1. SECURITY IN THE LINUX OPERATING SYSTEM
Andy Saxena
Somudra Chakraborty
CSE 258, Fall 1999

2. Introduction Linux is a multi-user operating system.
Security mechanisms must allow users to service their legitimate needs without compromising the server system security.
Private data must still remain private,
Resource and file management must maintain integrity;
User privileges must still be granted.
Overall system stability and correctness cannot suffer.

3. External Security User login Uses passwords for authentication
Passwords encrypted using DES
“Shadow Technique” to hide encrypted password -
/etc/shadow

4. External Security Password Shadowing /etc/passwd :
user:passwd:UID:GID:full_name:home_directory:shell
/etc/shadow
user:x:UID:GID:full_name:home_directory:shell
Shadowing does not change the structure of
/etc/passwd
It simply moves the passwords.

5. External Security Password Attacks Brute Force
Cycle through all 256 possible keys until one works
July 17,1998, Deep Crack cracked the 56-bit key 56 hours.
Dictionary Attack
Search key space that comprises weak passwords
Crack-

6. External Security Remote Access Precautions
Secure Shell -
Replaces telnet, rlogin, ftp
Provides secure means of data transmission
Insecure networks are no loner a problem

7. External Security SSH - 3 major components
Transport Layer [SSH-TRANS] - provide server authentication, confidentiality, and integrity
User authentication protocol [SSH-USERAUTH] - runs over the transport layer protocol, authenticates the client- side user to the server
Connection protocol [SSH-CONN] - runs over the user authentication protocol, multiplexes the encrypted tunnel into several logical channels

8. External Security Administering Trusted Users and Hosts
The .rhosts file exists in a user's home directory
Specifies trusted hosts based on the user’s choice
More headaches for administrator - loss of control
Solution - disable or monitor contents.
Limiting User Access to System Resources
In /etc/pam.d/limits.conf
Limit processes per user.
Limit memory usage.

9. Internal Security 5 functional mechanisms implemented
audit trail mechanisms,
discretionary access control,
information labels,
mandatory access control and
privilege.

10. Internal Security Audit Trail Functionality
System generated records - Include system and kernel messages
syslogd & klogd ---> /var/log/messages
syslogd -> program name, facility type, priority, etc. klogd -> intercepts and logs kernel messages
Application generated records - Store in user-managed space.
Limit User Access to System Resources
In /etc/pam.d/limits.conf
Limit processes per user.
Limit memory usage.

11. Internal Security Discretionary Access Control (DAC)
Limit a user’s access to a file.
Let owner determine file access permissions.
Information Labels
Contain file information about origin of file,
a release marking,
DAC advisories,
project related information, etc.
Purpose - tracking

12. Internal Security Privilege (Dynamic implementation)
Elevating subject permissions temporarily.
Let’s subject override access access control information for an object.
Set SUID bit
Mandatory Access Control
Protection decisions must not be decided by the object owner.
The system must enforce the protection decisions.

13. Internal Security File Access
Permission Bit Mechanism (Part of DAC) is implemented.
Read, write, execute permissions for owner, group and other.
10 bits used.
ACL object.
Groups
Grant privileges to a group of users.
Information stored in the groups vector in the task_struct(defined in sched.h).

14. Internal Security Memory Access Each process has its own page table.
All memory access via page table.
Easy for OS to terminate process which references an invalid memory address.
Access control information for page held in the page table entry (PTE).
Prevents executable code from being overwritten
Separates kernel code and user code.

15. Internal Security Memory Access
KRE Code running in kernel mode can read this page
URE Code running in user mode can read this page
KWE Code running in kernel mode can write to this page
UWE Code running in user mode can write to this page

16. Internal Security Process Execution Control
Kernel generates signals to executing processes.
Processes may choose to ignore most signals.
Cannot ignore SIGSTOP and SIGKILL
SIGSTOP - Halt execution.
SIGKILL - Terminate process.
This is a mandatory access control implementation.

17. Conclusion Linux is a versatile OS.
Security implementation in the OS is spread throughout the system - memory management, file management, process management, etc.
Therefore every aspect of security needs to be configured from scratch since default is not maximum security.