linux-kernel - Re: [PATCH v6 11/10] x86/retpoline: Avoid return buffer underflows on context switch II

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Message-ID: <1515460999.4423.104.camel@amazon.co.uk>
Date:   Tue, 9 Jan 2018 01:23:19 +0000
From:   "Woodhouse, David" <dwmw@...zon.co.uk>
To:     Andi Kleen <ak@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "tim.c.chen@...ux.intel.com" <tim.c.chen@...ux.intel.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "riel@...hat.com" <riel@...hat.com>,
        "keescook@...gle.com" <keescook@...gle.com>,
        "gnomes@...rguk.ukuu.org.uk" <gnomes@...rguk.ukuu.org.uk>,
        "pjt@...gle.com" <pjt@...gle.com>,
        "dave.hansen@...el.com" <dave.hansen@...el.com>,
        "luto@...capital.net" <luto@...capital.net>,
        "jikos@...nel.org" <jikos@...nel.org>,
        "gregkh@...ux-foundation.org" <gregkh@...ux-foundation.org>
Subject: Re: [PATCH v6 11/10] x86/retpoline: Avoid return buffer underflows
 on context switch II

On Mon, 2018-01-08 at 17:21 -0800, Andi Kleen wrote:
> On Mon, Jan 08, 2018 at 05:16:02PM -0800, Andi Kleen wrote:
> > > If we clear the registers, what the hell are you going to put in the
> > > RSB that helps you?
> > 
> > RSB allows you to control chains of gadgets.
> 
> I admit the gadget thing is a bit obscure.
> 
> There's another case we were actually more worried about: 
> 
> On Skylake and Broadwell when the RSB underflows it will fall back to the 
> indirect branch predictor, which can be poisoned and we try to avoid
> using with retpoline. So we try to avoid underflows, and this filling
> helps us with that.

That's no longer true for Broadwell with the latest microcode, right?

And is why we *were* talking about using retpoline only for pre-
Skylake, and using IBRS on Skylake onwards (where IBRS isn't quite so
horribly slow anyway).

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