1. Operating system Part four Introduction to computer, 2nd semester, 2010/2011 Mr.Nael Aburas nras@iugaza.edu.psnras@iugaza.edu.ps Faculty of Information Technology Islamic University of Gaza

2. Security The security of a computer system requires a well-designed operating system. The security takes multiple forms; for example: reliability (no fail). If a flaw in the file manager causes the loss of part of a file, then the file was not secure.

3. Attacks from the outside An important task performed by operating system is to protect the computer’s resources from any unauthorized access. ◦ Establishing accounts for each user by the administrator (username, password and privileges) ◦ The OS use this information during each login procedure. ◦ The administrator(highly privileged) is able to monitor activity within the computer system in an effort to detect destructive behavior. ◦ To assist in this; Some software (auditing software), record and then analyze the activities taking place within the computer system.  Incorrect passwords.  Sniffing software ( software that records activities and later reports them to intruder)

4. security One of the major obstacles to the security of computer systems is the carelessness of the users themselves. ◦ Easy to guess password ◦ Share password with friends

5. Attacks from within Once an intruder gain access to a computer system, the next step is to explore, looking for information or for places to insert destructive software. The intruder may try to trick the memory manager into allowing a process to access main memory cells outside its allotted area. Or may try to trick the file manager into retrieving files whose access should be denied.

6. security To protect against such actions, CPUs are designed to operate in one of two privilege levels: ◦ Privileged mode ◦ Non-privileged mode Privileged mode: the CPU is able to execute all the instructions in its machine language Non-privileged mode: the list of acceptable instructions is limited.

7. security The instructions that are available only in privileged mode are called privileged instructions. An attempt to execute a privileged instruction when the CPU is in non-privileged mode causes an interrupt. This interrupt convert the CPU to privileged mode. When first turned on, the CPU is in privileged mode. Thus, when the OS starts at the end of the boot process, all instructions are executable. However, each time the OS allows a process to start a time slice, it switches the CPU to non-privileged mode by executing a “change privilege mode” instructinos.

8. security Privileged instructions and the control of privilege levels is the major tools available to OS for maintaining security. A single flaw in privilege level control can open the door to disaster. If a process is allowed to alter the timer that controls the system’s multiprogramming system, that process can extend its time slice and dominate the machine. If a process is allowed to access peripheral devices directly, then it can read files without supervision by the system’s file manager. If a process is allowed to access memory cell outside its allotted area, it can read and even alter data being used by other process.