Date: Thu, 4 Jan 2018 12:49:17 +0100
From: Pavel Machek <pavel@ucw.cz>
To: Andi Kleen <andi@firstfloor.org>
Cc: tglx@linutronix.de, torvalds@linux-foundation.org,
        gregkh@linux-foundation.org, linux-kernel@vger.kernel.org,
        tim.c.chen@linux.intel.com
Subject: Re: Avoid speculative indirect calls in kernel
Message-ID: <20180104114917.GC1702@amd>
References: <20180104020019.1173-1-andi@firstfloor.org>
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Hi!

> This is a fix for Variant 2 in=20
> https://googleprojectzero.blogspot.com/2018/01/reading-privileged-memory-=
with-side.html
>=20
> Any speculative indirect calls in the kernel can be tricked=20
> 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.
>=20
> There's a special code sequence called a retpoline that can
> do indirect calls without speculation. We use a new compiler
> option -mindirect-branch=3Dthunk-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
--=20
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blo=
g.html

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