CIT 480: Securing Computer Systems Operating System Concepts
Topics 1. What is an OS? 2. Processes 3. Memory management 4. Filesystems 5. Virtual machines
A Computer Model An operating system has to deal with the fact that a computer is made up of a CPU, random access memory (RAM), input/output (I/O) devices, and long-term storage. I/O CPU 0 1 2 3 4 5 6 7 8 9... RAM Disk Drive
OS Concepts An operating system (OS) provides interface between the users of a computer and that computer s hardware. Handles multiple users Handles multiple programs per user Manages resource allocation CPU RAM Disk Network and other hardware access
The Kernel Core of operating system. Always in RAM. Layer between hardware and applications.
CPU Management Protection rings Lower number=higher privilege Certain CPU instructions only available in lower rings. Windows/Linux ring use Kernel runs in ring 0. User programs run in ring 3. Changing rings Interrupts change to ring 0. Interrupts also changes location of current CPU instruction to address in kernel.
Multitasking Multitasking Give each running program a slice of the CPU s time. The CPU is running so fast that to any user it appears that the computer is running all the programs simultaneously. A program runs until one of the following occurs: 1. It has used up its entire time slice. 2. It asks the kernel to access a resource. 3. An interrupt occurs.
System Calls User applications can t access hardware directly as it requires privileged CPU instructions, and thus must ask kernel to access hardware via system calls. System calls setup a data structure describing the request to make and then cause an interrupt. Examples: File I/O: open, close, read, write Request memory: brk Creation process: fork Running application: exec
Interrupts
What is a process? A process is a program in execution. Program code + dynamic execution context. Virtualization Processes provide virtual CPU + virtual memory. Kernel refers to processes as tasks.
What is in a process? A process consists of: Process ID (PID) Program code. Address space. Data. Resources: Open files Network connections If you run a program 3 times, you created 3 different processes.
Top: highest CPU processes
fork() and exec() model fork() creates a new process New PID New address space Same program code and data exec() replaces code with that of a new program $ ls fork() creates a copy of the bash shell exec() loads and runs the ls program exit() terminates ls program
Process Creation and Termination
The Process Tree OS kernel creates first process init, PID 1. All other processes created by init or by processes created by init via fork() and exec(). There init is the parent or great n - grandparent of all processes..
Viewing the Process Tree with ps
Multitasking Processes
Memory Management OS manages physical RAM. Gives each process a virtual address space. On a 32-bit machine, 2 32 bytes=4gb maximum RAM Process sees 3GB for itself. 1GB reserved for OS kernel. By creating a page table for each process. Memory is divided into pages of ~ 4KB each Address divided into page number + offset. Page table is a map from virtual pages to physical pages. CPU uses page table to translate virtual addresses to physical addresses. Only the kernel can modify a page table. A process cannot access memory of other processes since its page table does not contain mappings to their memory pages.
Virtual Address Translation
Virtual Memory OS gives each process 4GB Most processes do not use that much RAM. Many page table entries are blank. A single process cannot use more than 3GB (1GB reserved for OS kernel.) All processes together may require more RAM than is physically available. OS can map pages to the hard disk to handle that case.
Page Table Metadata Pages have permissions Read No execute (NX) A page fault interrupt is generated by kernel when Memory access attempted that would violate permissions. Page is marked as not valid (not mapped to a physical page.)
Page Faults 1. Process requests virtual address not in memory, causing a page fault. 2. Paging supervisor pages Process Blocks in physical memory read 0110101 Page fault, let me fix that. Paging supervisor out an old block of physical memory. old 3. Paging supervisor locates requested block on the disk and brings it into RAM memory. new External disk
Memory Layout of a Process
Input/Output The input/output devices of a computer include things like its keyboard, mouse, video display, and network card, as well as other more optional devices, like a scanner, Wi-Fi interface, video camera, USB ports, etc. Each such device is represented in an operating system using a device driver, which encapsulates the details of how interaction with that device should be done. The application programmer interface (API), which the device drivers present to application programs, allows those programs to interact with those devices at a fairly high level, while the operating system does the heavy lifting of performing the low-level interactions that make such devices actually work.
Filesystems A filesystem is an abstraction of how external storage of the computer is organized. An OS can support multiple filesystems. Examples: ext4fs, iso9660, YAFFS, etc. Operating systems typically organize files hierarchically into folders, also called directories. Each folder may contain files and/or subfolders. Thus, a filesystem consists of a collection of nested folders that form a tree. The topmost folder is the root of this tree and is also called the root folder.
File System Example
Virtual Machines Virtual machine: Software that emulates a computer system so that another OS can run on top of the existing OS. Benefits: Hardware Efficiency Portability Security Management Public domain image from http://commons.wikimedia.org/wiki/file:vmm-type2.jpg
Virtualization adds Hypervisor OS In a VM, apps run on guest OS. Guest OS runs on top of a hypervisor OS.
Each VM has own Guest OS Linux BSD W2k8 Virtual Machines Physical Machine
Hypervisor Security Vulnerability consequences Guest code execution with privilege VM Escape (Host code execution) Vendor CVEs KVM 32 QEMU 23 VirtualBox 9 VMware 126 Xen 86 Xen CVE-2008-1943 VBox CVE-2010-3583
Key Points 1. An OS is a layer btw applications and hardware 1. Manages users, processes, and hardware resources. 2. A process is a program in execution 1. PID identifies process. 2. fork() creates a copy of a process. 3. exec() runs a new program into address space. 3. A process runs until 1. Its time slice expires. 2. It requests OS help via a system call. 3. An interrupt occurs. 4. Each process has its own virtual address space 1. Setup by kernel created page table. 2. CPU translates virtual to physical addresses via table. 3. Page fault occurs when page is mapped to disk (or does not exist.)
References 1. Anderson, Security Engineering 2 nd Edition, Wiley, 2008. 2. Bishop, Computer Security: Art and Science, Addison-Wesley, 2002. 3. Goodrich and Tammasia, Introduction to Computer Security, Pearson, 2011. 4. Sudhakar Govindavajhala and Andrew W. Appel, Using Memory Errors to Attack a Virtual Machine, July 2003.
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