linux-kernel - Re: [RFC PATCH v3 6/8] x86/pti: don't mark the user PGD with _PAGE

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

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