linux-kernel - Re: [PATCH] x86/msr: do not warn on writes to OC

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Message-ID: <CACdnJusOJVb0xpecFgPQB4N2WhUORikv_1eXAcGfJ3xwBVTo9Q@mail.gmail.com>
Date:   Tue, 8 Sep 2020 15:32:28 -0700
From:   Matthew Garrett <mjg59@...gle.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Borislav Petkov <bp@...en8.de>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        Sultan Alsawaf <sultan@...neltoast.com>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        kitsunyan <kitsunyan@...mail.cc>,
        "Brown, Len" <len.brown@...el.com>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] x86/msr: do not warn on writes to OC_MAILBOX

On Tue, Sep 8, 2020 at 1:35 PM Andy Lutomirski <luto@...capital.net> wrote:

> Undervolting is a bit different. It’s a genuinely useful configuration that can affect system stability.  In general, I think it should be allowed, and it should have a real driver in tree.

Agree that this should be a proper driver rather than permitting
arbitrary poking (especially if this isn't an architecturally defined
MSR - there's no guarantee that it'll have the same functionality
everywhere).

> But this has a tricky interaction with lockdown.  An interface that allows root to destabilize a system may well allow root to escalate privileges.  But I think that making lockdown=integrity prevent tuning voltages and such would be quite obnoxious.

Indeed - plundervolt.com is a demonstration of this. Any realistic
attack involves being able to drop the voltage enough to interfere
with a calculation and then raise it again before everything else
falls over, so simply applying some rate limiting seems like it would
be sufficient.

> Should there perhaps be a separate lockdown bit for stability?

If it's a sysfs interface then I think it'd be easy enough for people
who care to just add an SELinux or Apparmor rule, tbh.