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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>
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To:     Andy Lutomirski <luto@kernel.org>
Sender: linux-kernel-owner@vger.kernel.org
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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.
>>=20
>> 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.
>>=20
>> 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.
>>=20
>> 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.
>>=20
>> 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.
>=20
> 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...=