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

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Date:   Thu, 4 Jan 2018 12:49:17 +0100
From:   Pavel Machek <pavel@....cz>
To:     Andi Kleen <andi@...stfloor.org>
Cc:     tglx@...utronix.de, torvalds@...ux-foundation.org,
        gregkh@...ux-foundation.org, linux-kernel@...r.kernel.org,
        tim.c.chen@...ux.intel.com
Subject: Re: Avoid speculative indirect calls in kernel

Hi!

> 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.

Ok.

> So we want to avoid speculative indirect calls in the kernel.
> 
> There's a special code sequence called a retpoline that can
> do indirect calls without speculation. We use a new compiler
> option -mindirect-branch=thunk-extern (gcc patch will be released
> separately) to recompile the kernel with this new sequence.

So... this "retpoline" code is quite tricky; I guess it does the right
on recent Intel CPUs. Does it also do the right thing on all the AMD,
Cyrix, ... variants?

Is it neccessary on all the CPUs? I guess 486 does not need this?

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

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