W^X ("Write XOR Execute"; spoken as double-u ex-or ex) is the name of a security feature present in the OpenBSDoperating system. It is a memory protection policy whereby every page in a process' address space is either writable or executable, but not both simultaneously. The name comes from the XORBoolean operator which outputs 'true' only when either one or the other operand is true, but not both. W^X does not prevent these permissions from being requested by applications, write and executable permissions simply are not used as a matter of policy. The OpenBSD base system has been modified to comply with it. This alleviates some buffer overflow attacks, including the most common stack-based attack: by ensuring that the stack is not executable, arbitrary code injected into it will not execute but instead cause the program to terminate. W^X first appeared in OpenBSD 3.3, released May 2003. Similar features are available for other operating systems, including the PaX and Exec Shield patches for Linux, and NetBSD 4+'s implementation of PaX.
W^X is relatively simple on processors which support fine-grained page permissions, such as Sun's SPARC and SPARC64, AMD's AMD64, Hewlett-Packard's PA-RISC, and HP's (originally Digital Equipment Corporation's) Alpha; some early Intel 64 processors lacked the NX bit required for W^X, but this appeared in later chips. On processors with more limited features, such as the Inteli386, W^X requires using the CS code segment limit as a "line in the sand", a point in the address space above which execution is not permitted and data is located, and below which it is allowed and executable pages are placed. On all platforms, linker changes were required to separate code (such as trampolines and other code needed for linker and libraryruntime functions) and data.
Although this feature has only protected userland programs for most of its existence, in late 2014 and early 2015, Mike Larkin made W^X protect the kernel itself on the AMD64 architecture, with Theo de Raadt aiding the effort.