Protection - Operating Systems - Lecture Slides, Slides of Operating Systems

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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

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.

Principles of Protection

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

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.

Domain Implementation (MULTICS)

• Let Di and Dj be any two domain rings.
• If j < I ⇒ Di ⊆ Dj

Access Matrix

Use of Access Matrix

• If a process in Domain Di tries to do “op” on object O (^) j , 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 O (^) i
    • copy op from O (^) i to O (^) j
    • control – Di can modify D (^) j access rights
    • transfer – switch from domain D (^) i to Dj

Protection models

• Access control lists
• Capability based models
• Role based models

Access control list

• List of permissions attached to an object (what user can do what to the object) - Can be seen as one slice of the access matrix - Stored with the object
• Object can be:
  • File system objects (files, directories)
  • Devices (in Unix, they are files!)
  • Processes
• From a practical point of view, it frequently does not list all the users, but refer to them as groups Docsity.com

Role-based Access Control (RBAC)

• Large organizations: it is difficult to specify the rights of every user / every object on a fine- grain level
• Users are not assigned permissions directly:
  • They can be assigned one or more roles.
  • Permissions are attached to active roles.
• The permissions are usually assigned at higher level than in ACLs - ACL: can read, can write - RBAC: can create a new account, etc. Docsity.com

Language-Based Protection

• What we have seen up to now is protection happening at the OS level
• Protection at the programming language level:
  • Objects of protection are finer grain: down to the level of objects and method calls in an object oriented language
  • What can you access?
• How is the protection enforced:
  • By the execution environment: you must trust that the VM does the right thing. Docsity.com

Stack Inspection

• End of Chapter