linux-kernel - Re: Improve retpoline for Skylake

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Date:   Mon, 15 Jan 2018 11:03:08 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Jon Masters <jcm@...masters.org>
cc:     Henrique de Moraes Holschuh <hmh@....eng.br>,
        Andi Kleen <andi@...stfloor.org>,
        David Woodhouse <dwmw2@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org, pjt@...gle.com,
        torvalds@...ux-foundation.org, gregkh@...ux-foundation.org,
        peterz@...radead.org, luto@...capital.net, thomas.lendacky@....com,
        arjan.van.de.ven@...el.com
Subject: Re: Improve retpoline for Skylake

On Mon, 15 Jan 2018, Jon Masters wrote:
> On 01/12/2018 05:03 PM, Henrique de Moraes Holschuh wrote:
> > On Fri, 12 Jan 2018, Andi Kleen wrote:
> >>> Skylake still loses if it takes an SMI, right? 
> >>
> >> SMMs are usually rare, especially on servers, and are usually
> >> not very predictible, and even if you have
> > 
> > FWIW, a data point: SMIs can be generated on demand by userspace on
> > thinkpad laptops, but they will be triggered from within a kernel
> > context.  I very much doubt this is a rare pattern...
> 
> Sure. Just touch some "legacy" hardware that the vendor emulates in a
> nasty SMI handler. It's definitely not acceptable to assume that SMIs
> can't be generated under the control of some malicious user code.

We all know that there are holes, but can we finally sit down and do a
proper analysis whether they are practically exploitable or not.

A laptop is single user, i.e. the most likely attack vector is java
script. So please elaborate how you abuse that from JS.

If the laptop is compromised in a way that malicious code is executed on it
outside JS, then the SMI hole is the least of your worries, really.

> Our numbers on Skylake weren't bad, and there seem to be all kinds of
> corner cases, so again, it seems as if IBRS is the safest choice.

Talk is cheap. Show numbers comparing the full retpoline/RBS mitigation
compared to IBRS.

Thanks,

	tglx