linux-kernel - Re: [PATCH v2] [x86] detect and report lack of NX protections

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

Hi,

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

-- 
Kees Cook
Ubuntu Security Team
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