Fwd: [PATCH v2 3/4] x86, head_32/64.S: Enable SMEP

[Kostik Belousov]

On Wed, May 18, 2011 at 10:50:30AM -0400, John Baldwin wrote:
> On Wednesday, May 18, 2011 8:31:15 am Oliver Pinter wrote:
> > On 5/18/11, Kostik Belousov <kostikbel at gmail.com> wrote:
> > > On Wed, May 18, 2011 at 02:03:07AM +0200, Oliver Pinter wrote:
> > >> ---------- Forwarded message ----------
> > >> From: Fenghua Yu <fenghua.yu at intel.com>
> > >> Date: Mon, 16 May 2011 14:34:44 -0700
> > >> Subject: [PATCH v2 3/4] x86, head_32/64.S: Enable SMEP
> > >> To: Ingo Molnar <mingo at elte.hu>, Thomas Gleixner <tglx at linutronix.de>,
> > >> H Peter Anvin <hpa at zytor.com>, Asit K Mallick
> > >> <asit.k.mallick at intel.com>, Linus Torvalds
> > >> <torvalds at linux-foundation.org>, Avi Kivity <avi at redhat.com>, Arjan
> > >> van de Ven <arjan at infradead.org>, Andrew Morton
> > >> <akpm at linux-foundation.org>, Andi Kleen <andi at firstfloor.org>
> > >> Cc: linux-kernel <linux-kernel at vger.kernel.org>, Fenghua Yu
> > >> <fenghua.yu at intel.com>
> > >>
> > >> From: Fenghua Yu <fenghua.yu at intel.com>
> > >>
> > >> Enable newly documented SMEP (Supervisor Mode Execution Protection) CPU
> > >> feature in kernel.
> > >>
> > >> SMEP prevents the CPU in kernel-mode to jump to an executable page that
> > >> does
> > >> not have the kernel/system flag set in the pte. This prevents the kernel
> > >> from executing user-space code accidentally or maliciously, so it for
> > >> example
> > >> prevents kernel exploits from jumping to specially prepared user-mode
> > >> shell
> > >> code. The violation will cause page fault #PF and will have error code
> > >> identical to XD violation.
> > >>
> > >> CR4.SMEP (bit 20) is 0 at power-on. If the feature is supported by CPU
> > >> (X86_FEATURE_SMEP), enable SMEP by setting CR4.SMEP. New kernel
> > >> option nosmep disables the feature even if the feature is supported by
> > >> CPU.
> > >>
> > >> Signed-off-by: Fenghua Yu <fenghua.yu at intel.com>
> > >
> > > So, where is the mentioned documentation for SMEP ? Rev. 38 of the
> > > Intel(R) 64 and IA-32 Architectures Software Developer's Manual does
> > > not contain the description, at least at the places where I looked and
> > > expected to find it.
> > 
> > http://www.intel.com/Assets/PDF/manual/325384.pdf
> > 
> > Intel? 64 and IA-32 Architectures Software Developer?s Manual
> >                    Volume 3 (3A & 3B):
> >              System Programming Guide
> 
> Which revision?  It is not documented in revision 38 from April 2011.
> 
> I just downloaded that link, and it is still revision 38 and has no mention 
> 'SMEP'.  Also, bit 20 of CR4 is still marked as Reserved in that manual 
> (section 2.5).
This is exactly what I said about rev. 38 in my original reply.
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