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Date:	Mon, 19 Oct 2009 21:44:45 -0700
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 v2] [x86] detect and report lack of NX protections


On Tue, Oct 20, 2009 at 11:18:43AM +0900, H. Peter Anvin wrote:
> On 10/20/2009 11:04 AM, Kees Cook wrote:
> >It is possible for x86_64 systems to lack the NX bit (see check_efer())
> >either due to the hardware lacking support or the BIOS having turned
> >off the CPU capability, so NX status should be reported.  Additionally,
> >anyone booting NX-capable CPUs in 32bit mode without PAE will lack NX
> >functionality, so this change provides feedback for that case as well.
> >
> >v2: use "Alert:" instead of "Warning:" to avoid confusiong with WARN_ON()
> >
> They're both wrong.  Both imply that the user needs to take an
> action, which is wrong because the kernel is working as intended.
> If you need to use any kind of alert word, it should be something
> like "Notice:", and it should be KERN_NOTICE or even KERN_INFO.

In the case of a system where the BIOS was shipped with XD not enabled,
the user needs to take an action.  I'm okay with switching to Notice:, but
I don't think KERN_INFO is right.  I would agree, "Alert:" would seem to
be a KERN_ALERT, which is above KERN_CRIT, which this is clearly not.
"Notice" it is.


Kees Cook
Ubuntu Security Team
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