Data Execution Prevention

In computer security, executable-space protection marks memory regions as non-executable, such that an attempt to execute machine code in these regions will cause an exception. It makes use of hardware features such as the NX bit (no-execute bit), or in some cases software emulation of those features. However, technologies that emulate or supply an NX bit will usually impose a measurable overhead while using a hardware-supplied NX bit imposes no measurable overhead.

The Burroughs 5000 offered hardware support for executable-space protection on its introduction in 1961; that capability remained in its successors until at least 2006. In its implementation of tagged architecture, each word of memory had an associated, hidden tag bit designating it code or data. Thus user programs cannot write or even read a program word, and data words cannot be executed.

If an operating system can mark some or all writable regions of memory as non-executable, it may be able to prevent the stack and heap memory areas from being executable. This helps to prevent certain buffer overflow exploits from succeeding, particularly those that inject and execute code, such as the Sasser and Blaster worms. These attacks rely on some part of memory, usually the stack, being both writeable and executable; if it is not, the attack fails.

OS implementations

Many operating systems implement or have an available executable space protection policy. Here is a list of such systems in alphabetical order, each with technologies ordered from newest to oldest.

For some technologies, there is a summary which gives the major features each technology supports. The summary is structured as below.

* Hardware Supported Processors: (Comma separated list of CPU architectures)
* Emulation: (No) or (Architecture Independent) or (Comma separated list of CPU architectures)
* Other Supported: (None) or (Comma separated list of CPU architectures)
* Standard Distribution: (No) or (Yes) or (Comma separated list of distributions or versions which support the technology)
* Release Date: (Date of first release)

A technology supplying Architecture Independent emulation will be functional on all processors which aren't hardware supported. The "Other Supported" line is for processors which allow some grey-area method, where an explicit NX bit doesn't exist yet hardware allows one to be emulated in some way.

Android

As of Android 2.3 and later, architectures which support it have non-executable pages by default, including non-executable stack and heap.

FreeBSD

Initial support for the NX bit, on x86-64 and IA-32 processors that support it, first appeared in FreeBSD -CURRENT on June 8, 2004. It has been in FreeBSD releases since the 5.3 release.

Linux

The Linux kernel supports the NX bit on x86-64 and IA-32 processors that support it, such as modern 64-bit processors made by AMD, Intel, Transmeta and VIA. The support for this feature in the 64-bit mode on x86-64 CPUs was added in 2004 by Andi Kleen, and later the same year, Ingo Molnar added support for it in 32-bit mode on 64-bit CPUs. These features have been part of the Linux kernel mainline since the release of kernel version 2.6.8 in August 2004.

The availability of the NX bit on 32-bit x86 kernels, which may run on both 32-bit x86 CPUs and 64-bit IA-32-compatible CPUs, is significant because a 32-bit x86 kernel would not normally expect the NX bit that an AMD64 or IA-64 supplies; the NX enabler patch assures that these kernels will attempt to use the NX bit if present.

Some desktop Linux distributions, such as Fedora, Ubuntu and openSUSE, do not enable the HIGHMEM64 option by default in their default kernels, which is required to gain access to the NX bit in 32-bit mode, because the PAE mode that is required to use the NX bit causes boot failures on pre-Pentium Pro (including Pentium MMX) and Celeron M and Pentium M processors without NX support. Other processors that do not support PAE are AMD K6 and earlier, Transmeta Crusoe, VIA C3 and earlier, and Geode GX and LX. VMware Workstation versions older than 4.0, Parallels Workstation versions older than 4.0, and Microsoft Virtual PC and Virtual Server do not support PAE on the guest. Fedora Core 6 and Ubuntu 9.10 and later provide a kernel-PAE package which supports PAE and NX.

NX memory protection has always been available in Ubuntu for any systems that had the hardware to support it and ran the 64-bit kernel or the 32-bit server kernel. The 32-bit PAE desktop kernel (linux-image-generic-pae) in Ubuntu 9.10 and later, also provides the PAE mode needed for hardware with the NX CPU feature. For systems that lack NX hardware, the 32-bit kernels now provide an approximation of the NX CPU feature via software emulation that can help block many exploits an attacker might run from stack or heap memory.

Non-execute functionality has also been present for other non-x86 processors supporting this functionality for many releases.

