operating system consists of a collection of object|s, 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.

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Domain Structure
• 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

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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.

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Multics Rings
• Let Di and Dj be any two domain rings.
• If j < I  Di  Dj

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Access Matrix
Figure 1

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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

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Use of Access Matrix (Cont.)
• Access matrix design separates mechanism from policy.
– Mechanism
 Operating system provides Access-matrix + rules.
 If 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.

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Implementation of Access Matrix
• Each column = Access-control list for one object
Defines who can perform what operation.
Domain 1 = Read, Write
Domain 2 = Read
Domain 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

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Access Matrix of Figure 1 With Domains as Objects
Figure 2

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Access Matrix with Copy Rights

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Access Matrix With Owner Rights

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Modified Access Matrix of Figure 2

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Revocation of Access Rights
• Access List – Delete access rights from access list.
– Simple
– Immediate
• Capability List – Scheme required to locate capability in the
system before capability can be revoked.
– Reacquisition
– Back-pointers
– Indirection
– Keys

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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.
• Cambridge CAP System
– Data capability - provides standard read, write, execute of
individual storage segments associated with object.
– Software capability -interpretation left to the subsystem,
through its protected procedures.

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Language-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.