Chapter 14: Protection

14.2 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Apr 11, 2005 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

14.3 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Apr 11, 2005 Objectives Discuss the goals and principles of protection in a modern computer system Explain how protection domains combined with an access matrix are used to specify the resources a process may access Examine capability and language-based protection systems

14.4 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Apr 11, 2005 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.

14.5 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Apr 11, 2005 Principles of Protection Guiding principle – principle of least privilege Programs, users and systems should be given just enough privileges to perform their tasks The military security rule of "need-to-know" is an example of this principle

14.6 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Apr 11, 2005 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

14.7 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Apr 11, 2005 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.

14.8 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Apr 11, 2005 Security of setuid Security Programs that use this bit must be carefully designed to be immune to buffer overrun attacks. Place privileged program in a special directory Less flexible

14.9 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Apr 11, 2005 Domain Implementation (MULTICS) Let D i and D j be any two domain rings. If j < I  D i  D j

14.10 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Apr 11, 2005 Access Matrix View protection as a matrix (access matrix) Rows represent domains Columns represent objects Access(i, j) is the set of operations that a process executing in Domain i can invoke on Object j

14.11 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Apr 11, 2005 Access Matrix

14.12 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Apr 11, 2005 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. 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 – D i can modify D j access rights  transfer – switch from domain D i to D j

14.13 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Apr 11, 2005 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.

14.14 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Apr 11, 2005 Implementation of Access Matrix Global table A set of ordered triples (domain, object, right-set) A large table It is difficult to take advantage of special grouping objects or domain Each column = Access-control list for one object Defines who can perform what operation. Domain 1 = Read, Write Domain 2 = Read Domain 3 = Read Capability refers to a value that references an object along with an associated set of access rights.referencesobjectaccess rights Each Row = Capability List (like a key) For each domain, what operations allowed on what objects. Object 1 – Read Object 4 – Read, Write, Execute Object 5 – Read, Write, Delete, Copy

14.15 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Apr 11, 2005 Capability Suppose that, in a user process's memory space, there exists the following string: /etc/passwd Although this identifies a unique object on the system, it does not specify access rights and hence is not a capability. Suppose there is instead the following two values: /etc/passwd O_RDWR This identifies an object along with a set of access rights. It, however, is still not a capability because the user process's possession of these values says nothing about whether that access would actually be legitimate. Now suppose that the user program successfully executes the following statement: int fd = open("/etc/passwd", O_RDWR); The variable fd now contains the index of a file descriptor in the process's file descriptor table. This file descriptor is a capability. Its existence in the process's file descriptor table is sufficient to know that the process does indeed have legitimate access to the object. A key feature of this arrangement is that the file descriptor table is in kernel memory and cannot be directly manipulated by the user program.kernel memory

14.16 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Apr 11, 2005 Implementation of Access Matrix A lock-key mechannism Each object has a list of unique bit patterns, called locks Each domain has a list of unique bit patterns, called keys

14.17 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Apr 11, 2005 Access Matrix of Figure A With Domains as Objects Figure B

14.18 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Apr 11, 2005 Access Matrix with Copy Rights

14.19 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Apr 11, 2005 Access Matrix With Owner Rights

14.20 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Apr 11, 2005 Modified Access Matrix of Figure B

14.21 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Apr 11, 2005 Access Control Protection can be applied to non-file resources Solaris 10 provides role-based access control to implement least privilege Privilege is right to execute system call or use an option within a system call Can be assigned to processes Users assigned roles granting access to privileges and programs Roles-based Within an organization, roles are created for various job functions. The permissions to perform certain operations are assigned to specific roles. Members of staff (or other system users) are assigned particular roles, and through those role assignments acquire the permissions to perform particular system functions. Since users are not assigned permissions directly, but only acquire them through their role (or roles), management of individual user rights becomes a matter of simply assigning appropriate roles to the user; this simplifies common operations, such as adding a user, or changing a user's departmentroles

14.22 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Apr 11, 2005 Role-based Access Control in Solaris 10

14.23 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Apr 11, 2005 Revocation of Access Rights Access List – Delete access rights from access list. Simple Immediate Capability List – Scheme required to locate capability in the system before capability can be revoked. Reacquisition: be periodically deleted, reacquire again Back-pointers Indirection: points to entry in global table. Keys: a key is associate with each object. When a capability is created, a key is defined. Revocation replace the master key with anew value.

14.24 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Apr 11, 2005 Capability-Based Systems Hydra Fixed set of access rights known to and interpreted by the system. Interpretation of user-defined rights performed solely by user's 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.

14.25 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Apr 11, 2005 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.

14.26 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Apr 11, 2005 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.

14.27 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Apr 11, 2005 Stack Inspection

End of Chapter 14