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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Wed, 10 Jan 2018 11:54:17 -0800
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Subject: Re: [RFC PATCH v3 6/8] x86/pti: don't mark the user PGD with _PAGE_NX.
To: Willy Tarreau <w@1wt.eu>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
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On Wed, Jan 10, 2018 at 11:28 AM, Willy Tarreau <w@1wt.eu> 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.

Yeah, no. This is wrong.

Sure, SMEP gives the same thing in most cases, but not for older CPU's.

So NX is a really nice way to make sure that PTI really does protect
against user-space gadgets.

We don't break that, and we definitely don't break that just because
of some broken notion of "let's make page table isolation per-thread".

                  Linus