Access control matrix in Operating Systems - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an Access Control Matrix?

An Access Control Matrix is a table that shows which users (subjects) have what kind of access (permissions) to which resources (objects) in a system.

beginner

What are the main components of an Access Control Matrix?

The main components are subjects (like users or processes), objects (like files or devices), and the rights or permissions (like read, write, execute) that subjects have on objects.

beginner

How does an Access Control Matrix help in security?

It helps by clearly defining who can do what with each resource, preventing unauthorized access and protecting system resources.

intermediate

What is a limitation of using a full Access Control Matrix in real systems?

The matrix can become very large and sparse, making it inefficient to store and manage directly for systems with many users and resources.

intermediate

Name two common ways to implement Access Control Matrices in operating systems.

Access Control Lists (ACLs), which list permissions per object, and Capability Lists, which list permissions per subject.

What does an Access Control Matrix primarily represent?

A User passwords

B Network connections between computers

C File storage locations

D Permissions of subjects on objects

An Access Control Matrix shows which subjects have what permissions on which objects.

In an Access Control Matrix, what are 'subjects'?

A Resources like files or devices

B Users or processes that access resources

C Security policies

D Encryption keys

Subjects are entities like users or processes that request access to resources.

Which of the following is NOT a typical permission in an Access Control Matrix?

A Delete user account

B Write

C Read

D Execute

Deleting user accounts is usually managed by system policies, not typical object permissions.

Why is a full Access Control Matrix often not stored directly in systems?

A It is illegal to store

B It is too small to be useful

C It is too large and mostly empty

D It requires special hardware

The matrix can be very large and sparse, so storing it directly is inefficient.

Which method lists permissions per object rather than per subject?

A Access Control List (ACL)

B Capability List

C Encryption

D Firewall

ACLs list which subjects have permissions on a particular object.

Explain what an Access Control Matrix is and why it is important in operating systems.

Think about how a system controls who can do what with files or devices.

Describe two common ways to implement access control based on the Access Control Matrix concept.

One focuses on objects, the other on subjects.

Practice

1. What does an access control matrix primarily represent in operating systems?

easy

A. Which users have what permissions on which resources

B. The order in which processes are executed

C. The memory allocation for each program

D. The list of installed software on a system

Solution

1. Step 1: Understand the purpose of access control matrix

   An access control matrix is a table that shows permissions users have on resources.

2. Step 2: Match the description to the options

   Only Which users have what permissions on which resources correctly describes permissions of users on resources.

3. Final Answer:

   Which users have what permissions on which resources -> Option A

4. Quick Check:

   Access control matrix = user permissions on resources [OK]

Hint: Think: Who can do what on which resource? [OK]

Common Mistakes:

• Confusing access control matrix with process scheduling
• Thinking it shows memory or software lists
• Mixing permissions with system hardware details

2. Which of the following correctly represents a row in an access control matrix?

easy

A. Process1 -> Memory address 0x1000

B. User1: read, write

C. File size = 1024 bytes

D. CPU usage = 50%

Solution

1. Step 1: Identify what a row in access control matrix shows

   A row lists a user and their permissions on various resources.

2. Step 2: Check which option matches this format

   User1: read, write shows a user with permissions, matching the row concept.

3. Final Answer:

   User1: read, write -> Option B

4. Quick Check:

   Row = user permissions list [OK]

Hint: Rows list users and their permissions [OK]

Common Mistakes:

• Confusing resource details with user permissions
• Mixing process or CPU info with access control
• Choosing options unrelated to permissions

3. Given the access control matrix below:

Users\Files | File1 | File2
---------|--------|-------
Alice | read | write
Bob | write | execute

What permission does Bob have on File2?

medium

A. execute

B. write

C. read

D. no permission

Solution

1. Step 1: Locate Bob's row in the matrix

   Bob's permissions are listed in the second row.

2. Step 2: Find the permission under File2 column for Bob

   Bob has 'execute' permission on File2.

3. Final Answer:

   execute -> Option A

4. Quick Check:

   Bob on File2 = execute [OK]

Hint: Find user row, then resource column [OK]

Common Mistakes:

• Mixing up rows and columns
• Choosing Alice's permissions instead of Bob's
• Assuming default permissions if not listed

4. Identify the error in this access control matrix snippet:

Users\Resources | FileA | FileB
--------------|--------|-------
Carol | read | readwriteexecute

What is wrong with this matrix?

medium

A. The matrix must list users vertically, not horizontally

B. Carol cannot have read permission

C. Multiple permissions should be separated, not combined in one cell

D. FileB should not have any permissions listed

Solution

1. Step 1: Check how permissions are listed in each cell

   Permissions should be clearly separated, usually by commas or listed distinctly.

2. Step 2: Identify if combining multiple permissions in one cell is acceptable

   Combining multiple permissions in one cell without clear separation can cause confusion.

3. Final Answer:

   Multiple permissions should be separated, not combined in one cell -> Option C

4. Quick Check:

   Permissions must be clearly separated [OK]

Hint: Permissions must be clearly separated in matrix cells [OK]

Common Mistakes:

• Thinking users must be listed horizontally
• Assuming permissions can't be combined at all
• Ignoring formatting clarity in matrix

5. You have an access control matrix with users and resources. If you want to quickly check which users can write to a resource named Report.doc, what is the best approach?

hard

A. Look across the row for Report.doc and note users with write permission

B. Check the entire matrix for any write permission

C. Check the diagonal cells only for write permission

D. Look down the column for Report.doc and note users with write permission

Solution

1. Step 1: Understand matrix layout

   Rows represent users, columns represent resources.

2. Step 2: To find users who can write to Report.doc, check the column for that resource

   Looking down the column shows which users have write permission.

3. Final Answer:

   Look down the column for Report.doc and note users with write permission -> Option D

4. Quick Check:

   Users with write on resource = column check [OK]

Hint: Check down the resource column for permissions [OK]

Common Mistakes:

• Looking across a resource row instead of column
• Checking only diagonal cells
• Scanning entire matrix unnecessarily