linux-kernel - Re: Improve retpoline for Skylake

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Date:   Mon, 15 Jan 2018 19:07:52 +0100
From:   David Woodhouse <dwmw2@...radead.org>
To:     Andy Lutomirski <luto@...capital.net>,
        Andrew Cooper <andrew.cooper3@...rix.com>
Cc:     Jon Masters <jcm@...masters.org>,
        Henrique de Moraes Holschuh <hmh@....eng.br>,
        Andi Kleen <andi@...stfloor.org>, tglx@...utronix.de,
        x86@...nel.org, linux-kernel@...r.kernel.org, pjt@...gle.com,
        torvalds@...ux-foundation.org, gregkh@...ux-foundation.org,
        peterz@...radead.org, thomas.lendacky@....com,
        arjan.van.de.ven@...el.com
Subject: Re: Improve retpoline for Skylake

On Mon, 2018-01-15 at 10:06 -0800, Andy Lutomirski wrote:
> 
> > Refill or not, you are aware that a correctly timed SMI in a leaf
> > function will cause the next ret to speculate into userspace, because
> > there is guaranteed peturbance in the RSB?  (On the expectation that the
> > SMM handler isn't entirely devoid of function calls).
> 
> Couldn't firmware fill the RSB with a some known safe address, maybe
> even 0, and then immediately do RSM?

Why don't we just unconditionally declare that *all* firmware that uses
SMI for anything at all is broken?


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