What is the difference between user and kernel modes in operating systems

Question

What are the differences between User Mode and Kernel Mode  why and how do you activate either of them  and what are their use cases

User · Answer

Other answers already explained the difference between user and kernel mode  If you really want to get into detail you should get a copy of  Windows Internals  an excellent book written by Mark Russinovich and David Solomon describing the architecture and inside details of the various Windows operating systems

User · Answer

CPU rings are the most clear distinction  In x86 protected mode  the CPU is always in one of 4 rings  The Linux kernel only uses 0 and 3    0 for kernel 3 for users   This is the most hard and fast definition of kernel vs userland   Why Linux does not use rings 1 and 2  CPU Privilege Rings  Why rings 1 and 2 aren  39 t used   How is the current ring determined   The current ring is selected by a combination of    global descriptor table  a in-memory table of GDT entries  and each entry has a field Privl which encodes the ring   The LGDT instruction sets the address to the current descriptor table   See also  http   wiki osdev org Global Descriptor Table the segment registers CS  DS  etc   which point to the index of an entry in the GDT   For example  CS   0 means the first entry of the GDT is currently active for the executing code      What can each ring do   The CPU chip is physically built so that    ring 0 can do anything ring 3 cannot run several instructions and write to several registers  most notably    cannot change its own ring  Otherwise  it could set itself to ring 0 and rings would be useless   In other words  cannot modify the current segment descriptor  which determines the current ring  cannot modify the page tables  How does x86 paging work   In other words  cannot modify the CR3 register  and paging itself prevents modification of the page tables   This prevents one process from seeing the memory of other processes for security   ease of programming reasons  cannot register interrupt handlers  Those are configured by writing to memory locations  which is also prevented by paging   Handlers run in ring 0  and would break the security model   In other words  cannot use the LGDT and LIDT instructions  cannot do IO instructions like in and out  and thus have arbitrary hardware accesses   Otherwise  for example  file permissions would be useless if any program could directly read from disk   More precisely thanks to Michael Petch  it is actually possible for the OS to allow IO instructions on ring 3  this is actually controlled by the Task state segment   What is not possible is for ring 3 to give itself permission to do so if it didn t have it in the first place   Linux always disallows it  See also  Why doesn  39 t Linux use the hardware context switch via the TSS     How do programs and operating systems transition between rings    when the CPU is turned on  it starts running the initial program in ring 0  well kind of  but it is a good approximation   You can think this initial program as being the kernel  but it is normally a bootloader that then calls the kernel still in ring 0    when a userland process wants the kernel to do something for it like write to a file  it uses an instruction that generates an interrupt such as int 0x80 or syscall to signal the kernel  x86-64 Linux syscall hello world example    data hello world       ascii  hello world n      hello world len     - hello world  text  global  start  start         write        mov  1   rax     mov  1   rdi     mov  hello world   rsi     mov  hello world len   rdx     syscall         exit        mov  60   rax     mov  0   rdi     syscall   compile and run   as -o hello world o hello world S ld -o hello world out hello world o   hello world out   GitHub upstream   When this happens  the CPU calls an interrupt callback handler which the kernel registered at boot time  Here is a concrete baremetal example that registers a handler and uses it   This handler runs in ring 0  which decides if the kernel will allow this action  do the action  and restart the userland program in ring 3  x86 64  when the exec system call is used  or when the kernel will start  init   the kernel prepares the registers and memory of the new userland process  then it jumps to the entry point and switches the CPU to ring 3 If the program tries to do something naughty like write to a forbidden register or memory address  because of paging   the CPU also calls some kernel callback handler in ring 0   But since the userland was naughty  the kernel might kill the process this time  or give it a warning with a signal  When the kernel boots  it setups a hardware clock with some fixed frequency  which generates interrupts periodically   This hardware clock generates interrupts that run ring 0  and allow it to schedule which userland processes to wake up   This way  scheduling can happen even if the processes are not making any system calls    What is the point of having multiple rings   There are two major advantages of