Intel MPX (Memory Protection Extensions) was a set of extensions to the

x86 x86 (also known as 80x86 or the 8086 family) is a family of complex instruction set computer (CISC) instruction set architectures initially developed by Intel based on the Intel 8086 microprocessor and its 8088 variant. The 8086 was intr ...

instruction set architecture. With

compiler In computing, a compiler is a computer program that translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primarily used for programs tha ...

, runtime library and

operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may also i ...

support, Intel MPX claimed to enhance security to

software Software is a set of computer programs and associated software documentation, documentation and data (computing), data. This is in contrast to Computer hardware, hardware, from which the system is built and which actually performs the work. ...

by checking pointer references whose normal compile-time intentions are maliciously exploited at runtime due to

buffer overflow In information security and programming, a buffer overflow, or buffer overrun, is an anomaly whereby a program, while writing data to a buffer, overruns the buffer's boundary and overwrites adjacent memory locations. Buffers are areas of memo ...

s. In practice, there have been too many flaws discovered in the design for it to be useful, and support has been deprecated or removed from most compilers and operating systems. Intel has listed MPX as removed in 2019 and onward hardware in section 2.5 of its Intel® 64 and IA-32 Architectures Software Developer's Manual Volume 1.

Extensions

Intel MPX introduces new bounds registers, and new instruction set extensions that operate on these registers. Additionally, there is a new set of "bound tables" that store bounds beyond what can fit in the bounds registers. MPX uses four new 128-bit bounds registers, BND0 to BND3, each storing a pair of 64-bit lower bound (LB) and upper bound (UB) values of a buffer. The upper bound is stored in

ones' complement The ones' complement of a binary number is the value obtained by inverting all the bits in the binary representation of the number (swapping 0s and 1s). The name "ones' complement" (''note this is possessive of the plural "ones", not of a sin ...

form, with BNDMK (create bounds) and BNDCU (check upper bound) performing the conversion. The architecture includes two configuration registers BNDCFGx (BNDCFGU in user space and BNDCFGS in kernel mode), and a status register BNDSTATUS, which provides a memory address and error code in case of an exception. Two-level address translation is used for storing bounds in memory. The top layer consists of a Bounds Directory (BD) created on the application startup. Each BD entry is either empty or contains a pointer to a dynamically created Bounds Table (BT), which in turn contains a set of pointer bounds along with the linear addresses of the pointers. The bounds load (BNDLDX) and store (BNDSTX) instructions transparently perform the address translation and access bounds in the proper BT entry. Intel MPX was introduced as part of the Skylake microarchitecture. Intel

Goldmont Goldmont is a microarchitecture for low-power Atom, Celeron and Pentium branded processors used in systems on a chip (SoCs) made by Intel. They allow only one thread per core. The ''Apollo Lake'' platform with 14 nm Goldmont core was un ...

microarchitecture also supports Intel MPX.

Software support

* glibc removed support in version 2.35. *

GNU Compiler Collection The GNU Compiler Collection (GCC) is an optimizing compiler produced by the GNU Project supporting various programming languages, hardware architectures and operating systems. The Free Software Foundation (FSF) distributes GCC as free softwar ...

(GCC) 5.0 added support for MPX. In 2018, support for these extensions waned due to maintenance burdens and

Intel Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California. It is the world's largest semiconductor chip manufacturer by revenue, and is one of the developers of the x86 seri ...

developers intermittently contributing patches, resulting in a proposal to drop support in GCC 9.0. Support was removed in GCC 9.1. * Intel C++ Compiler (icc) 15.0 added support for Intel MPX. *

Kernel Kernel may refer to: Computing * Kernel (operating system), the central component of most operating systems * Kernel (image processing), a matrix used for image convolution * Compute kernel, in GPGPU programming * Kernel method, in machine learn ...

-level software support for Intel MPX was merged into the

Linux kernel mainline The Linux kernel is a free and open-source, monolithic, modular, multitasking, Unix-like operating system kernel. It was originally authored in 1991 by Linus Torvalds for his i386-based PC, and it was soon adopted as the kernel for the GNU o ...

in kernel version 3.19, which was released on February 8, 2015. In 2018, Thomas Gleixner proposed removing MPX support from Linux kernel 4.18. The pull request with its removal was posted in December 2018, during 4.20 development cycle, but wasn't accepted. The second attempt was made in July 2019. MPX support was removed in 5.6. * QEMU supported MPX since version 2.6 and dropped its support in 4.0 release. *

Microsoft Visual Studio 2015 Visual Studio is an integrated development environment (IDE) from Microsoft. It is used to develop computer programs including websites, web apps, web services and mobile apps. Visual Studio uses Microsoft software development platforms such a ...

Update 1 added experimental support for MPX.

Analysis of Intel MPX

A study examined a detailed cross-layer dissection of the MPX system stack and comparison with three prominent software-based memory protection mechanisms (

AddressSanitizer AddressSanitizer (or ASan) is an open source programming tool that detects memory corruption bugs such as buffer overflows or accesses to a dangling pointer (use-after-free). AddressSanitizer is based on compiler instrumentation and directly ma ...

, SAFECode, and SoftBound) and presents the following conclusions. * Even though Intel MPX is a specially designed hardware-assisted approach with its own added set of hardware registers, it is not faster than any of the software-based approaches. New Intel MPX instructions can cause up to 4× slowdown in the worst case, although compiler optimizations amortize it and lead to runtime overheads of ~50% on average. * In contrast to the other software-based solutions, Intel MPX provides no protection against temporal memory safety errors. * Intel MPX does not support multithreading inherently, which can lead to unsafe data races in legacy threaded programs and if compilers do not synchronize bounds explicitly. * Intel MPX does not support several common C/

C++ C++ (pronounced "C plus plus") is a high-level general-purpose programming language created by Danish computer scientist Bjarne Stroustrup as an extension of the C programming language, or "C with Classes". The language has expanded significan ...

programming idioms due to restrictions on the allowed memory layout. * Intel MPX is conflicting with some other ISA extensions resulting in performance and security issues. More specifically, these issues arise when Intel MPX is used in combination with other hardware based protection mechanisms such as

Intel TSX Transactional Synchronization Extensions (TSX), also called Transactional Synchronization Extensions New Instructions (TSX-NI), is an extension to the x86 instruction set architecture (ISA) that adds hardware transactional memory support, speedin ...

and Intel SGX. * Lastly, MPX instructions incur significant performance penalty (15+%) even on Intel CPUs without MPX support. In addition, a review concluded MPX was not production ready, and

was a better option. A review by Kostya Serebryany at Google, AddressSanitizer's developer, had similar findings.

Meltdown

Another study exploring the scope of Spectre and Meltdown security vulnerabilities discovered that Meltdown can be used to bypass Intel MPX, using the Bound Range Exceeded (#BR) hardware exception. According to their publication, the researchers were able to leak information through a Flush+Reload covert channel from an out-of-bound access on an array safeguarded by the MPX system. Their Proof Of Concept has not been publicly disclosed.

References

External links