[Security] [PATCH v5 0/4] x86: clear XD_DISABLED flag on Intel to regain NX

[Kees Cook]

Resending with Alan Cox's Acks, at Ingo's request.

Intel CPUs have an additional MSR bit to indicate if the BIOS was
configured to disable NX. This bit was traditionally used for operating
systems that did not understand how to handle the NX bit. Since Linux
understands this, this BIOS flag should be ignored by default.

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.

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.

-Kees

[1] http://www.outflux.net/blog/archives/2010/02/18/data-mining-for-nx-bit/

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Changelog:
v2 - rearranged use of verify_cpu()
v3 - resent, show stats on systems that need it
v4 - expanded commit messages, added acks
v5 - added acks
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