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

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Date:   Mon, 8 Jan 2018 15:56:30 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     David Woodhouse <dwmw@...zon.co.uk>,
        Andi Kleen <ak@...ux.intel.com>, Paul Turner <pjt@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Kees Cook <keescook@...gle.com>,
        Rik van Riel <riel@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...capital.net>,
        Jiri Kosina <jikos@...nel.org>,
        One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
Subject: Re: [PATCH v6 11/10] x86/retpoline: Avoid return buffer underflows on
 context switch

On Mon, Jan 8, 2018 at 3:44 PM, David Woodhouse <dwmw2@...radead.org> wrote:
>
> To guard against this fill the return buffer with controlled
> content during context switch. This prevents any underflows.

Ugh. I really dislike this patch. Everything else in the retpoline
patches makes me go "ok, that's reasonable". This one makes me go
"Eww".

It's hacky, it's ugly, and it looks pretty expensive too.

Is there really nothing more clever we can do?

              Linus