linux-kernel - Re: [PATCH v4] [x86] detect and report lack of NX protections

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Date:	Tue, 10 Nov 2009 08:57:38 -0800
From:	Kees Cook <kees.cook@...onical.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Arjan van de Ven <arjan@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	Pekka Enberg <penberg@...helsinki.fi>,
	Jan Beulich <jbeulich@...ell.com>,
	Vegard Nossum <vegardno@....uio.no>,
	Yinghai Lu <yinghai@...nel.org>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	linux-kernel@...r.kernel.org
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

-- 
Kees Cook
Ubuntu Security Team
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