linux-kernel - Re: [PATCH RFC 3/4] x86/pti: don't mark the user PGD with _PAGE

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Date:   Tue, 9 Jan 2018 05:22:01 +0100
From:   Willy Tarreau <w@....eu>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Dave Hansen <dave.hansen@...el.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, tglx@...utronix.de, gnomes@...rguk.ukuu.org.uk,
        torvalds@...ux-foundation.org, Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH RFC 3/4] x86/pti: don't mark the user PGD with _PAGE_NX.

Hi Andy,

On Mon, Jan 08, 2018 at 03:05:48PM -0800, Andy Lutomirski wrote:
> On 01/08/2018 09:03 AM, Dave Hansen wrote:
> > On 01/08/2018 08:12 AM, Willy Tarreau wrote:
> I vote per-thread.

The per-mm approach that Linus suggested doesn't look bad either and
makes quite some sense.

> Anyway, we can easily sync the NX-clearing: just catch the spurious page
> fault and clear the bit.  Avoiding infinite loops will need a bit of
> thought, but it's surely doable.

That's an excellent idea, eventhough I have no idea how to implement it :-)

> Or we set a per-mm flag saying "no NX", then do synchronize_sched() or
> similar if we were the first to set it (or take the pagetable lock), then
> clear all the NX bits.  Again, needs some care, but doable.
> 
> FWIW, the NX trick quite nicely emulates SMEP on non-SMEP hardware, which is
> fantastic for Spectre resistance and general hardening.

Yes I figured exactly this when I faced this protection!

> Turning it off totally defeats that, which hurts a bit.

I agree, that's why I'd like it to be conditional. Probably that with
your idea of catching the page fault and the per-mm flag it would work
quite well, but before being able to do this I still have a lot to
explore :-/

> Also, Kees should be CC'd here.

Yes I've added him and you (and a few others) in CC of all forthcoming
patches. Sorry for not adding you initially, I simply wanted to share
a quick experiment and initiate a discussion.

Willy