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Computer System Security and Management
Reference Monitors
Three fundamental concepts in computer security:
•Reference Monitors: An access control concept that refers to an abstract machine
that mediates all accesses to objects by subjects.
•Security Kernel: The hardware, firmware, and software elements of a trusted
computing base that implement the Reference Monitor concept.
•Trusted Computing Base (TCB): The totality of protection mechanisms within a
computer system – including hardware, firmware – the combination of which is
responsible for enforcing a security policy.
Operating system
OS kernel
Different layers in a computer system
Computer System Security and Management
Reference Monitors
Placing the Reference Monitor
The reference monitor can be placed:
• In hardware: Access control to mechanisms in microprocessors.
• In the operating system: Access control in Linux.
• In the services layer: Access control in databases.
• In the application: In the application code.
Operating system integrity
The goal of an attacker is to disable the security control by modifying the operating
When securing an operating system two requirements have to be addressed:
• Users should be able to use the operating system.
• Users should not be able to misuse the operating system.
In Linux there is 2 modes:
• user mode: protected mode.
• supervisor mode: root mode.
To execute a command in supervisor mode sudo can be used in Linux.
Computer System Security and Management
Reference Monitors
Hardware security features
Hardware is the lowest layer in an IT architecture:
Operating system
OS kernel
Protection in
the Security
• It may be possible to evaluate security to a higher level of assurance
• Putting security mechanisms into the core of the system increase the
performance. No overheads caused by security checks.
• Access control decisions are removed from applications.
Computer System Security and Management
Reference Monitors
A brief overview of Computer Architecture
Input devices: Keyboards
Output devices: Monitor
CPU components:
• General purpose registers.
• Dedicated register.
Program counter: Points to memory location that
contains the next instruction to be executed.
Stack pointe: Points to the top of the system stack.
Status register: Allows the CPU to keep essential
state information.
Arithmetic Logic Unit (ALU): Executes instructions
given in a machine level language or sets bits in the
status register.
To switch between different programs, the CPU
perform a context switch and saves the state of the
current process.
RAM (Random Access Memory):
stores data temporarily.
ROM (Read Only Memory): Store data
EPROM (Erasable & Programmable
Read Only Memory): The data can be
erased or kept permanently.
WROM (Write Once memory): Freeze
the data once for all.
Keep the OS or cryptographic keys on
RAM: volatile memory.
ROM, EPROM, WROM: non-volatile
Computer System Security and Management
Reference Monitors
Processes and Threads
A process is a program in execution.
A process is an important unit of control for the OS and for the security.
A process consists of:
• Executable code.
• Data.
• The execution context.
Processes communicate with each other through primitives provided by the OS, IPC.
Less secure because of the context switch between processes.
Expensive operation in the OS
Treads are execute within a process.
Share the process address space.
More secure.
Less expensive operation in the OS.
Computer System Security and Management
Reference Monitors
Interrupts (traps, exceptions)
Interrupts are created by processes when:
• There is error in the program.
• User sends a request.
• Hardware failure.
A trap is a special input to the CPU which includes an ad
Interrupt vector table
contains addresses
Interrupt table has to be protected.
Viruses can change an entry in the
table so that it points to attack
Redirecting pointer is a very
efficient attack method.
Interrupt vector
Process of an interrupt