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Commit a5445576 authored by Linus Torvalds's avatar Linus Torvalds
Browse files

Merge branch 'x86-mpx-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 

Pull x86 MPX fixes from Thomas Gleixner:
 "Three updates for the new MPX infrastructure:
   - Use the proper error check in the trap handler
   - Add a proper config option for it
   - Bring documentation up to date"

* 'x86-mpx-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  x86, mpx: Give MPX a real config option prompt
  x86, mpx: Update documentation
  x86_64/traps: Fix always true condition
parents 1092b596 72e9b5fe

Documentation/x86/intel_mpx.txt

+14 −4
Original line number Diff line number Diff line
@@ -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
@@ -30,9 +34,15 @@ 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

arch/x86/Kconfig

+26 −4
Original line number Diff line number Diff line
@@ -249,10 +249,6 @@ 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
@@ -1594,6 +1590,32 @@ 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

arch/x86/kernel/traps.c

+1 −1
Original line number Diff line number Diff line
@@ -331,7 +331,7 @@ dotraplinkage void do_bounds(struct pt_regs *regs, long error_code) 
		break; /* Success, it was handled */

	case 1: /* Bound violation. */

		info = mpx_generate_siginfo(regs, xsave_buf);

		if (PTR_ERR(info)) {

		if (IS_ERR(info)) {

			/*

			 * We failed to decode the MPX instruction.  Act as if

			 * the exception was not caused by MPX.