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

Andy Lutomirski
2022-08-13T16:05:27Z

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?