Operating-System Protection Mechanisms — 10 Minutes of OS, Day 17

📅

loading..🐩🐩🐩🐩

Before getting started 🔗

If security is about stopping external threats,

protection

prevents internal users and programs from misusing system resources.

Operating systems provide fine-grained protection mechanisms for this purpose.

This article walks through protection principles, domain structure, access-control methods, and protection policies.

Basic protection principles 🔗

Operating-system protection starts with two questions:

Who?
What are they allowed to do?

To answer, the OS explicitly defines

which permissions each user has for which resources

, then allows or denies access accordingly.

Example 🔗

Consider a school library.

Teachers may borrow every book, while students may borrow only some.

Staff may enter every reading room, while students may enter only room A.

Defining who can do what is the core of protection.

Domain structure 🔗

A domain is the

set of permissions

available to a user or program.

When a user signs in or a program runs, the OS allows actions only within that domain.

Domain examples 🔗

User A's domain: Can read files but cannot change system settings.
Administrator domain: Can access all resources.
Program domain: Can access only part of memory; network access is blocked.

Restricted access

Access control 🔗

The OS carefully manages who can access each resource and how.

Several access-control techniques support this.

1. Access-control matrix 🔗

Rows: Users or domains.
Columns: Resources such as files, directories, or printers.
Cells: Allowed permissions, such as read, write, or execute.

예: 사용자 A가 파일 X를 읽고 쓸 수 있음 → 행 A, 열 X = read/write

In practice, this matrix becomes too large, so the following two approaches optimize it.

2. Access-control list (ACL) 🔗

Stores a list of allowed users for each resource.
Example: File X → {A: read/write, B: read only}.

3. Capability list 🔗

Stores a list of accessible resources for each user.
Example: User A → {File X: read/write, Printer: execute}.

4. Bitmap 🔗

Represents resource-access permissions as bits.
Example: One bit for access to file X, with 0 meaning denied and 1 meaning allowed.

예: 파일 X에 대한 접근 권한 비트맵
파일 X: 1100 (사용자 A: 읽기, 쓰기 가능, 사용자 B: 읽기 불가)

Protection policies 🔗

Operating systems design protection systems around the following principles.

Least privilege 🔗

Users should have
only the minimum permissions required for their work
.
Example: A user who only writes posts should not be allowed to change settings.

Isolation 🔗

Separate resources so users and programs do not affect one another.
Example: A failure in one app does not affect another.

Auditing 🔗

Record and analyze access events so problems can be traced.
Example: A system log records that user B accessed file X.

Summary 🔗

Operating-system protection goes beyond setting permissions: It is a complete system for safely separating, controlling, and recording resource use.

Understanding protection principles, domains, access-control methods, and policy design strengthens your security foundations.

The next post covers

virtual machines and hardware virtualization

References 🔗

Operating System Concepts (10th Edition)
https://en.wikipedia.org/wiki/Access_control↗
https://man7.org/linux/man-pages/man5/acl.5.html↗

Operating-System Protection Mechanisms — 10 Minutes of OS, Day 17 | PromleeBlog