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
This commit is contained in:
Linus Torvalds 2014-12-19 13:22:42 -08:00
commit a54455766b
3 changed files with 41 additions and 9 deletions

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@ -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

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@ -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

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@ -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.