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W^X ("Write XOR Execute"; spoken as W xor X) is the name of a security feature in operating systems and virtual machines. It is a memory protection policy whereby every page in a process's address space may be either writable or executable, but not both. The name comes from the XOR Boolean operator which puts out 'true' only when either one or the other operand is true, but not both. Without such protection, a program can write (as data) CPU instructions in an area of memory intended for data and then arrange to run (as executable) those instructions. This can be dangerous if the writer of the memory is malicious.
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 Intel i386, 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 library runtime functions) and data.
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.
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 OpenBSD kernel itself on the AMD64 architecture, with Theo de Raadt aiding the effort.
- OpenBSD 3.3 release notes
- "i386 W^X". 2003-04-17. Retrieved 19 June 2014.
- "W^X protection for the AMD64 kernel".
- "W^X JIT-code enabled in Firefox". Retrieved 2016-04-29.
- OpenBSD-3.3 announcement, public release of W^X
- Slides from a presentation by OpenBSD lead developer Theo de Raadt covering W^X
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