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

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Message-ID: <4AF9E851.2000406@zytor.com>
Date:	Tue, 10 Nov 2009 14:25:21 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Kees Cook <kees.cook@...onical.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 11/10/2009 02:15 PM, Kees Cook wrote:
> On Tue, Nov 10, 2009 at 01:22:18PM -0800, H. Peter Anvin wrote:
>> Yes, it should be.  set_nx() and check_efer() are doing the same thing,
>> except in different ways, and they are - IMO - *both* doing something
>> dumb -- although check_efer() is saner.
> 
> BTW, it seems like set_nx() should move from init_memory_mapping to
> setup_arch, since init_memory_mapping can be called twice, but I don't
> think set_nx needs to be.
> 

We already have call points in setup_arch for check_efer(), which are
where this code needs to be called.

As I said, check_efer() and set_nx() are the 64- and 32-bit versions of
*exactly the same thing*.  Furthermore, it looks to me that set_nx() is
simply broken; it forces the bit set in EFER even though it is already
set in head_32.S.

Thus, what we *should* do is:

a) remove set_nx() completely;
b) enable check_efer() on 32 bits;
c) rename check_efer() to something sane, like x86_configure_nx();

Optionally we should just use cpu_has_nx instead of reading EFER.

	-hpa
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