Chapter 14: Protection
Chapter 14: Protection
Goals of Protection Principles of Protection Domain of Protection
Access Matrix
Implementation of Access Matrix Access Control Revocation of Access Rights
Capability-Based Systems
Language-Based Protection
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Objectives
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
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Goals of Protection
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.
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Principles of Protection
Guiding principle principle of least privilege
Programs, users and systems should be given just enough privileges to perform their tasks
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Domain 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
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Domain Implementation (UNIX)
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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Domain Implementation (Multics)
Let Di and Dj be any two domain rings. If j < I
Di
Dj
Multics Rings
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Access 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
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Access Matrix
Figure A
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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 Dj 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. User dictates policy. Who can access what object and in what mode.
Policy
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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 A With Domains as Objects
Figure B
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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 B
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Access Control
Protection can be applied to non-file resources
Solaris 10 provides role-based access control to implement least
privilege
Privilege is right to execute system call or use an option within a system call Can be assigned to processes Users assigned roles granting access to privileges and programs
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Role-based Access Control in Solaris 10
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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.
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Protection 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.
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 library.
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Stack Inspection
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End of Chapter 14
