Page 279 - DCAP103_Principle of operating system

Principles of Operating Systems

Notes Access control is concerned with determining the allowed activities of legitimate users, mediating
every attempt by a user to access a resource in the system. A given information technology
(IT) infrastructure can implement access control systems in many places and at different levels.
Operating systems use access control to protect files and directories. Database management
systems DBMS apply access control to regulate access to tables and views. Most commercially
available application systems implement access control, often independent of the operating
systems and/or DBMSs on which they are installed.

The objectives of an access control system are often described in terms of protecting system
resources against inappropriate or undesired user access. From a business perspective, this
objective could just as well be described in terms of the optimal sharing of information. After
all, the main objective of IT is to make information available to users and applications. A greater
degree of sharing may get in the way of resource protection; in reality, a well-managed and
effective access control system actually facilitates sharing. A sufficiently fine-grained access
control mechanism can enable selective sharing of information where, in its absence, sharing
may be considered too risky altogether.
8.3.1 Concepts

This section introduces some of the concepts that are commonly used in the access control
research community and are also used throughout this document.
• Object: An entity that contains or receives information. Access to an object potentially
implies access to the information it contains. Examples of objects are records, fields (in a
database record), blocks, pages, segments, files, directories, directory trees, process, and
programs, as well as processors, video displays, keyboards, clocks, printers, and network
nodes. Devices such as electrical switches, disc drives, relays, and mechanical components
connected to a computer system may also be included in the category of objects.
• Subject: An active entity, generally in the form of a person, process, or device that causes
information to flow through objects (see below) or changes the system state.
• Operation: An active process invoked by a subject; for example, when an automatic teller
machine (ATM) user enters a card and correct personal identification number (PIN), the
control program operation on the user’s behalf is a process, but the subject can initiate
more than one operation-deposit, withdrawal, balance inquiry, etc.
• Permission (privilege): An authorization to perform some action on the system. In most
computer security literature, the term permission refers to some combination of objects
and operations. A particular operation used on two different objects represents two distinct
permissions, and similarly, two different operations applied to a single object represent
two distinct permissions. For example, a bank teller may have permissions to execute debit
and credit operations on customer records through transactions, while an accountant may
have of accounting data.
• Access Control List (ACL): A list associated with an object that specifies all the subjects
that can access the object, along with their rights to the object. Each entry in the list is a
pair (subject, set of rights). An ACL corresponds to a column of the access control matrix
(described next). ACLs are frequently implemented directly or as an approximation in
modern operating systems.
• Access Control Matrix: A table in which each row represents a subject, each column represents
an object, and each entry is the set of access rights for that subject to that object. In general,
the access control matrix is sparse—most subjects do not have access rights to most objects.
Therefore, different representations have been proposed. The access control matrix can be
represented as a list of triples, having the form <subject, rights, object>. Searching a large
number of these triples is inefficient enough that this implementation is seldom used. Rather,
the matrix is typically subdivided into columns (ACLs) or rows (capabilities).

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