linux-kernel - Re: Bricked x86 CPU with software?

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Message-ID: <20180108233235.GB25349@amd>
Date:   Tue, 9 Jan 2018 00:32:35 +0100
From:   Pavel Machek <pavel@....cz>
To:     Tim Mouraveiko <tim.ml@...opper.com>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: Bricked x86 CPU with software?

On Mon 2018-01-08 07:58:33, Tim Mouraveiko wrote:
> > Hi!
> >
> > > > Is the sequence available from ring 3, or does it need ring 0?
> > > >
> > > > Can we get the code? Extraordinary claims and all that...
> > > >
> > >
> > > I did not test privilege level. Are you suggesting that I put the code out there for everyone to
> > > see or what?
> >
> > Yes, that's what I'm suggesting.
> >
> 
> That would be neither prudent nor practical.
> 
> Perhaps you did not consider the consequences. What if it is compatible with your
> processor? Would you send me a handwritten thank you card if that processor stops
> processing? Would you be a happy replacement-sale customer of Intel? I think you did not
> put much thought into why we are talking about it a year later or at
> all.

Actually, yes, thank you card. Not handwritten -- plenty of CPUs here :-).

> Unlike the now-oh-so-scary feature that was in existence for decades, that is only so scary
> because of a "clever" idea to "cloud" host different customers on bare metal, without any
> consideration to their security, this could affect real people not just oh-so-clever computer
> farmers.

I don't believe you actually have a way to brick CPUs.

Yes, it is possible to brick some computers -- overwriting BIOS will
do the trick, for example; doable from ring 0. There is more firmware
that can be overwritten... That's old news. Worth mentioning on
bugtraq, so manufacturer can fix it, but...

If you had something that worked directly on CPU, that would be news;
and yes, there are fuses there, but I really doubt they can be
manipulated by software. And I believe it would be news worth more
than price of a CPU... Still not exactly dangerous. Usually data are
worth more than hardware.

Ouch, and if it worked from ring 3... That would be newsworthy. That
would be actually quite dangerous. OTOH... people did try to fuzz CPU
instruction sets, so my bet is someone would have noticed.

Best regards,
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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