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

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

Hi Alan,

On Tue, Nov 09, 2010 at 04:43:47PM -0800, Kees Cook wrote:
> 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 will report if I find anything bad.

I have reviewed all the spec updates for all the processors I could find
(for a total of 45 PDFs). The only mention of XD is for the Dual-Core Xeon
5100[1]:

AG29. #GP Fault is Not Generated on Writing IA32_MISC_ENABLE [34]
      When Execute Disable (XD) is Not Supported
Problem: A #GP fault is not generated on writing to IA32_MISC_ENABLE [34]
         bit in a processor which does not support Execute Disable (XD)
         functionality.
Implication: Writing to IA32_MISC_ENABLE [34] bit is silently ignored
             without generating a fault.
Workaround: None identified.
Status: For the steppings affected, see the Summary Tables of Changes.

But this case is already handled (and doesn't matter) because
my patch already avoids this (we only clear IA32_MISC_ENABLE, not set it).

So, based on that, there doesn't seem to be an errata related to _needing_
to disable the XD cpu feature.

-Kees

[1] ftp://download.intel.com/design/Xeon/specupdt/313356.pdf

-- 
Kees Cook
Ubuntu Security Team