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Protection Goals of Protection Goals of Protection Domain of Protection Domain of Protection Access Matrix Access Matrix Implementation of Access Matrix.

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Presentation on theme: "Protection Goals of Protection Goals of Protection Domain of Protection Domain of Protection Access Matrix Access Matrix Implementation of Access Matrix."— Presentation transcript:

1 Protection Goals of Protection Goals of Protection Domain of Protection Domain of Protection Access Matrix Access Matrix Implementation of Access Matrix Implementation of Access Matrix Revocation of Access Rights Revocation of Access Rights

2 Protection Operating system consists of a collection of objects, hardware or software. 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. 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. Protection problem - ensure that each object is accessed correctly and only by those processes that are allowed to do so.

3 Domain Structure Access-right = where rights-set is a subset of all valid operations that can be performed on the object. Access-right = where rights-set is a subset of all valid operations that can be performed on the object. Domain = set of access-rights Domain = set of access-rights

4 Domain Implementation (UNIX) System consists of 2 domains: System consists of 2 domains: UserUser SupervisorSupervisor UNIX UNIX Domain = user-idDomain = user-id Domain switch accomplished via file system.Domain switch accomplished via file system. Each file has associated with it a domain bit (setuid bit). 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. 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.

5 Domain Implementation (Multics) Let D i and D j be any two domain rings. Let D i and D j be any two domain rings. If j < I D i D j If j < I D i D j Multics Rings

6 Access Matrix View protection as a matrix (access matrix) View protection as a matrix (access matrix) Rows represent domains Rows represent domains Columns represent objects Columns represent objects Access(i, j) is the set of operations that a process executing in Domain i can invoke on Object j Access(i, j) is the set of operations that a process executing in Domain i can invoke on Object j

7 Access Matrix Figure A

8 Use of Access Matrix If a process in Domain D i tries to do op on object O j, then op must be in the access matrix. If a process in Domain D i tries to do op on object O j, then op must be in the access matrix. Can be expanded to dynamic protection. Can be expanded to dynamic protection. Operations to add, delete access rights.Operations to add, delete access rights. Special access rights:Special access rights: owner of O i owner of O i copy op from O i to O j copy op from O i to O j control – D i can modify D j access rights control – D i can modify D j access rights transfer – switch from domain D i to D j transfer – switch from domain D i to D j

9 Use of Access Matrix (Cont.) Access matrix design separates mechanism from policy. Access matrix design separates mechanism from policy. MechanismMechanism Operating system provides access-matrix + rules. Operating system provides access-matrix + rules. If ensures that the matrix is only manipulated by authorized agents and that rules are strictly enforced. If ensures that the matrix is only manipulated by authorized agents and that rules are strictly enforced. PolicyPolicy User dictates policy. User dictates policy. Who can access what object and in what mode. Who can access what object and in what mode.

10 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 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. 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

11 Access Matrix of Figure A With Domains as Objects Figure B

12 Access Matrix with Copy Rights

13 Access Matrix With Owner Rights

14 Modified Access Matrix of Figure B

15 END


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