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 16:06:26 -0800
From:   Andi Kleen <ak@...ux.intel.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     David Woodhouse <dwmw2@...radead.org>,
        David Woodhouse <dwmw@...zon.co.uk>,
        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 08, 2018 at 03:56:30PM -0800, Linus Torvalds wrote:
> 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.

Modern cores are quite fast at executing calls. 

> 
> Is there really nothing more clever we can do?

We could be a cleverer in selecting how many dummy calls to do.
But that would likely be fragile and hard to maintain
and likely be more complicated, and I doubt it would buy that much.

Don't really have a better proposal, sorry.

-Andi