Re: [RFC PATCH 30/39] KVM: guest_memfd: Handle folio preparation for guest_memfd mmap

[Ackerley Tng]

Elliot Berman <quic_eberman@xxxxxxxxxxx> writes:

> On Tue, Sep 10, 2024 at 11:44:01PM +0000, Ackerley Tng wrote:
>> Since guest_memfd now supports mmap(), folios have to be prepared
>> before they are faulted into userspace.
>>
>> When memory attributes are switched between shared and private, the
>> up-to-date flags will be cleared.
>>
>> Use the folio's up-to-date flag to indicate being ready for the guest
>> usage and can be used to mark whether the folio is ready for shared OR
>> private use.
>
> Clearing the up-to-date flag also means that the page gets zero'd out
> whenever it transitions between shared and private (either direction).
> pKVM (Android) hypervisor policy can allow in-place conversion between
> shared/private.
>
> I believe the important thing is that sev_gmem_prepare() needs to be
> called prior to giving page to guest. In my series, I had made a
> ->prepare_inaccessible() callback where KVM would only do this part.
> When transitioning to inaccessible, only that callback would be made,
> besides the bookkeeping. The folio zeroing happens once when allocating
> the folio if the folio is initially accessible (faultable).
>
> From x86 CoCo perspective, I think it also makes sense to not zero
> the folio when changing faultiblity from private to shared:
>  - If guest is sharing some data with host, you've wiped the data and
>    guest has to copy again.
>  - Or, if SEV/TDX enforces that page is zero'd between transitions,
>    Linux has duplicated the work that trusted entity has already done.
>
> Fuad and I can help add some details for the conversion. Hopefully we
> can figure out some of the plan at plumbers this week.

Zeroing the page prevents leaking host data (see function docstring for
kvm_gmem_prepare_folio() introduced in [1]), so we definitely don't want
to introduce a kernel data leak bug here.

In-place conversion does require preservation of data, so for
conversions, shall we zero depending on VM type?

+ Gunyah: don't zero since ->prepare_inaccessible() is a no-op
+ pKVM: don't zero
+ TDX: don't zero
+ SEV: AMD Architecture Programmers Manual 7.10.6 says there is no
  automatic encryption and implies no zeroing, hence perform zeroing
+ KVM_X86_SW_PROTECTED_VM: Doesn't have a formal definition so I guess
  we could require zeroing on transition?

This way, the uptodate flag means that it has been prepared (as in
sev_gmem_prepare()), and zeroed if required by VM type.

Regarding flushing the dcache/tlb in your other question [2], if we
don't use folio_zero_user(), can we relying on unmapping within core-mm
to flush after shared use, and unmapping within KVM To flush after
private use?

Or should flush_dcache_folio() be explicitly called on kvm_gmem_fault()?

clear_highpage(), used in the non-hugetlb (original) path, doesn't flush
the dcache. Was that intended?

> Thanks,
> Elliot
>
>>
>> <snip>

[1] https://lore.kernel.org/all/20240726185157.72821-8-pbonzini@xxxxxxxxxx/
[2] https://lore.kernel.org/all/diqz34ldszp3.fsf@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/