separating kernel and userland    it is easier to make programs as you are more certain one won t interfere with the other  E g   one userland process does not have to worry about overwriting the memory of another program because of paging  nor about putting hardware in an invalid state for another process  it is more secure  E g  file permissions and memory separation could prevent a hacking app from reading your bank data  This supposes  of course  that you trust the kernel    How to play around with it   I ve created a bare metal setup that should be a good way to manipulate rings directly  https   github com cirosantilli x86-bare-metal-examples  I didn t have the patience to make a userland example unfortunately  but I did go as far as paging setup  so userland should be feasible  I d love to see a pull request   Alternatively  Linux kernel modules run in ring 0  so you can use them to try out privileged operations  e g  read the control registers  How to access the control registers cr0 cr2 cr3 from a program  Getting segmentation fault  Here is a convenient QEMU   Buildroot setup to try it out without killing your host   The downside of kernel modules is that other kthreads are running and could interfere with your experiments  But in theory you can take over all interrupt handlers with your kernel module and own the system  that would be an interesting project actually   Negative rings  While negative rings are not actually referenced in the Intel manual  there are actually CPU modes which have further capabilities than ring 0 itself  and so are a good fit for the  negative ring  name   One example is the hypervisor mode used in virtualization   For further details see    https   security stackexchange com questions 129098 what-is-protection-ring-1 https   security stackexchange com questions 216527 ring-3-exploits-and-existence-of-other-rings   ARM  In ARM  the rings are called Exception Levels instead  but the main ideas remain the same   There exist 4 exception levels in ARMv8  commonly used as    EL0  userland EL1  kernel   supervisor  in ARM terminology    Entered with the svc instruction  SuperVisor Call   previously known as swi before unified assembly  which is the instruction used to make Linux system calls  Hello world ARMv8 example   hello S   text  global  start  start         write        mov x0  1     ldr x1   msg     ldr x2   len     mov x8  64     svc 0         exit        mov x0  0     mov x8  93     svc 0 msg       ascii  hello syscall v8 n  len     - msg   GitHub upstream   Test it out with QEMU on Ubuntu 16 04   sudo apt-get install qemu-user gcc-arm-linux-gnueabihf arm-linux-gnueabihf-as -o hello o hello S arm-linux-gnueabihf-ld -o hello hello o qemu-arm hello   Here is a concrete baremetal example that registers an SVC handler and does an SVC call  EL2  hypervisors  for example Xen   Entered with the hvc instruction  HyperVisor Call    A hypervisor is to an OS  what an OS is to userland   For example  Xen allows you to run multiple OSes such as Linux or Windows on the same system at the same time  and it isolates the OSes from one another for security and ease of debug  just like Linux does for userland programs   Hypervisors are a key part of today s cloud infrastructure  they allow multiple servers to run on a single hardware  keeping hardware usage always close to 100  and saving a lot of money   AWS for example used Xen until 2017 when its move to KVM made the news  EL3  yet another level  TODO example   Entered with the smc instruction  Secure Mode Call    The ARMv8 Architecture Reference Model DDI 0487C a - Chapter D1 - The AArch64 System Level Programmer s Model - Figure D1-1 illustrates this beautifully     The ARM situation changed a bit with the advent of ARMv8 1 Virtualization Host Extensions  VHE   This extension allows the kernel to run in EL2 efficiently     VHE was created because in-Linux-kernel virtualization solutions such as KVM have gained ground over Xen  see e g  AWS  move to KVM mentioned above   because most clients only need Linux VMs  and as you can imagine  being all in a single project  KVM is simpler and potentially more efficient than Xen  So now the host Linux kernel acts as the hypervisor in those cases   Note how ARM  maybe due to the benefit of hindsight  has a better naming convention for the privilege levels than x86  without the need for negative levels  0 being the lower and 3 highest  Higher levels tend to be created more often than lower ones   The current EL can be queried with the MRS instruction  what is the current execution mode exception level  etc   ARM does not require all exception levels to be present to allow for implementations that don t need the feature to save chip area  ARMv8  Exception levels  says      An implementation might not include all of the Exception levels  All implementations must include EL0 and EL1    EL2 and EL3 are optional    QEMU for example defaults to EL1  but EL2 and EL3 can be enabled with command line options  qemu-system-aarch64 entering el1 when emulating a53 power up  Code snippets tested on Ubuntu 18 10

User · Answer

A processor in a computer running Windows has two different modes  user mode and kernel mode  The processor switches between the two modes depending on what type of code is running on the processor  Applications run in user mode  and core operating system components run in kernel mode  While many drivers run in kernel mode  some drivers may run in user mode   When you start a user-mode application  Windows creates a process for the application  The process provides the application with a private virtual address space and a private handle table  Because an application s virtual address space is private  one application cannot alter data that belongs to another application  Each application runs in isolation  and if an application crashes  the crash is limited to that one application  Other applications and the operating system are not affected by the crash   In addition to being private  the virtual address space of a user-mode application is limited  A processor running in user mode cannot access virtual addresses that are reserved for the operating system  Limiting the virtual address space of a user-mode application prevents the application from altering  and possibly damaging  critical operating system data   All code that runs in kernel mode shares a single virtual address space  This means that a kernel-mode driver is not isolated from other drivers and the operating system itself  If a kernel-mode driver accidentally writes to the wrong virtual address  data that belongs to the operating system or another driver could be compromised  If a kernel-mode driver crashes  the entire operating system crashes   If you are a Windows user once go through this link you will get more   Communication between user mode and kernel mode

User · Answer

Kernel Mode      In Kernel mode  the executing code has complete and unrestricted   access to the underlying hardware  It   can execute any CPU instruction and   reference any memory address  Kernel   mode is generally reserved for the   lowest-level  most trusted functions   of the operating system  Crashes in   kernel mode are catastrophic  they   will halt the entire PC    User Mode      In User mode  the executing code has no ability to directly access   hardware or reference memory  Code   running in user mode must delegate to   system APIs to access hardware or   memory  Due to the protection afforded   by this sort of isolation  crashes in   user mode are always recoverable  Most   of the code running on your computer   will execute in user mode       Read more   Understanding User and Kernel Mode

User · Answer

I m going to take a stab in the dark and guess you re talking about Windows   In a nutshell  kernel mode has full access to hardware  but user mode doesn t   For instance  many if not most device drivers are written in kernel mode because they need to control finer details of their hardware   See also this wikibook

User · Answer

These are two different modes in which your computer can operate  Prior to this  when computers were like a big room  if something crashes     it halts the whole computer  So computer architects decide to change it  Modern microprocessors implement in hardware at least 2 different states   User mode    mode where all user programs execute  It does not have access to RAM and hardware  The reason for this is because if all programs ran in kernel mode  they would be able to overwrite each other   s memory  If it needs to access any of these features     it makes a call to the underlying API   Each process started by windows except of system process runs in user mode    Kernel mode    mode where all kernel programs execute  different drivers   It has access to every resource and underlying hardware  Any CPU instruction can be executed and every memory address can be accessed  This mode is reserved for drivers which operate on the lowest level   How the switch occurs   The switch from user mode to kernel mode is not done automatically by CPU  CPU is interrupted by interrupts  timers  keyboard  I O   When interrupt occurs  CPU stops executing the current running program  switch to kernel mode  executes interrupt handler  This handler saves the state of CPU  performs its operations  restore the state and returns to user mode   http   en wikibooks org wiki Windows Programming User Mode vs Kernel Mode  http   tldp org HOWTO KernelAnalysis-HOWTO-3 html  http   en wikipedia org wiki Direct memory access  http   en wikipedia org wiki Interrupt request

User · Answer

What  Basically the difference between kernel and user modes is not OS dependent and is achieved only by restricting some instructions to be run only in kernel mode by means of hardware design  All other purposes like memory protection can be done only by that restriction    How  It means that the processor lives in either the kernel mode or in the user mode  Using some mechanisms the architecture can guarantee that whenever it is switched to the kernel mode the OS code is fetched to be run   Why  Having this hardware infrastructure these could be achieved in common OSes    Protecting user programs from accessing whole the memory  to not let programs overwrite the OS for example  preventing user programs from performing sensitive instructions such as those that change CPU memory pointer bounds  to not let programs break their memory bounds for example

[operating-system] What is the difference between user and kernel modes in operating systems?

Examples related to operating-system