User space and kernel space
Way of using computer memory
A modern computer operating system usually uses virtual memory to provide separate address spaces or regions of a single address space, called user space and kernel space. This separation primarily provides memory protection and hardware protection from malicious or errant software behaviour.
Kernel space is strictly reserved for running a privileged operating system kernel, kernel extensions, and most device drivers. In contrast, user space is the memory area where application software, daemons, and some drivers execute, typically with one address space per process.
Exploits targeting the user space are colloquially referred to as "userland exploits".
01Overview
The term user space (or userland) refers to all code that runs outside the operating system's kernel. User space usually refers to the various programs and libraries that the operating system uses to interact with the kernel: software that performs input/output, manipulates file system objects, application software, etc.
Each user space process usually runs in its own virtual memory space, and, unless explicitly allowed, cannot access the memory of other processes. This is the basis for memory protection in today's mainstream operating systems, and a building block for privilege separation. A separate user mode can also be used to build efficient virtual machines, see Popek and Goldberg's virtualization requirements. With enough privileges, processes can request the kernel to map part of another process's memory space to their own, as is the case for debuggers. Programs can also request shared memory regions with other processes, although other techniques are also available to allow inter-process communication.
| User mode | User applications | bash, LibreOffice, GIMP, Blender, 0 A.D., Mozilla Firefox, ... | ||||
|---|---|---|---|---|---|---|
| System components | init daemon: OpenRC, runit, systemd, ... |
System daemons: polkitd, smbd, sshd, udevd, ... |
Windowing system: X11, Wayland, SurfaceFlinger (Android) |
Graphics: Mesa, AMD Software, ... |
Other libraries: GTK, Qt, EFL, SDL, SFML, FLTK, GNUstep, ... | |
| C standard library | fopen, execv, malloc, memcpy, localtime, pthread_create, ... (up to 2000 subroutines) glibc aims to be fast, musl aims to be lightweight, uClibc targets embedded systems, bionic was written for Android, etc. All aim to be POSIX/SUS-compatible. | |||||
| Kernel mode | Linux kernel | stat, splice, dup, read, open, ioctl, write, mmap, close, exit, etc. (about 380 system calls) The Linux kernel System Call Interface (SCI), aims to be POSIX/SUS-compatible | ||||
| Process scheduling subsystem | IPC subsystem | Memory management subsystem | Virtual files subsystem | Networking subsystem | ||
| Other components: ALSA, DRI, evdev, klibc, LVM, device mapper, Linux Network Scheduler, Netfilter Linux Security Modules: SELinux, TOMOYO, AppArmor, Smack | ||||||
| Hardware (CPU, main memory, data storage devices, etc.) | ||||||
02Implementation
The most common way of implementing a user mode separate from kernel mode involves operating system protection rings. Protection rings, in turn, are implemented using CPU modes. Typically, kernel space programs run in kernel mode, also called supervisor mode; standard applications in user space run in user mode.
Some operating systems are single address space operating systems, with a single address space for all user-mode code. (The kernel-mode code may be in the same address space, or it may be in a second address space). Other operating systems have per-process address spaces, with a separate address space for each user-mode process.
Sources and credits
This article is adapted from the Wikipedia article “User space and kernel space”, written by its contributors and licensed under CC BY-SA 4.0. Fathomly has changed the layout, removed citation markers, navigation and maintenance notices, and adjusted punctuation. This adapted version is shared under the same license. For references, see the original article.
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