Re: [GIT pull] x86 mpx support for 3.19

From: Dave Hansen
Date: Thu Dec 11 2014 - 17:04:55 EST

On 12/10/2014 10:19 PM, Ingo Molnar wrote:
> * Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> wrote:
>> @@ -1575,6 +1571,27 @@ config X86_SMAP
>> If unsure, say Y.
>> +config X86_INTEL_MPX
>> + prompt "Intel MPX (Memory Protection Extensions)" if EXPERT
> I think the 'if EXPERT' needs to be dropped.

Fine with me.

>> + def_bool y
>> + depends on CPU_SUP_INTEL
> On the one hand, the 'def_bool y' might be acceptable, if we
> think of MPX as X32 or SECCOMP: ABI extensions that are only
> really useful if all distros enable it.

It's a _bit_ different than X32 or SECCOMP, though. An x32 app is a
doorstop if the kernel's config option is off. An MPX-instrumented app
ends up looking like a dumb app running a bunch of noops (for the MPX
instructions and prefixes) and being mean to the icache, but it's not a

That said, we *really* want distros enabling it. (Putting my Intel hat
on firmly now...) MPX already requires recompiling binaries, which is a
fairly large burden. If an application writer goes to that trouble, we
really want to make sure that the kernel support is there.

It'd be a real shame to have an app compiled with MPX support (and all
those noops and their overhead), running on a CPU with silicon that
supports MPX, and a kernel that *can* support MPX... and then not use it.

>> + Enabling this option will make the kernel larger and
>> + slightly increase the size of some kernel data
>> + structures.
> And will add a few branches to critical code paths, right?

Yep, new patch attached.

From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>

Give MPX a real config option. The CPUs that support it
(referenced here):

are not available publicly, so we need to make it somewhat
easy to disable.

Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>

b/arch/x86/Kconfig | 28 ++++++++++++++++++++++++----
1 file changed, 24 insertions(+), 4 deletions(-)

diff -puN arch/x86/Kconfig~x86-mpx-real-config-option-v3 arch/x86/Kconfig
--- a/arch/x86/Kconfig~x86-mpx-real-config-option-v3 2014-12-11 07:37:05.147501580 -0800
+++ b/arch/x86/Kconfig 2014-12-11 07:38:55.864495123 -0800
@@ -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 +1571,30 @@ config X86_SMAP

If unsure, say Y.

+config X86_INTEL_MPX
+ prompt "Intel MPX (Memory Protection Extensions)"
+ def_bool y
+ 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().
+ If unsure, say Y.
config EFI
bool "EFI runtime service support"
depends on ACPI