linux-kernel - Re: [RFC PATCH] x86/msr: Filter MSR writes

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Date:   Sat, 13 Jun 2020 11:39:14 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Peter Zijlstra <peterz@...radead.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        x86-ml <x86@...nel.org>, lkml <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] x86/msr: Filter MSR writes

On Fri, Jun 12, 2020 at 10:39:35PM +0200, Peter Zijlstra wrote:
> 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

Damn, that has fallen off my radar completely and the reason for me
requesting the sysfs interface is the *same* - kill the direct MSR
access.

Rafael, how about I refresh those patches and teach cpupower to access
the sysfs interface too and we drop that MSR from the whitelist too?

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette