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

[David Hildenbrand <david@redhat.com>]

>   
> -	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?

-- 
Cheers,

David / dhildenb