Page 228 - DCAP103_Principle of operating system

Unit 6: File Management

6.9.1 Protection of Memory Notes
In a multiprogramming environment, protection of main memory is essential. The concern here is
not just security but the correct functioning of the various processes that are active. The separation
of the memory space of various processes is easily accomplished with a virtual-memory scheme.
Segmentation or Paging, or two in combination, provides an effective tools of managing main
memory. If complete isolation is sought, then the OS must simply ensure that each segment or
page accessible only by the process to which it is assigned. This is accomplished by requiring
that there be no duplicate entries in page and/or segment tables. If sharing is to be allowed then
the same segment or page may appear in more than one table. Segmentation specially lends
itself to the implementation of protection and sharing policies. Because each segment table entry
includes a length as well as a base address. A program can not access a main memory location
beyond the limit of a segment. To achieve sharing, it is possible for a segment to be referenced
in the segment tables of more than on process. In the paging system, the page structure of the
programs and data is not visible to the programmer. The measures taken to control access in a
data processing systems fall into two categories:
1. User-oriented
2. Data-oriented

A program can not access a main memory location beyond the limit of a
segment. To achieve sharing, it is possible for a segment to be referenced in
the segment tables of more than on process.
6.9.2 User-oriented Access Control

User control of access is sometimes referred to as Authentication. The most common technique for
user access control on a shared system or server is the user log, which requires ID and Password.
User access control in distributed environment can be either centralized or decentralized in a
centralized approach network provides a log on service, determining who is allowed to use the
network and to whom the user is allowed to connect. Decentralized user access control treats
the network as a transport communication link, and the destination host carries out the usual
log on procedure. In many networks, two levels of access control may be used. Data-Oriented
Access Control Following successful log on , the user has been granted access to one or set
of hosts and applications. At this time we need Data access control. In this regard real world
operating system protection models fall basically into one of two types:

1. Mandatory Access Controls (MAC)
2. Discretionary Access Controls (DAC)
In computer security passive resources are called objects and active entities that utilize the
resources are called subjects. Typical objects include—files, directories, memory, printers and
typical subjects include: users, processes. The roles depend on situation—for example, a process
can request access to some resource (act as a subject) and later be a target of access request (act
as an object).

In Mandatory access controls, also called multilevel access control, Objects (information) are
classified on hierarchical levels of security sensitivity (typically, top secrets, secret, confidential).
Subjects (Users) are assigned their security clearance. Access of a subject to an object is
granted or denied depending on the relation between the clearance of the subject and the
security classification of the object. Lattice model and Bell-LaPadula model are based on MAC
Discretionary access controls (DAC). Each object has its unique owner. The owner exercises its
discretion over the assignment of access permissions. Lampson introduced the access matrix
model for DAC. The core of this model is a matrix whose rows are indexed by subjects and
columns by objects.

LOVELY PROFESSIONAL UNIVERSITY 221