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Message-ID: <CA+55aFxyx2-m8Rd5vBoncd2u-UOF=QQ-By7XwzWEfHtA=Lqp5A@mail.gmail.com>
Date: Wed, 10 Jan 2018 11:54:17 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Willy Tarreau <w@....eu>
Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
"the arch/x86 maintainers" <x86@...nel.org>,
Andy Lutomirski <luto@...nel.org>,
Borislav Petkov <bp@...en8.de>,
Brian Gerst <brgerst@...il.com>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Ingo Molnar <mingo@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Thomas Gleixner <tglx@...utronix.de>,
Josh Poimboeuf <jpoimboe@...hat.com>,
"H. Peter Anvin" <hpa@...or.com>,
David Woodhouse <dwmw@...zon.co.uk>,
Kees Cook <keescook@...omium.org>
Subject: Re: [RFC PATCH v3 6/8] x86/pti: don't mark the user PGD with _PAGE_NX.
On Wed, Jan 10, 2018 at 11:28 AM, Willy Tarreau <w@....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
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