Re: [PATCH 06/18] x86, barrier: stop speculation for failed access_ok

Thomas Gleixner
2018-01-07T18:31:41Z

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On Sun, 7 Jan 2018, James Bottomley wrote:

> On Sat, 2018-01-06 at 20:36 -0500, David Miller wrote:
> > From: Willy Tarreau <w@1wt.eu>
> > Date: Sat, 6 Jan 2018 21:42:29 +0100
> >
> > > On Sat, Jan 06, 2018 at 06:38:59PM +0000, Alan Cox wrote:
> > >> Normally people who propose security fixes don't have to argue
> > about the
> > >> fact they added 30 clocks to avoid your box being 0wned.
> > >
> > > In fact it depends, because if a fix makes the system unusable for
> > its
> > > initial purpose, this fix will simply not be deployed at all, which
> > is
> > > the worst that can happen.
> >
> > +1
> >
> > I completely agree with Willy and Alexei.
> >
> > And the scale isn't even accurate, we're talking about at least
> > hundreds upon hundreds of clocks, not 30, if we add an operation
> > whose side effect is to wait for all pending loads to complete.  So
> > yeah this is going to be heavily scrutinized.
>
> Plus this is the standard kernel code review MO: we've never blindly
> accepted code just because *security* (otherwise we'd have grsec in by
> now).  We use the pushback to get better and more performant code.
>  What often happens is it turns out that the "either security or
> performance" position was a false dichotomy and there is a way of
> fixing stuff that's acceptable (although not usually perfect) for
> everyone.  I'm not saying this always happens, but it is reasonable to
> let the iterative pushback see if we can get to better code in this
> case rather than trying to cut it of with the "because *security*"
> argument.

In principle I agree, though there are a few things to consider:

1) We have not the time to discuss that for the next 6 month

2) Alexei's analyis is purely based on the public information of the google
zero folks. If it would be complete and the only attack vector all fine.

If not and I doubt it is, we're going to regret this decision faster
than we made it and this is not the kind of play field where we can
afford that.

The whole 'we know better and performance is key' attitude is what led to
this disaster in the first place. We should finaly start to learn.

Can we please stop that and live with the extra cycles for a few month up
to the point where we get more informed answers to all these questions?

Thanks,

tglx