linux-kernel - On the issue of CPU model-specific registers write protection in UEFI secure boot mode

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Message-ID: <8d1f8adc-222b-40df-4573-67f28819126c@gmx.com>
Date:   Thu, 7 Feb 2019 01:37:16 +0000
From:   "Artem S. Tashkinov" <ultrafine@....com>
To:     linux-kernel@...r.kernel.org
Subject: On the issue of CPU model-specific registers write protection in UEFI
 secure boot mode

Hello LKML,

Is there a serious reason why CPU MSR is write protected in UEFI secure 
boot mode in Linux?

  * In order to even use MSR you have to be root to `modprobe msr`.
  * In order to read/write from/to MSR you have to be root as 
/dev/cpu/*/msr is accessible only by root.
  * CPU registers don't survive reboot/power cycles.
  * I'm not a CPU designer but if I'm not mistaken MSR cannot be used to 
create any sort of stealth malware.

So, I'm asking this question because these registers allow to fine tune 
Intel CPU power parameters ( https://github.com/georgewhewell/undervolt 
) like voltage and others and make it possible to run your system both 
faster and cooler and right now it's not possible under Linux and 
perfectly possible under competing proprietary OSes.

Of course, the user can

  * fetch his distro kernel sources
  * apply a patch from ( 
https://github.com/intel/intel-cmt-cat/wiki/UEFI-Secure-Boot-Compatibility )
  * install his own UEFI certificate
  * compile, sign and install a patched MSR kernel module

However this all has to be done for each new kernel release and many 
Linux users just cannot do anything on this list.

Best regards,
Artem