From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1752622AbeBPEzd (ORCPT <rfc822;w@1wt.eu>);
        Thu, 15 Feb 2018 23:55:33 -0500
Received: from mail-pg0-f43.google.com ([74.125.83.43]:39376 "EHLO
        mail-pg0-f43.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1752450AbeBPEzb (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Thu, 15 Feb 2018 23:55:31 -0500
X-Google-Smtp-Source: AH8x225l63kQabZ17gkm7xY+7nIFTd5JAbYziMZUK6wDLlx9VRQehL75IA0hhxltdQxBU5A/zClJsg==
Content-Type: text/plain; charset=us-ascii
Mime-Version: 1.0 (Mac OS X Mail 10.3 \(3273\))
Subject: Re: [PATCH RFC v2 4/6] x86: Disable PTI on compatibility mode
From: Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <CALCETrVF8kjEUhgMQh=BT483dzQ0amfo0ruim4MOSD7DMnEVBw@mail.gmail.com>
Date: Thu, 15 Feb 2018 20:55:16 -0800
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Pavel Emelyanov <xemul@parallels.com>,
        Cyrill Gorcunov <gorcunov@openvz.org>, Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Dave Hansen <dave.hansen@linux.intel.com>, Willy Tarreau <w@1wt.eu>,
        X86 ML <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>
Message-Id: <F6703B5D-19B6-4A29-B2D5-1DAD14435934@gmail.com>
References: <20180215163602.61162-1-namit@vmware.com>
 <20180215163602.61162-5-namit@vmware.com>
 <CALCETrUY4uZeG2fz=OQWGuyCOcBBNbep6TaJz82CZSP89=g3-A@mail.gmail.com>
 <9EB804CA-0EC9-4CBB-965A-F3C8520201E7@gmail.com>
 <CALCETrVP3BaOapL3eS8Tf=KFeM8VFPqvMoECGRkFa7WPLq=Asw@mail.gmail.com>
 <CA+55aFypED6i6pESAnk_RPUon_cyvdP3g-QZg2gspEBNAQWvmQ@mail.gmail.com>
 <B723A40E-EFA4-4CCE-80F6-65960213761F@gmail.com>
 <CA+55aFzNcbCxwimCokH2BPihc=vrusbz0NqCnHK3s4aUSLwe-A@mail.gmail.com>
 <CALCETrVF8kjEUhgMQh=BT483dzQ0amfo0ruim4MOSD7DMnEVBw@mail.gmail.com>
To: Andy Lutomirski <luto@kernel.org>
X-Mailer: Apple Mail (2.3273)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org
Content-Transfer-Encoding: 8bit
X-MIME-Autoconverted: from quoted-printable to 8bit by mail.home.local id w1G4tbwM007895

Andy Lutomirski <luto@kernel.org> wrote:

> On Fri, Feb 16, 2018 at 12:42 AM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
>> On Thu, Feb 15, 2018 at 4:22 PM, Nadav Amit <nadav.amit@gmail.com> wrote:
>>> It is not too pretty, I agree, but it should do the work. There is only one
>>> problematic descriptor that can be used to switch from compatibility-mode to
>>> long-mode in the GDT (LDT descriptors always have the L-bit cleared).
>>> Changing the descriptor's present bit on context switch when needed can do
>>> the work.
>> 
>> Sure, I can see it working, but it's some really shady stuff, and now
>> the scheduler needs to save/restore/check one more subtle bit.
>> 
>> And if you get it wrong, things will happily work, except you've now
>> defeated PTI. But you'll never notice, because you won't be testing
>> for it, and the only people who will are the black hats.
>> 
>> This is exactly the "security depends on it being in sync" thing that
>> makes me go "eww" about the whole model. Get one thing wrong, and
>> you'll blow all the PTI code out of the water.
>> 
>> So now you tried to optimize one small case that most people won't
>> use, but the downside is that you may make all our PTI work (and all
>> the overhead for all the _normal_ cases) pointless.
> 
> There's also the fact that, if this stuff goes in, we'll be
> encouraging people to deploy 32-bit binaries.  Then they'll buy
> Meltdown-fixed CPUs (or AMD CPUs!) and they may well continue running
> 32-bit binaries.  Sigh.  I'm not totally a fan of this.

Ok, ok. Stop kicking the dead body...