Exec Shield

Red Hat kernel developer Ingo Molnar released a Linux kernel patch named Exec Shield to approximate and utilize NX functionality on 32-bit x86 CPUs. The Exec Shield patch was released to the Linux kernel mailing list on May 2, 2003, but was rejected for merging with the base kernel because it involved some intrusive changes to core code in order to handle the complex parts of the emulation. Exec Shield's legacy CPU support approximates NX emulation by tracking the upper code segment limit. This imposes only a few cycles of overhead during context switches, which is for all intents and purposes immeasurable. For legacy CPUs without an NX bit, Exec Shield fails to protect pages below the code segment limit; an mprotect() call to mark higher memory, such as the stack, executable will mark all memory below that limit executable as well. Thus, in these situations, Exec Shield's schemes fails. This is the cost of Exec Shield's low overhead. Exec Shield checks for two ELF header markings, which dictate whether the stack or heap needs to be executable. These are called PT_GNU_STACK and PT_GNU_HEAP respectively. Exec Shield allows these controls to be set for both binary executables and for libraries; if an executable loads a library requiring a given restriction relaxed, the executable will inherit that marking and have that restriction relaxed.

* Hardware Supported Processors: All that Linux supports NX on
* Emulation: NX approximation using the code segment limit on IA-32 (x86) and compatible
* Other Supported: None
* Standard Distribution: Fedora Core and Red Hat Enterprise Linux
* Release Date: May 2, 2003

PaX

The PaX NX technology can emulate NX functionality, or use a hardware NX bit. PaX works on x86 CPUs that do not have the NX bit, such as 32-bit x86. The Linux kernel still does not ship with PaX (as of May, 2007); the patch must be merged manually.

PaX provides two methods of NX bit emulation, called SEGMEXEC and PAGEEXEC. The SEGMEXEC method imposes a measurable but low overhead, typically less than 1%, which is a constant scalar incurred due to the virtual memory mirroring used for the separation between execution and data accesses. SEGMEXEC also has the effect of halving the task's virtual address space, allowing the task to access less memory than it normally could. This is not a problem until the task requires access to more than half the normal address space, which is rare. SEGMEXEC does not cause programs to use more system memory (i.e. RAM), it only restricts how much they can access. On 32-bit CPUs, this becomes 1.5 GB rather than 3 GB.

PaX supplies a method similar to Exec Shield's approximation in the PAGEEXEC as a speedup; however, when higher memory is marked executable, this method loses its protections. In these cases, PaX falls back to the older, variable-overhead method used by PAGEEXEC to protect pages below the CS limit, which may become quite a high-overhead operation in certain memory access patterns. When the PAGEEXEC method is used on a CPU supplying a hardware NX bit, the hardware NX bit is used, thus no significant overhead is incurred.

PaX supplies mprotect() restrictions to prevent programs from marking memory in ways that produce memory useful for a potential exploit. This policy causes certain applications to cease to function, but it can be disabled for affected programs.

PaX allows individual control over the following functions of the technology for each binary executable:
*PAGEEXEC
*SEGMEXEC
* mprotect() restrictions
* Trampoline emulation
* Randomized executable base
* Randomized mmap() base

PaX ignores both PT_GNU_STACK and PT_GNU_HEAP. In the past, PaX had a configuration option to honor these settings but that option has been removed for security reasons, as it was deemed not useful. The same results of PT_GNU_STACK can normally be attained by disabling mprotect() restrictions, as the program will normally mprotect() the stack on load. This may not always be true; for situations where this fails, simply disabling both PAGEEXEC and SEGMEXEC will effectively remove all executable space restrictions, giving the task the same protections on its executable space as a non-PaX system.

* Hardware Supported Processors: Alpha, AMD64, IA-64, MIPS (32 and 64 bit), PA-RISC, PowerPC, SPARC
* Emulation: IA-32 (x86)
* Other Supported: PowerPC (32 and 64 bit), SPARC (32 and 64 bit)
* Standard Distribution: Alpine Linux
* Release Date: October 1, 2000

macOS

macOS for Intel supports the NX bit on all CPUs supported by Apple (from Mac OS X 10.4.4 – the first Intel release – onwards). Mac OS X 10.4 only supported NX stack protection. In Mac OS X 10.5, all 64-bit executables have NX stack and heap; W^X protection. This includes x86-64 (Core 2 or later) and 64-bit PowerPC on the G5 Macs.

NetBSD

As of NetBSD 2.0 and later (December 9, 2004), architectures which support it have non-executable stack and heap.

Architectures that have per-page granularity consist of: alpha, amd64, hppa, i386

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