TAILIEUCHUNG - Operating System Concepts (19)

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. Silberschatz and Galvin 1999 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 Silberschatz and Galvin 1999 Domain Structure.• Access-right = . Rights-set is a subset of all valid operations that can be. performed on the object• Domain = set of access-rights. Silberschatz and Galvin 1999 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 Silberschatz and Galvin 1999 Multics Rings• Let Di and Dj be any two domain rings.• If j < I Di Dj. Silberschatz and Galvin 1999 Access Matrix. Figure 1. Silberschatz and Galvin 1999 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. Silberschatz and Galvin 1999 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 Silberschatz and Galvin 1999 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. Silberschatz and Galvin 1999 Access Matrix of Figure 1 With Domains as Objects. Figure 2 Silberschatz and Galvin 1999 Access Matrix with Copy Rights. Silberschatz and Galvin 1999 Access Matrix With Owner Rights. Silberschatz and Galvin 1999 Modified Access Matrix of Figure 2. Silberschatz and Galvin 1999 Revocation of A

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