Fundamentals of computer system security

1.

Fundamentals of computer system
security
Mandatory access and integrity control
March 17, 2026

2.

Training questions
1) Mandatory access control (MAC)
2) Mandatory integrity monitoring (MIC)
3) MAC + MIC

3.

Why DAC not enough?
Top Secret file
Petrov
Ivanov
The owner can give access to anyone, even without a LEAK
tolerance!

4.

Mandatory access control (MAC)
Mandatory access control – a method of access differentiation in which the system forcibly checks
whether the security labels of the subject and object match.
✅ Rules are set by the administrator
✅ The user can't change them
✅ Each object and subject is assigned a label
✅ Access only if the placemarks match

5.

Security Label
LEVEL : CATEGORIES
Example: 2:1,3-level 2 (Top secret), categories 1
and 3
Access levels:
0
Unclassified
1
Classified
2
Top secret
3
Of particular importance

6.

Mandatory access rules
No Read Up (don't read above): a subject can only read an object if its level is greater than the
object level and the categories include the object categories
No Write Down (don't write down): a subject can write to an object only if its level is less than the
object's level
Reading: you can go up or at your own level
Record: you can go down or at your own level
Forbidden: read from top to bottom, write from bottom to top

7.

Placemark Management commands
pdpl-file – managing file labels
sudo pdpl-file 2:0 secret.txt
# assign level 2, category 0
sudo pdpl-file -c 2:0 script.sh
# assign an integrity level
ls -l --pdfl file # viewing placemarks
pdpl-user – managing user placemarks
sudo pdpl-user -l 2 username
# assign an access level
sudo pdpl-user -C 1,3 username
# assign categories
sudo pdpl-user -v username
# viewing user placemarks

8.

Practice: Assigning labels to files
Create files with different security levels:
echo 'Publicly available' > public.txt
echo 'Secret' > secret.txt
echo 'Sov. secret' > topsecret.txt
sudo pdpl-file 0:0 public.txt
sudo pdpl-file 1:0 secret.txt
sudo pdpl-file 2:0 topsecret.txt
ls -l --pdfl *.txt
# check

9.

Practice: Checking access rules
Check No Read Up: a user with level 1 is trying to read a level 2 file
sudo su - user2
cat /home/student/topsecret.txt
# Mistake! Permission denied
Check No Write Down: a user with level 2 tries to write to a level 0 file
sudo su - user3
echo 'test' >> /home/student/public.txt
denied
# Mistake! Permission

10.

Mandatory integrity monitoring (MIC)
Protection from damaging effects-viruses, modifications, and system file spoofing
MAC (privacy policy):
MCC (Integrity):
High Level = Secret
High Level = trusted
No Read Up, No Write Down
No Read Down, No Write Up
Example: system files (/bin, /etc) have a high level of integrity
Temporary files (/tmp) – low level
Virus from /tmp can't change it /bin/bash

11.

Comprehensive scenario: /securespace
work
incoming
integrity 0
forbidden
level 2
integrity 2
Rules:
• From incoming you can't write to work (MCC – low integrity ? high)
• From work you can't write to outgoing (MAC – high level ? low)
• From work can't read from incoming (MCC – No Read Down)
• Only through a special declassification script
outgoing
forbidden
level 1
integrity 1

12.

Practical tasks
1. Assign labels to files (0:0, 1:0, 2:0)
2. Assign placemarks to users (user1=0, user2=1, user3=2)
3. Check the rule No Read Up
4. Check the rule No Write Down
5. Create a folder /opt/trusted with a high level of integrity
6. Check the write ban on the trusted directory
7. Create a structure /securespace
8. Check information flows in /securespace
9. Create a declassification script

13.

Key findings
✅ Security label: level (0-3) + categories
✅ No Read Up – you can't read above your own level
✅ No Write Down – you can't write below your own level
✅ МК MCC protects system integrity
✅ MCC Rules: No Read Down, No Write Up
✅ Team: pdpl-file, pdpl-user
✅ Tomorrow: Auditing and Cryptography