tailieunhanh - Lecture Operating system concepts (Fifth edition): Module 19 - Avi Silberschatz, Peter Galvin

The various processes in an operating system must be protected from one another’s activities. For that purpose, various mechanisms exist that can be used to ensure that the files, memory segments, CPU, and other resources can be operated on by only those processes that have gained proper authorization from the operating system. In this chapter, we examine the problem of protection in great detail and develop a unifying model for implementing protection. | 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 Operating System Concepts 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. Operating System Concepts 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 Operating System Concepts 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 . | 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 Operating System Concepts 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. Operating System Concepts 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 Operating System Concepts 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. Operating System Concepts Multics Rings Let Di and Dj be any two domain rings. If j < I Di Dj Operating System Concepts Access Matrix Figure 1 Operating System Concepts 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 Operating System Concepts 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. Operating System Concepts Implementation of Access Matrix Each column