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Date:	Tue, 10 Nov 2009 08:57:38 -0800
From:	Kees Cook <>
To:	"H. Peter Anvin" <>
Cc:	Arjan van de Ven <>,
	Thomas Gleixner <>,
	Ingo Molnar <>,,
	Pekka Enberg <>,
	Jan Beulich <>,
	Vegard Nossum <>,
	Yinghai Lu <>,
	Jeremy Fitzhardinge <>,
Subject: Re: [PATCH v4] [x86] detect and report lack of NX protections

On Tue, Nov 10, 2009 at 08:47:23AM -0800, H. Peter Anvin wrote:
> On 11/10/2009 07:49 AM, Kees Cook wrote:
> >>
> >> The second clause can only get executed if CONFIG_X86_PAE is unset,
> >> which in turn means _PAGE_NX == 0... so that piece of code is meaningless.
> > 
> > CONFIG_X86_PAE is unset for x86_64, where _PAGE_NX is valid.  (This was
> > the main situation I was trying to address.)  So that chunk runs for
> > non-pae 32bit, and all 64bit:
> > 
> In reality, X86_PAE really should have been defined for 64 bits, since
> 64 bits really is PAE in most aspects.
> Anyway, for the 64-bit case it looks like the proper place for any of
> this is in check_efer() just below, not in this null routine.

64-bit does not set nx_enabled to "1" by default anywhere.  And setting
the default to 1 in check_efer() seemed out of place to me.


Kees Cook
Ubuntu Security Team
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