linux-kernel - Re: Avoid speculative indirect calls in kernel

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Date:   Thu, 4 Jan 2018 03:11:08 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Alan Cox <gnomes@...rguk.ukuu.org.uk>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andi Kleen <andi@...stfloor.org>, tglx@...utronix.de,
        Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
        dwmw@...zon.co.uk, Tim Chen <tim.c.chen@...ux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: Avoid speculative indirect calls in kernel

On 04/01/2018 02:59, Alan Cox wrote:
>> But then, exactly because the retpoline approach adds quite some cruft
>> and leaves something to be desired, why even bother?
>
> Performance

Dunno.  If I care about mitigating this threat, I wouldn't stop at
retpolines even if the full solution has pretty bad performance (it's
roughly in the same ballpark as PTI).  But if I don't care, I wouldn't
want retpolines either, since they do introduce a small slowdown (10-20
cycles per indirect branch, meaning that after a thousand such papercuts
they become slower than the full solution).

A couple manually written asm retpolines may be good as mitigation to
block the simplest PoCs (Linus may disagree), but patching the compiler,
getting alternatives right, etc. will take a while.  The only redeeming
grace of retpolines is that they don't require a microcode update, but
the microcode will be out there long before these patches are included
and trickle down to distros...  I just don't see the point in starting
from retpolines or drawing the line there.

Paolo