From 010e593b140decdc16bb0c1c754b07b1fccb6999 Mon Sep 17 00:00:00 2001 From: Dave Hansen Date: Fri, 12 Dec 2014 10:38:35 -0800 Subject: x86, mpx: Update documentation I was writing some MPX test programs and realized that the current design makes it tricky. I did something like: bndcfgu |= bnd_dir | BNDCFGU_ENABLE; xrstor(); printf("xrstor done"); // #BR bounds exception here prctl(MPX_ENABLE_MANAGEMENT); and then compiled the app with "-fcheck-pointer-bounds -mmpx" to enable MPX instrumentation. The problem is that there is MPX instrumentation inserted in to the area of the printf(). The kernel gets a bounds exception and since management isn't yet enabled, it SIGSEGV's. Add a bit to the documentation to explain a way around this and where apps need to be careful. Signed-off-by: Dave Hansen Cc: Dave Hansen Link: http://lkml.kernel.org/r/20141212183835.8C581B3E@viggo.jf.intel.com Signed-off-by: Thomas Gleixner --- Documentation/x86/intel_mpx.txt | 12 +++++++++--- 1 file changed, 9 insertions(+), 3 deletions(-) (limited to 'Documentation') diff --git a/Documentation/x86/intel_mpx.txt b/Documentation/x86/intel_mpx.txt index 4472ed2ad921..6ca6e2bd9ae9 100644 --- a/Documentation/x86/intel_mpx.txt +++ b/Documentation/x86/intel_mpx.txt @@ -30,9 +30,15 @@ is how we expect the compiler, application and kernel to work together. instrumentation as well as some setup code called early after the app starts. New instruction prefixes are noops for old CPUs. 2) That setup code allocates (virtual) space for the "bounds directory", - points the "bndcfgu" register to the directory and notifies the kernel - (via the new prctl(PR_MPX_ENABLE_MANAGEMENT)) that the app will be using - MPX. + points the "bndcfgu" register to the directory (must also set the valid + bit) and notifies the kernel (via the new prctl(PR_MPX_ENABLE_MANAGEMENT)) + that the app will be using MPX. The app must be careful not to access + the bounds tables between the time when it populates "bndcfgu" and + when it calls the prctl(). This might be hard to guarantee if the app + is compiled with MPX. You can add "__attribute__((bnd_legacy))" to + the function to disable MPX instrumentation to help guarantee this. + Also be careful not to call out to any other code which might be + MPX-instrumented. 3) The kernel detects that the CPU has MPX, allows the new prctl() to succeed, and notes the location of the bounds directory. Userspace is expected to keep the bounds directory at that locationWe note it -- cgit v1.2.3-59-g8ed1b From 72e9b5fe9bee0826e7ce7599adbdc64e544780ef Mon Sep 17 00:00:00 2001 From: Dave Hansen Date: Fri, 12 Dec 2014 10:38:36 -0800 Subject: x86, mpx: Give MPX a real config option prompt Give MPX a real config option. The CPUs that support it (referenced here): https://software.intel.com/en-us/forums/topic/402393 are not available publicly yet. Right now only the software emulator provides MPX for the general public. [ tglx: Make it default off. There is no point in having it on right now as no hardware and no proper tooling support are available ] Signed-off-by: Dave Hansen Cc: Dave Hansen Link: http://lkml.kernel.org/r/20141212183836.2569D58D@viggo.jf.intel.com Signed-off-by: Thomas Gleixner --- Documentation/x86/intel_mpx.txt | 6 +++++- arch/x86/Kconfig | 30 ++++++++++++++++++++++++++---- 2 files changed, 31 insertions(+), 5 deletions(-) (limited to 'Documentation') diff --git a/Documentation/x86/intel_mpx.txt b/Documentation/x86/intel_mpx.txt index 6ca6e2bd9ae9..818518a3ff01 100644 --- a/Documentation/x86/intel_mpx.txt +++ b/Documentation/x86/intel_mpx.txt @@ -7,11 +7,15 @@ that can be used in conjunction with compiler changes to check memory references, for those references whose compile-time normal intentions are usurped at runtime due to buffer overflow or underflow. +You can tell if your CPU supports MPX by looking in /proc/cpuinfo: + + cat /proc/cpuinfo | grep ' mpx ' + For more information, please refer to Intel(R) Architecture Instruction Set Extensions Programming Reference, Chapter 9: Intel(R) Memory Protection Extensions. -Note: Currently no hardware with MPX ISA is available but it is always +Note: As of December 2014, no hardware with MPX is available but it is possible to use SDE (Intel(R) Software Development Emulator) instead, which can be downloaded from http://software.intel.com/en-us/articles/intel-software-development-emulator diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig index 967dfe0ba85e..cc91e5e55d13 100644 --- a/arch/x86/Kconfig +++ b/arch/x86/Kconfig @@ -248,10 +248,6 @@ config HAVE_INTEL_TXT def_bool y depends on INTEL_IOMMU && ACPI -config X86_INTEL_MPX - def_bool y - depends on CPU_SUP_INTEL - config X86_32_SMP def_bool y depends on X86_32 && SMP @@ -1575,6 +1571,32 @@ config X86_SMAP If unsure, say Y. +config X86_INTEL_MPX + prompt "Intel MPX (Memory Protection Extensions)" + def_bool n + depends on CPU_SUP_INTEL + ---help--- + MPX provides hardware features that can be used in + conjunction with compiler-instrumented code to check + memory references. It is designed to detect buffer + overflow or underflow bugs. + + This option enables running applications which are + instrumented or otherwise use MPX. It does not use MPX + itself inside the kernel or to protect the kernel + against bad memory references. + + Enabling this option will make the kernel larger: + ~8k of kernel text and 36 bytes of data on a 64-bit + defconfig. It adds a long to the 'mm_struct' which + will increase the kernel memory overhead of each + process and adds some branches to paths used during + exec() and munmap(). + + For details, see Documentation/x86/intel_mpx.txt + + If unsure, say N. + config EFI bool "EFI runtime service support" depends on ACPI -- cgit v1.2.3-59-g8ed1b