From: Kees Cook <kees.cook@canonical.com>
To: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: x86@kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [Security] [PATCH v3 0/4] x86: clear XD_DISABLED flag on Intel to regain NX
Date: Tue, 9 Nov 2010 15:53:14 -0800 [thread overview]
Message-ID: <20101109235314.GD5876@outflux.net> (raw)
In-Reply-To: <20101109225000.1b90e720@lxorguk.ukuu.org.uk>
Hi Alan,
On Tue, Nov 09, 2010 at 10:50:00PM +0000, Alan Cox wrote:
> On Tue, 9 Nov 2010 10:56:04 -0800
> Kees Cook <kees.cook@canonical.com> wrote:
>
> > On Tue, Nov 09, 2010 at 06:31:42PM +0000, Alan Cox wrote:
> > > > In a review[1] of reported hardware being used by Ubuntu bug reporters,
> > > > almost 10% of systems had an incorrectly configured BIOS, leaving their
> > > > systems unable to use the NX features of their CPU.
> > >
> > > Ouch
> >
> > Yeah :(
> >
> > > > This change will clear the MSR_IA32_MISC_ENABLE_XD_DISABLE bit so that NX
> > > > cannot be inappropriately controlled by the BIOS on Intel CPUs. If, under
> > > > very strange hardware configurations, NX actually needs to be disabled,
> > > > "noexec=off" can be used to restore the prior behavior.
> > >
> > > Have you done an audit of CPU errata to ensure that none of these cases
> > > are ones where the BIOS has disabled it to avoid an erratum. I'd hate to
> > > turn it on and find out the BIOS actually turned it off for good reason !
> >
> > Where can I find those details?
>
> In the errata manuals for each processor released by the relevant
> vendors. As they are aggregated into families and they have an index in
> the front it shouldn't take too long to check the ones your 10% scan
> found.
Only Intel has this problem (it's the only CPU that defines this MSR), so
that'll reduce it. But Google is of no help to me on this; where can I find
these documents?
> I'm not specifically aware of any but I do know for example that there
> are other CPU BIOS disablable features that some systems disable in the
> BIOS for good reason (an ancient example being the original Pentium
> REP MOVS optimisation on some steppings)
By definition there should be fewer errata CPUs than those than need their
BIOS ignored, so I still think this patch makes sense (especially since it
can trivially be worked around with the noexec=off boot option).
-Kees
--
Kees Cook
Ubuntu Security Team
next prev parent reply other threads:[~2010-11-09 23:53 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-11-09 18:11 [Security] [PATCH v3 0/4] x86: clear XD_DISABLED flag on Intel to regain NX Kees Cook
2010-11-09 18:14 ` [PATCH 1/4] x86: rename verify_cpu_64.S to verify_cpu.S Kees Cook
2010-11-09 18:46 ` Pekka Enberg
2010-11-09 19:00 ` Kees Cook
2010-11-09 19:59 ` Pekka Enberg
2010-11-09 19:02 ` Kees Cook
2010-11-09 19:11 ` Pekka Enberg
2010-11-09 18:15 ` [PATCH 2/4] x86: clear XD_DISABLED flag on Intel to regain NX Kees Cook
2010-11-10 16:11 ` Andi Kleen
2010-11-10 16:47 ` Kees Cook
2010-11-10 17:42 ` Andi Kleen
2010-11-10 18:15 ` Kees Cook
2010-11-09 18:15 ` [PATCH 3/4] x86: call verify_cpu during 32bit CPU startup Kees Cook
2010-11-09 19:09 ` Pekka Enberg
2010-11-09 19:19 ` Kees Cook
2010-11-09 19:46 ` Pekka Enberg
2010-11-09 19:56 ` Kees Cook
2010-11-09 20:28 ` Pekka Enberg
2010-11-09 20:48 ` Kees Cook
2010-11-09 20:50 ` Pekka Enberg
2010-11-09 18:15 ` [PATCH 4/4] x86: only CPU features determine NX capabilities Kees Cook
2010-11-09 18:31 ` [Security] [PATCH v3 0/4] x86: clear XD_DISABLED flag on Intel to regain NX Alan Cox
2010-11-09 18:56 ` Kees Cook
2010-11-09 22:50 ` Alan Cox
2010-11-09 23:53 ` Kees Cook [this message]
2010-11-10 0:21 ` Alan Cox
2010-11-10 0:43 ` Kees Cook
2010-11-10 1:10 ` Kees Cook
2010-11-10 11:11 ` Alan Cox
2010-11-10 11:15 ` Ingo Molnar
2010-11-11 15:15 ` Rogier Wolff
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