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 23:58:22 +0000
From:   "Woodhouse, David" <dwmw@...zon.co.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
CC:     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, 2018-01-08 at 15:56 -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.
> 
> Is there really nothing more clever we can do?

You get this part in the IBRS/microcode solution too. The IBRS MSR
doesn't catch everything; you still need to stuff the RSB in very
similar places (and/or use the IBPB MSR in some).
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