Re: [RFC PATCH v3 6/8] x86/pti: don't mark the user PGD with _PAGE_NX.

[Andy Lutomirski]

On Wed, Jan 10, 2018 at 11:54 AM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Wed, Jan 10, 2018 at 11:28 AM, Willy Tarreau <w@xxxxxx> 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".
>

If we're going to have a thread without PTI off, that thread needs to
run with the same page tables for kernel and user, so it needs NX off
on the user part.  I don't see any way around it.

We could nix the entire concept of fine-grained PTI control, or we
could make it require SMEP, I suppose.