linux-kernel - Re: [PATCH 07/34] x86/entry/32: Restore segments before int registers

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Date:   Tue, 6 Mar 2018 09:38:17 +0100
From:   Joerg Roedel <joro@...tes.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Brian Gerst <brgerst@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Peter Anvin <hpa@...or.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Andrew Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jürgen Groß <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>, Jiri Kosina <jkosina@...e.cz>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        David Laight <David.Laight@...lab.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Eduardo Valentin <eduval@...zon.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Will Deacon <will.deacon@....com>,
        "Liguori, Anthony" <aliguori@...zon.com>,
        Daniel Gruss <daniel.gruss@...k.tugraz.at>,
        Hugh Dickins <hughd@...gle.com>,
        Kees Cook <keescook@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Waiman Long <llong@...hat.com>, Pavel Machek <pavel@....cz>,
        Joerg Roedel <jroedel@...e.de>
Subject: Re: [PATCH 07/34] x86/entry/32: Restore segments before int registers

On Mon, Mar 05, 2018 at 01:58:32PM -0800, Linus Torvalds wrote:
> On Mon, Mar 5, 2018 at 1:35 PM, Joerg Roedel <joro@...tes.org> wrote:
> > I could probably add some debug instrumentation to check for that in my
> > future testing, as there is no NX protection in the user address-range
> > for the kernel-cr3.
> 
> Does not NX work with PAE?
> 
> Oh, it looks like the NX bit is marked as "RSVD (must be 0)" in the
> PDPDT. Oh well.

I had a version of these patches running which implemented NX on the PDE
level by allocating 8k PMD pages. But that ended up needing 5 order-1
allocations for each page-table, which I got to fail pretty easily after
some time. So I abandoned this approach for now.

Maybe it can be implemented with order-0 allocations for PMD pages, the
open problem is how to link the user and kernel PMD page-pairs together
then.

Regards,

	Joerg