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

[Sean Christopherson <seanjc@google.com>]

On Thu, Oct 03, 2024, Ackerley Tng wrote:
> Ackerley Tng <ackerleytng@google.com> writes:
> 
> > Elliot Berman <quic_eberman@quicinc.com> writes:
> >> 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.
> 
> Actually it seems like filemap_grab_folio() already gets a zeroed page.
> 
> filemap_grab_folio() eventually calls __alloc_pages_noprof()
> -> get_page_from_freelist()
>    -> prep_new_page()
>       -> post_alloc_hook()
> 
> and post_alloc_hook() calls kernel_init_pages(), which zeroes the page,
> depending on kernel config.
> 
> Paolo, was calling clear_highpage() in kvm_gmem_prepare_folio() zeroing an
> already empty page returned from filemap_grab_folio()?

Yes and no.  CONFIG_INIT_ON_ALLOC_DEFAULT_ON and init_on_alloc are very much
hardening features, not functional behavior that other code _needs_ to be aware
of.  E.g. enabling init-on-alloc comes with a measurable performance cost.

Ignoring hardening, the guest_memfd mapping specifically sets the gfp_mask to
GFP_HIGHUSER, i.e. doesn't set __GFP_ZERO.

That said, I wouldn't be opposed to skipping the clear_highpage() call when
want_init_on_alloc() is true.

Also, the intended behavior (or at least, what  intended) of kvm_gmem_prepare_folio()
was it would do clear_highpage() if and only if a trusted entity does NOT zero
the page.  Factoring that in is a bit harder, as it probably requires another
arch hook (or providing an out-param from kvm_arch_gmem_prepare()).  I.e. the
want_init_on_alloc() case isn't the only time KVM could shave cycles by not
redundantly zeroing memory.