linux-kernel - Re: [PATCH RFC v2 4/6] x86: Disable PTI on compatibility mode

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Message-Id: <F6703B5D-19B6-4A29-B2D5-1DAD14435934@gmail.com>
Date:   Thu, 15 Feb 2018 20:55:16 -0800
From:   Nadav Amit <nadav.amit@...il.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Pavel Emelyanov <xemul@...allels.com>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Willy Tarreau <w@....eu>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC v2 4/6] x86: Disable PTI on compatibility mode

Andy Lutomirski <luto@...nel.org> wrote:

> On Fri, Feb 16, 2018 at 12:42 AM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>> On Thu, Feb 15, 2018 at 4:22 PM, Nadav Amit <nadav.amit@...il.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...