linux-kernel - Re: [Security] [PATCH v3 0/4] x86: clear XD

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Date:	Tue, 9 Nov 2010 18:31:42 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Kees Cook <kees.cook@...onical.com>
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [Security] [PATCH v3 0/4] x86: clear XD_DISABLED flag on Intel
 to regain NX

> 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

> 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 !

Alan
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