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

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Date:   Mon, 8 Jan 2018 17:53:53 -0800
From:   Paul Turner <pjt@...gle.com>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        "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>,
        "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 8, 2018 at 5:21 PM, Andi Kleen <ak@...ux.intel.com> 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

(Broadwell without Microcode)

> 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?

The majority of the confusion does not stem from this being complicated.
It's that there's been great reluctance to document the details which
are different -- or changed by the microcode -- even at a high level
such as this.
Because of this, some of the details instead include vague "things are
different on Skylake" notes, with no exposition.

>
> -Andi