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

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Date:   Mon, 8 Jan 2018 18:21:02 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Willy Tarreau <w@....eu>, linux-kernel@...r.kernel.org,
        x86@...nel.org, tglx@...utronix.de, gnomes@...rguk.ukuu.org.uk,
        torvalds@...ux-foundation.org, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH RFC 3/4] x86/pti: don't mark the user PGD with _PAGE_NX.


* Dave Hansen <dave.hansen@...el.com> wrote:

> On 01/08/2018 08:12 AM, Willy Tarreau wrote:
> > Since we're going to keep running on the same PGD when returning to
> > userspace for certain performance-critical tasks, we'll need the user
> > pages to be executable. So this code disables the extra protection
> > that was added consisting in marking user pages _PAGE_NX so that this
> > pgd remains usable for userspace.
> > 
> > Note: it isn't necessarily the best approach, but one way or another
> >       if we want to be able to return to userspace from the kernel,
> >       we'll have to have this executable anyway. Another approach
> >       might consist in using another pgd for userland+kernel but
> >       the current core really looks like an extra careful measure
> >       to catch early bugs if any.
> 
> I don't like this.
> 
> I think the prctl() should apply to an entire process, not to a thread.
> If it applies to a process, you can unpoison the PGD.  I even had code
> to do this in an earlier version of the (whole system) runtime PTI
> on/off stuff.
> 
> Why are you even posting half-baked hacks like this now?  Is there
> something super-pressing about this set that we need to lock in a new
> ABI now?

Arguably it was posted as an RFC patch-set, to get feedback early on.

The motivation is clear enough from the announcement I think: to speed up the 
haproxy performance almost two-fold, without sacrificing the overall security 
given by PTI against the Meltdown attack. haproxy does not require PTI, as it 
never executes untrusted code.

Thanks,

	Ingo