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

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Message-ID: <20190207162411.GA1144@linux.intel.com>
Date:   Thu, 7 Feb 2019 08:24:12 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     "Artem S. Tashkinov" <ultrafine@....com>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: On the issue of CPU model-specific registers write protection in
 UEFI secure boot mode

On Thu, Feb 07, 2019 at 01:37:16AM +0000, Artem S. Tashkinov wrote:
> 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.

Rewriting SYSENTER MSRs allows arbitrary ring0 code execution.  That's
the most obvious example, there are undoubtedly other attacks that can
be concocted by modifying MSRs.

https://lkml.kernel.org/r/20130208191213.GA25081@www.outflux.net
https://lkml.kernel.org/r/149141201983.30815.1240162780237131881.stgit@warthog.procyon.org.uk

> 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