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

[Andy Lutomirski]

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