Hi David,
On Tue, 6 Aug 2024 at 14:51, David Hildenbrand <david@xxxxxxxxxx> 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
>
