linux-kernel - Re: Avoid speculative indirect calls in kernel

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Date:   Thu, 4 Jan 2018 12:26:14 +0100
From:   Pavel Machek <pavel@....cz>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andi Kleen <andi@...stfloor.org>, tglx@...uxtronix.de,
        Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
        dwmw@...zon.co.uk, Tim Chen <tim.c.chen@...ux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: Avoid speculative indirect calls in kernel

On Wed 2018-01-03 15:51:35, Linus Torvalds wrote:
> On Wed, Jan 3, 2018 at 3:09 PM, Andi Kleen <andi@...stfloor.org> wrote:
> > This is a fix for Variant 2 in
> > https://googleprojectzero.blogspot.com/2018/01/reading-privileged-memory-with-side.html
> >
> > Any speculative indirect calls in the kernel can be tricked
> > to execute any kernel code, which may allow side channel
> > attacks that can leak arbitrary kernel data.
> 
> Why is this all done without any configuration options?
> 
> A *competent* CPU engineer would fix this by making sure speculation
> doesn't happen across protection domains. Maybe even a L1 I$ that is
> keyed by CPL.

Would that be enough?

AFAICT this will be pretty tricky to fix; it looks like you could
"attack" one userland application from another. Probing does not have
to work on L1 cache level; even main memory has "state".

It seems that complete fix would be considering any cache modification
and any memory access as a "side effect" -- so not okay to do
speculatively.

But that sounds... quite expensive for the performance...?

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

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