Re: [Security] [PATCH v3 0/4] x86: clear XD_DISABLED flag on Intel to regain NX

[Kees Cook <kees.cook@canonical.com>]

On Wed, Nov 10, 2010 at 12:21:53AM +0000, Alan Cox wrote:
> > Only Intel has this problem (it's the only CPU that defines this MSR), so
> > that'll reduce it. But Google is of no help to me on this; where can I find
> > these documents?
> 
> www.intel.com 8)

Gee, thanks. :P

I looks like there is nothing that actually lists errata directly, so
walking the entire site for lists of all processor types, e.g.:
http://www.intel.com/products/processors/previousgeneration/index.htm
http://www.intel.com/design/celeron/documentation.htm
and then checking spec updates is the only way to go.

> > > I'm not specifically aware of any but I do know for example that there
> > > are other CPU BIOS disablable features that some systems disable in the
> > > BIOS for good reason (an ancient example being the original Pentium
> > > REP MOVS optimisation on some steppings)
> > 
> > By definition there should be fewer errata CPUs than those than need their
> > BIOS ignored, so I still think this patch makes sense (especially since it
> > can trivially be worked around with the noexec=off boot option).
> 
> We need to be sure because the last thing you want is even 1% of your 10%
> of users to sudden get magical random memory corruption from overriding
> something wrongly !
> 
> It's definitely worth checking and worth doing because even if there is
> one (and I have no idea if there is) it'll tell you which steppings

I will report if I find anything bad.

-Kees

-- 
Kees Cook
Ubuntu Security Team