Re: [PATCHv7 10/14] x86/mm: Avoid load_unaligned_zeropad() stepping into unaccepted memory

Kirill A. Shutemov
2022-08-13T20:55:16Z

On Sat, Aug 13, 2022 at 09:04:58AM -0700, Andy Lutomirski wrote:
>
>
> On Wed, Aug 3, 2022, at 7:02 AM, Dave Hansen wrote:
> > On 8/2/22 16:46, Dave Hansen wrote:
> >> To sum it all up, I'm not happy with the complexity of the page
> >> acceptance code either but I'm not sure that it's bad tradeoff compared
> >> to greater #VE complexity or fragility.
> >>
> >> Does anyone think we should go back and really reconsider this?
> >
> > One other thing I remembered as I re-read my write up on this.
> >
> > In the "new" mode, guests never get #VE's for unaccepted memory. They
> > just exit to the host and can never be reentered. They must be killed.
> >
> > In the "old" mode, I _believe_ that the guest always gets a #VE for
> > non-EPT-present memory. The #VE is basically the same no matter if the
> > page is unaccepted or if the host goes out and makes a
> > previously-accepted page non-present.
> >
> > One really nasty implication of this "old" mode is that the host can
> > remove *accepted* pages that are used in the syscall gap. That means
> > that the #VE handler would need to be of the paranoid variety which
> > opens up all kinds of other fun.
> >
> > * "Old" - #VE's can happen in the syscall gap
> > * "New" - #VE's happen at better-defined times. Unexpected ones are
> > fatal.
> >
> > There's a third option which I proposed but doesn't yet exist. The TDX
> > module _could_ separate the behavior of unaccepted memory #VE's and
> > host-induced #VEs. This way, we could use load_unaligned_zeropad() with
> > impunity and handle it in the #VE handler. At the same time, the host
> > would not be allowed to remove accepted memory and cause problems in the
> > syscall gap. Kinda the best of both worlds.
> >
> > But, I'm not sure how valuable that would be now that we have the
> > (admittedly squirrelly) code to avoid load_unaligned_zeropad() #VE's.
>
> How would that be implemented? It would need to track which GPAs *were*
> accepted across a host-induced unmap/remap cycle. This would involve
> preventing the host from ever completely removing a secure EPT table
> without the guest’s help, right?
>
> Admittedly this would IMO be better behavior. Is it practical to implement?

I don't think it is better if you look from host POV. It owns resources of
the machine and it has to have a way to pull memory from uncooperative TD
at any point.

It also would require more complicated private->shared conversion as guest
has to notify TDX module about the change, not only host as we do now.

--
Kiryl Shutsemau / Kirill A. Shutemov