Re: [GIT pull] x86 mpx support for 3.19

[Dave Hansen]

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
doorstop.

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):

	https://software.intel.com/en-us/forums/topic/402393

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
_