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Date:   Fri, 12 Jun 2020 22:39:35 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Borislav Petkov <bp@alien8.de>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        x86-ml <x86@kernel.org>, lkml <linux-kernel@vger.kernel.org>
Subject: Re: [RFC PATCH] x86/msr: Filter MSR writes
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On Fri, Jun 12, 2020 at 07:48:01PM +0200, Borislav Petkov wrote:
> On Fri, Jun 12, 2020 at 10:20:03AM -0700, Linus Torvalds wrote:
> > Since you already added the filtering, this looks fairly sane.
> > 
> > IOW, what MSR's do we expect people to maybe write to normally? You
> > added MSR_IA32_ENERGY_PERF_BIAS as an allowed MST, maybe there are
> > others?
> 
> Right, this MSR is being written by cpupower in tools/. My search was
> confined within the kernel source only so there very likely are others.

So that tool writing to /dev/msr has already caused pain; the direct
result is that the intel pstate driver doesn't want to use an MSR shadow
variable to avoid RDMSR because that'd loose input.

https://lkml.org/lkml/2019/3/25/310

(sorry, that's what google found me)

So ideally we'd just disallow it too. It already has a sysfs file (per
those patches):

  Documentation/admin-guide/pm/intel_epb.rst