operatingsystem《操作系统》ch.ppt

Chapter 14:Protection,Chapter 14:Protection,Goals of Protection Principles of ProtectionDomain of Protection Access Matrix Implementation of Access Matrix Access ControlRevocation of Access Rights Capability-Based Systems Language-Based Protection,Objectives,Discuss the goals and principles of protection in a modern computer systemExplain how protection domains combined with an access matrix are used to specify the resources a process may accessExamine capability and language-based protection systems,Goals of Protection,Operating system consists of a collection of objects,hardware or softwareEach 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 privilegePrograms,users and systems should be given just enough privileges to perform their tasks,Domain Structure,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 Implementation(UNIX),System consists of 2 domains:UserSupervisorUNIX Domain=user-idDomain 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,View protection as a matrix(access matrix)Rows represent domainsColumns represent objectsAccess(i,j)is the set of operations that a process executing in Domaini can invoke on Objectj,Access Matrix,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 Oicopy op from Oi to Ojcontrol Di can modify Dj access rightstransfer switch from domain Di to Dj,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.PolicyUser dictates policy.Who can access what object and in what mode.,Implementation of Access Matrix,Each column=Access-control list for one object Defines who can perform what operation.Domain 1=Read,WriteDomain 2=ReadDomain 3=Read Each Row=Capability List(like a key)Fore each domain,what operations allowed on what objects.Object 1 ReadObject 4 Read,Write,ExecuteObject 5 Read,Write,Delete,Copy,Access Matrix of Figure A With Domains as Objects,Figure B,Access Matrix with Copy Rights,Access Matrix With Owner Rights,Modified Access Matrix of Figure B,Access Control,Protection can be applied to non-file resourcesSolaris 10 provides role-based access control to implement least privilegePrivilege is right to execute system call or use an option within a system callCan be assigned to processesUsers assigned roles granting access to privileges and programs,Role-based Access Control in Solaris 10,Revocation of Access Rights,Access List Delete access rights from access list.Simple ImmediateCapability List Scheme required to locate capability in the system before capability can be revoked.ReacquisitionBack-pointersIndirectionKeys,Capability-Based Systems,HydraFixed set of access rights known to and interpreted by the system.Interpretation of user-defined rights performed solely by users 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.,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.,Protection in Java 2,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.,Stack Inspection,End of Chapter 14,