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From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
Date:   Tue, 8 Sep 2020 19:29:11 +0200
Message-ID: <CAHmME9pKfvPGf97BM1=VdUL1uU_8aOoc4+QOu6b51XnPz3SkRA@mail.gmail.com>
Subject: Re: [PATCH] x86/msr: do not warn on writes to OC_MAILBOX
To:     Borislav Petkov <bp@alien8.de>
Cc:     Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
        kitsunyan <kitsunyan@airmail.cc>,
        "Brown, Len" <len.brown@intel.com>, X86 ML <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Sultan Alsawaf <sultan@kerneltoast.com>
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On Tue, Sep 8, 2020 at 7:26 PM Borislav Petkov <bp@alien8.de> wrote:
>
> On Tue, Sep 08, 2020 at 07:12:44PM +0200, Jason A. Donenfeld wrote:
> > > Overclocking is not architectural I/F and is supported by some special
> > > CPU skews. I can't find any public document to specify the commands
> > > which can be used via this OC mailbox. I have to check internally to
> > > see if there is any. To add a proper sysfs interface we have to make
> > > sure that we are not allowing some random commands to hardware and
> > > crash the system.
> >
> > Well you can definitely crash the system this way -- undervolting can
> > result in all sorts of nice glitching. You might be able to even
> > programmatically undervolt to compromise the kernel in clever ways (a
> > lockdown bypass, I guess, but who cares).
> >
> > That's why I initially suggested this was pretty squarely in the realm
> > of hobbyists and should just be added to that whitelist.
>
> If that MSR can cause all kinds of crazy, I'd prefer writes to it from
> userspace to be completely forbidden, actually. And if force-enabled,
> with a BIG FAT WARNING each time userspace writes to it.

Well that's not cool. And it's sure to really upset the fairly sizable
crowd of people who rely on undervolting and related things to make
their laptops remotely usable, especially in light of the crazy
thermal designs for late-era 14nm intel cpus. Tools like
intel-undervolt have been a godsend in that regard. I came here
posting a patch to remove the annoying message you added for that use
case. Now you want to just totally remove that feature all together
from the kernel? Sounds like a regression in functionality I simply
can't get behind. I know that my laptop, at least, would suffer.

Jason