Re: [PATCH RFC 4/4] mm: guest_memfd: Add ability for mmap'ing pages

[Fuad Tabba <tabba@google.com>]

Hi David,

On Tue, 6 Aug 2024 at 14:51, David Hildenbrand <david@redhat.com> wrote:
>
> >
> > -     if (gmem_flags & GUEST_MEMFD_FLAG_NO_DIRECT_MAP) {
> > +     if (!ops->accessible && (gmem_flags & GUEST_MEMFD_FLAG_NO_DIRECT_MAP)) {
> >               r = guest_memfd_folio_private(folio);
> >               if (r)
> >                       goto out_err;
> > @@ -107,6 +109,82 @@ struct folio *guest_memfd_grab_folio(struct file *file, pgoff_t index, u32 flags
> >   }
> >   EXPORT_SYMBOL_GPL(guest_memfd_grab_folio);
> >
> > +int guest_memfd_make_inaccessible(struct file *file, struct folio *folio)
> > +{
> > +     unsigned long gmem_flags = (unsigned long)file->private_data;
> > +     unsigned long i;
> > +     int r;
> > +
> > +     unmap_mapping_folio(folio);
> > +
> > +     /**
> > +      * We can't use the refcount. It might be elevated due to
> > +      * guest/vcpu trying to access same folio as another vcpu
> > +      * or because userspace is trying to access folio for same reason
>
> As discussed, that's insufficient. We really have to drive the refcount
> to 1 -- the single reference we expect.
>
> What is the exact problem you are running into here? Who can just grab a
> reference and maybe do nasty things with it?

I was wondering, why do we need to check the refcount? Isn't it enough
to check for page_mapped() || page_maybe_dma_pinned(), while holding
the folio lock?

Thanks!
/fuad

> --
> Cheers,
>
> David / dhildenb
>