linux-kernel - Re: [PATCH v4 6/7] x86/cpufeature: Blacklist SPEC

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Date:   Thu, 25 Jan 2018 11:20:13 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     arjan@...ux.intel.com, karahmed@...zon.de, x86@...nel.org,
        linux-kernel@...r.kernel.org, tim.c.chen@...ux.intel.com,
        bp@...en8.de, peterz@...radead.org, pbonzini@...hat.com,
        ak@...ux.intel.com, torvalds@...ux-foundation.org,
        gregkh@...ux-foundation.org, dave.hansen@...el.com,
        gnomes@...rguk.ukuu.org.uk, ashok.raj@...el.com, mingo@...nel.org
Subject: Re: [PATCH v4 6/7] x86/cpufeature: Blacklist SPEC_CTRL on early
 Spectre v2 microcodes

On Thu, 2018-01-25 at 11:54 +0100, Thomas Gleixner wrote:
> On Thu, 25 Jan 2018, David Woodhouse wrote:
> 
> > 
> > On Thu, 2018-01-25 at 09:23 +0000, David Woodhouse wrote:
> > > 
> > > 
> > > +/*
> > > + * Early microcode releases for the Spectre v2 mitigation were broken.
> > > + * Information taken from;
> > > + * • https://newsroom.intel.com/wp-content/uploads/sites/11/2018/01/microcode-update-guidance.pdf
> > Oh look, Intel have released a *third* version of that document
> > already, and they've bumped the bad Kabylake versions to 0x84, although
> > they're *still* missing out the KBL 906EA SKU which was updated to 0x80
> > in the public 20180108 microcode release. I'll bump them all in my tree
> > to 0x84.
> > 
> > Thomas, want another copy in email now, or were we waiting for another
> > round of these before you merge them anyway?
> Looking at this mess makes me even less convinced that a blacklist is a
> good idea. We have already at least 3 different variants of blacklists.
> 
> So I rather see a whitelist approach which says:
> 
>    if (ucode_version < known_good(family, model))
>        return;
> 
> I know it would require that Intel releases a set of known good ucodes at
> some point in the future, but that's a reasonable request.
> 
> I rather take a few patches which add the cutoff for family/model (and NO,
> I don't want to add stepping into the mix at all) than having this
> blacklist mess which keeps changing every other day.

I don't know why they added KBL 0x84 microcode; I'm not aware that it's
been released. Maybe it escaped somewhere so that's why they added it
to the doc.

If they ship another bad microcode revision to the public, newer than
the ones which are already in that list, then there is something
*SEVERELY* wrong. Wronger than we already know.

And if we take this 'piecemeal whitelist' approach, they're going to
have to add the newly-released microcodes to the kernel each time they
do a release which covers a new SKU — so it's not like it'll even help,
because if they *do* ship another bad one, we'll *already* have added
it to the kernel by the time we realise.

I don't expect them to *keep* doing daily releases of the blacklist
doc; it's just been a bit rushed, that's all. I think we're much better
off doing it this way than saying that we'll keep taking incremental
updates as they do new releases.

Also, I don't think we can avoid looking at the stepping. Not unless
you want to make Intel synchronise the microcode version numbers for
different steppings of the same model? Because they aren't at the
moment.
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