Messages in this thread |  | | | Date | Tue, 9 Nov 2010 10:11:57 -0800 | | From | Kees Cook <> | | Subject | [Security] [PATCH v3 0/4] x86: clear XD_DISABLED flag on Intel to regain NX |
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Hi,
This is a re-send of an earlier patch series that got side-tracked. I'd like to get this into the kernel in some form, so I'd like to try submission again, as I think it is important.
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.
Based on feedback from HPA, this was reworked to extend the existing "verify_cpu" routines, and to more tightly confine which CPUs will call MSR_IA32_MISC_ENABLE. Since it includes some re-arrangements of files, I tried to break the patches up into their logical steps.
-Kees
[1] http://www.outflux.net/blog/archives/2010/02/18/data-mining-for-nx-bit/
-- Kees Cook Ubuntu Security Team
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