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

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Message-ID: <20180109012122.GA18313@tassilo.jf.intel.com>
Date:   Mon, 8 Jan 2018 17:21:22 -0800
From:   Andi Kleen <ak@...ux.intel.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "Woodhouse, David" <dwmw@...zon.co.uk>,
        "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, 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.

Does that make more sense?

-Andi