Re: Avoid speculative indirect calls in kernel

[Willy Tarreau <w@1wt.eu>]

Hi Thomas,

On Mon, Jan 08, 2018 at 10:18:09AM +0100, Thomas Gleixner wrote:
> On Sun, 7 Jan 2018, Willy Tarreau wrote:
> > On Sun, Jan 07, 2018 at 07:55:11PM +0100, Borislav Petkov wrote:
> > > > Just like you have to trust your plane's pilot eventhough you don't
> > > > know him personally.
> > > 
> > > Funny you should make that analogy. Remember that germanwings pilot?
> > > People trusted him too.
> > > 
> > > Now imagine if the plane had protection against insane pilots... some of
> > > those people might still be alive, who knows...
> > 
> > Sure but despite this case many people continue to take the plane because
> > it's their only option to cross half of the world in a reasonable time.
> > 
> > Boris, I'm *not* contesting the performance resulting from the fixes,
> > and I would never have been able to produce them myself had I to, so
> > I'm really glad we have them. I just want to be clear that the big drop
> > some of us are facing is not an option *at all* for certain processes
> > in certain environments and that we'll either continue to run with
> > pti=off or with pti=on + a finer grained setting ASAP.
> 
> No argument about that. We've looked into per process PTI very early and
> decided not to go that route because of the time pressure and the risk. I'm
> glad that we managed to pull it off at all without breaking the world
> completely. It's surely doable and we all know that it has to be done, just
> not right now as we have to fast track at least the basic protections for
> the other two attack vectors.

I know that most people with the skills to do it are very busy, which is
why I started to take a look at it, not being involved at all in this and
having interest in seeing it done. For me the road is long, progressively
discovering asid/pcid etc in the code, you can guess I won't come up with
something testable any time soon ;-)

My idea would be to use a privileged prctl() call to set a new TIF_NOPTI
on the task and to see where to check for this to avoid switching to the
user-only PGD when returning to userspace. I have no idea if this is
doable at all nor if this would be sufficient (I hope so) but reading
the code to try to figure whether it makes sense cannot hurt.

> You can be sure, that all people involved hate it more than you do.

I'm definitely convinced about this, we're all proud to save one CPU
cycle here and there from time to time and having to suddenly flush TLBs
and throw hundreds or thousands of cycles at once down the drain must be
a very hard decision to take. And by the way I don't hate what was done
because there's a config option and I still have the choice. Other OS
users probably don't even have this choice, so thanks to all involved
for this!

Willy