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

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Date:	Tue, 10 Nov 2009 07:49:56 -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 Mon, Nov 09, 2009 at 03:16:16PM -0800, H. Peter Anvin wrote:
> On 11/09/2009 02:10 PM, Kees Cook wrote:
> > diff --git a/arch/x86/mm/setup_nx.c b/arch/x86/mm/setup_nx.c
> > index 513d8ed..1b93231 100644
> > --- a/arch/x86/mm/setup_nx.c
> > +++ b/arch/x86/mm/setup_nx.c
> > @@ -53,6 +53,9 @@ void __init set_nx(void)
> >  #else
> >  void set_nx(void)
> >  {
> > +	/* notice if _PAGE_NX exists and was removed during check_efer() */
> > +	if (_PAGE_NX && ((__supported_pte_mask & _PAGE_NX) == _PAGE_NX))
> > +		nx_enabled = 1;
> >  }
> >  #endif
> >  
> 
> 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:

config X86_PAE
        bool "PAE (Physical Address Extension) Support"
        depends on X86_32 && !HIGHMEM4G

> It also looks to me that there is no message distinguishing the case
> when nx_enabled == 1 but disable_nx == 1, and instead we say NX is
> "active" when in fact it is disabled in the kernel.

That's true -- I had overlooked that part.  New patch on the way...

-Kees

-- 
Kees Cook
Ubuntu Security Team
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