Lecture 3 Privilege based Isolation and Batch Processing

Outline

1. The relationship between OS and hardware

2. The relationship between OS and applications

3. Isolation mechanism

Computer System

Computer architecture is the design of the abstraction layers that allow us to implement information processing applications efficiently using available manufacturing technologies.


-- cs-152 berkeley

Abstraction Levels of Computer System

HARDWARE SUPPORTS OS, and OS SUPPORTS APPLICATIONS

• OS is located between the hardware (HW) and applications (APP)
• Understanding the relationship between OS and HW/APP can help you better master OS

Instruction Set Architecture(ISA): hardware and software interface

Boundary between hardware and OS: ISA + registers

OS: Virtualization and Abstraction of Hardware

Architecture of RISC-V Processors

Framework of u/rCore

Outline

1. The relationship between OS and hardware

2. The relationship between OS and applications

3. Isolation mechanism

OS Support for Application Execution

• Provide services
• Provide system calls
• Manage address space layout

Provides services

• Provide services through system calls
• System call: OS/APP interface (one of the boundaries)
  

Implementation details of system calls

• What happens when we call ssize_t read(int fd, void *buf, size_t count);?
• Can programs in user space call functions in kernel space?
• Can programs in kernel space call functions in user space?

System calls : Enhance system security and reliability

• Characteristics of function calls
  • Advantages: fast execution;
  • Advantages: flexible - easy to pass and return complex data types;
  • Advantages: Familiar mechanisms for programmers,...
  • Disadvantages: Unreliable/malicious app may crash the whole system.

Address space of a Process

The address space (memory layout) of the process defines the boundary between OS and APP.

Address Space Division

The address space (memory layout) of the process defines the boundary between OS and APP.

Outline

1. The relationship between OS and hardware

2. The relationship between OS and applications

3. Isolation mechanism

• Why we need isolation?
• What problems can we solve through isolation?
• How to achieve isolation?

Isolation can solve the following problems:

• prevent program X from destroying or monitoring program Y
  • read/write memory, 100% CPU usage, change file descriptors
• Prevent processes from interfering with the OS
• Protection from malicious programs, viruses, trojans and bugs
  • erroneous processes may try to trick the hardware or kernel

What is Isolation?

• Definition of isolation
  • Applications can not affect (or disrupt) the normal execution of other applications/OS. There is no information leakage in the whole system.
• The Essence of Isolation
  • Appears only when there is a need to exchange information or share resources
• Isolation doesn't mean to forbid resource/information share

Isolation Boundary

Isolation needs clear boundary.

• Boundaries determine protection domain of each process
  • Crossing protection domains is risky for resource sharing
• Mandatory isolation
  • Prevent the faulty part destroying the whole system
• Isolated unit
  • Usually running programs

Isolation methods

• Classification of isolation methods
  • Software based isolation
  • Hardware based isolation
  • Network based isolation

Classification of isolation

• Control Isolation : privilege
  • User mode vs Kernel mode
• Data Isolation : address space
  • User address space vs kernel address space
• Time Isolation : interrupt handling
  • Interrupt the running App in user mode at any time
• Handling of isolation violation: exception handling
  • OS handles the abnormal behaviors of user-mode App in kernel mode timely

Data Isolation: Virtual Memory

• Virtual Memory
  • Security issues of memory read/write
  • Security issues of Inter-Process communication
  • The problem of memory space utilization
  • The efficiency of memory read/write
• Address spaces
  • A program can only access its own memory
  • A memory access request to other process's space will be denied without permission

How virtual memory works

Control Isolation: Privilege Modes

• Privilege modes in CPU hardware
  • Prevent applications from accessing device and sensitive CPU registers
    • Address Space Configuration Register
    • Shutdown related instructions or registers
    • ...

Privilege Modes

• CPU/hardware supports different privilege modes
  • Kernel Mode vs User Mode
  • Kernel mode can perform privileged operations forbidden in user mode
    • Accessing peripherals
    • Configure address space (virtual memory)
    • Read/write special system-level registers
• OS kernel runs in kernel mode, Apps run in user mode
• Every microprocessor has similar user/kernel mode flags

Time Isolation: Interrupt/Exception Mechanism

• CPU/hardware supports interrupt/exception handling
  • Respond to and handle apps' abnormal behaviors timely
  • Prevent apps occupying CPU for a long time.
• Interrupt occurs asynchronously, as a result of signals from external I/O devices.
  • Asynchronization means that hardware interrupt is not caused by any dedicated CPU instruction.

Interrupt handler

• Interrupt handler (programs for handling hardware interrupts or exceptions):
  1. I/O device sends a signal to the processor and places the exception number on the system bus to trigger interrupt;
  2. After finishing execution of the current instruction, the processor reads the exception number from system bus, saves context, and switches to kernel mode;
  3. The interrupt handler is called. After it is finished, the CPU will begin to execute the next instruction in the program.

Timer Interrupt

• Timer can generate interrupts in a stable and periodical manner
  • Prevent apps from occupying the CPU for a long time
  • Allow OS kernel to manage resource periodically

Interrupt Handler

• Trigger interrupt
• Save context, switch to kernel mode.
• Return to next instruction before interrupt occurs, resume context.
  

Exception Handler

• Get exception handling service according to the exception id
• Save context
• Process: Kill the program which causes exception, re-execute the exception instruction.
• Resume context
  

System call Handler

• Get system call service program
• Switch from User Mode to Kernel Mode
• Switch stack, save context
• Execute in Kernel Mode
• Return to User Mode

Interrupt vs Exception vs System call

中断 vs 异常 vs 系统调用

Process Switching vs Function Switching

Summary

• Understand the relationship between computer hardware and OS: interface/boundary
• Understand the relationship between OS and applications: interface/boundary
• Understand how OS achieves isolation for applications