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CSC 4320/6320 Operating Systems Lecture 10 Protection

Saurav Karmakar. CSC 4320/6320 Operating Systems Lecture 10 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

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CSC 4320/6320 Operating Systems Lecture 10 Protection

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  1. SauravKarmakar CSC 4320/6320Operating SystemsLecture 10Protection

  2. 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

  3. 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

  4. Goals of Protection • Protection problem - ensure that each object is accessed correctly and only by those processes that are allowed to do so. • Ensuring that a system uses resources only in ways consistent with stated policies (Access restriction employment) • Providing mechanism for the enforcement of the policies governing resource use. • Not only the concern of OS designer, but also application programmers need to use protection mechanism as well. [Note : Mechanism describes how things will be done and policies describes what will be done ]

  5. Principles of Protection • Guiding principle – Principle of least privilege • Programs, users and systems should be given just enough privileges to perform their tasks

  6. Domain of Protection • Operating system consists of a collection of objects, hardware or software • Requirement– Need-To-Know Principle • Useful in limiting the amount of damage a faulty process can cause in the system • Similar to “least privileged principle” .

  7. Domain Structure • Protection Domain : Resources, a process can access. • Access Right : Ability to execute operation on object • Representing 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 Association between domain and process may be static/dynamic

  8. Domain Structure • Dynamic association : Domain Switching • Domain Realization– • Each user may be a domain • Each process may be a domain • Each procedure may be a domain

  9. Domain Implementation (UNIX) • System consists of 2 domains in dual-mode: • User • Supervisor • UNIX • Domain = user-id • Domain switch accomplished via file system. • Each file has associated with it a domain bit (setuid bit) and an owner identification. • 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.

  10. Domain Implementation (MULTICS) • Let Di and Djbe any two domain rings. • If j < i Di  Dj

  11. 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 Domainican invoke on Objectj

  12. Access Matrix

  13. Use of Access Matrix • If a process in Domain Ditries 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: • copy op from Oi to Oj • transfer – switch from domain Di to Dj • owner of Oi • control – Di can modify Dj access rights

  14. Access Matrix with Copy Rights

  15. Access Matrix With Owner Rights

  16. Access Matrix of Figure A With Domains as Objects Figure B

  17. Modified Access Matrix of Figure B

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

  19. Implementation of Access Matrix • GLOBAL TABLE : • Consisting of set of ordered triplets <domain, object, right-set> • Operation M, Domain Di, Object Oj: search <Di, Oj, Rk>, with M in Rk  • Access Lists for Objects : • Each column : Access-control list for one object Defines who can perform what operation <domain, right-set>say for object F :<Domain 1 , Read, Write> <Domain 2 ,Read> • Can be extended to define a list plus default set of access rights.

  20. Implementation of Access Matrix • Capability Lists for Domains : • Each Row :Capability List (like a key)Fore each domain, what operations allowed on what objects. • Viz. Domain D : <Object 1 – Read> <Object 4 – Read, Write, Execute> <Object 5 – Read, Write, Delete, Copy> • Lock-Key Scheme : • Compromise between lists and capability based system • Each Object has a unique bit patterns, called locks • Each Domain has a uniquebit pattern lists, called keys • Lock and key pattern has to match for execution.

  21. Access Control • Protection can be applied to non-file resources as well • 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/roles • Users assigned roles granting access to privileges and programs

  22. Role-based Access Control in Solaris 10

  23. Revocation of Access Rights • Question about Revocation: • Immediate Vs Delayed • Selective Vs General • Partial Vs Total • Temporary Vs Permanent

  24. 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

  25. 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.

  26. 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.

  27. Protection in Java • 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.

  28. End of Lecture 10

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