Re: [PATCH v8 1/8] mm/memfd: Introduce userspace inaccessible memfd

[Chao Peng]

On Fri, Sep 23, 2022 at 04:19:46PM +0100, Fuad Tabba wrote:
> > Regarding pKVM's use case, with the shim approach I believe this can be done by
> > allowing userspace mmap() the "hidden" memfd, but with a ton of restrictions
> > piled on top.
> >
> > My first thought was to make the uAPI a set of KVM ioctls so that KVM could tightly
> > tightly control usage without taking on too much complexity in the kernel, but
> > working through things, routing the behavior through the shim itself might not be
> > all that horrific.
> >
> > IIRC, we discarded the idea of allowing userspace to map the "private" fd because
> > things got too complex, but with the shim it doesn't seem _that_ bad.
> >
> > E.g. on the memfd side:
> >
> >   1. The entire memfd must be mapped, and at most one mapping is allowed, i.e.
> >      mapping is all or nothing.
> >
> >   2. Acquiring a reference via get_pfn() is disallowed if there's a mapping for
> >      the restricted memfd.
> >
> >   3. Add notifier hooks to allow downstream users to further restrict things.
> >
> >   4. Disallow splitting VMAs, e.g. to force userspace to munmap() everything in
> >      one shot.
> >
> >   5. Require that there are no outstanding references at munmap().  Or if this
> >      can't be guaranteed by userspace, maybe add some way for userspace to wait
> >      until it's ok to convert to private?  E.g. so that get_pfn() doesn't need
> >      to do an expensive check every time.
> >
> >   static int memfd_restricted_mmap(struct file *file, struct vm_area_struct *vma)
> >   {
> >         if (vma->vm_pgoff)
> >                 return -EINVAL;
> >
> >         if ((vma->vm_end - vma->vm_start) != <file size>)
> >                 return -EINVAL;
> >
> >         mutex_lock(&data->lock);
> >
> >         if (data->has_mapping) {
> >                 r = -EINVAL;
> >                 goto err;
> >         }
> >         list_for_each_entry(notifier, &data->notifiers, list) {
> >                 r = notifier->ops->mmap_start(notifier, ...);
> >                 if (r)
> >                         goto abort;
> >         }
> >
> >         notifier->ops->mmap_end(notifier, ...);
> >         mutex_unlock(&data->lock);
> >         return 0;
> >
> >   abort:
> >         list_for_each_entry_continue_reverse(notifier &data->notifiers, list)
> >                 notifier->ops->mmap_abort(notifier, ...);
> >   err:
> >         mutex_unlock(&data->lock);
> >         return r;
> >   }
> >
> >   static void memfd_restricted_close(struct vm_area_struct *vma)
> >   {
> >         mutex_lock(...);
> >
> >         /*
> >          * Destroy the memfd and disable all future accesses if there are
> >          * outstanding refcounts (or other unsatisfied restrictions?).
> >          */
> >         if (<outstanding references> || ???)
> >                 memfd_restricted_destroy(...);
> >         else
> >                 data->has_mapping = false;
> >
> >         mutex_unlock(...);
> >   }
> >
> >   static int memfd_restricted_may_split(struct vm_area_struct *area, unsigned long addr)
> >   {
> >         return -EINVAL;
> >   }
> >
> >   static int memfd_restricted_mapping_mremap(struct vm_area_struct *new_vma)
> >   {
> >         return -EINVAL;
> >   }
> >
> > Then on the KVM side, its mmap_start() + mmap_end() sequence would:
> >
> >   1. Not be supported for TDX or SEV-SNP because they don't allow adding non-zero
> >      memory into the guest (after pre-boot phase).
> >
> >   2. Be mutually exclusive with shared<=>private conversions, and is allowed if
> >      and only if the entire gfn range of the associated memslot is shared.
> 
> In general I think that this would work with pKVM. However, limiting
> private<->shared conversions to the granularity of a whole memslot
> might be difficult to handle in pKVM, since the guest doesn't have the
> concept of memslots. For example, in pKVM right now, when a guest
> shares back its restricted DMA pool with the host it does so at the
> page-level. pKVM would also need a way to make an fd accessible again
> when shared back, which I think isn't possible with this patch.

But does pKVM really want to mmap/munmap a new region at the page-level,
that can cause VMA fragmentation if the conversion is frequent as I see.
Even with a KVM ioctl for mapping as mentioned below, I think there will
be the same issue.

> 
> You were initially considering a KVM ioctl for mapping, which might be
> better suited for this since KVM knows which pages are shared and
> which ones are private. So routing things through KVM might simplify
> things and allow it to enforce all the necessary restrictions (e.g.,
> private memory cannot be mapped). What do you think?
> 
> Thanks,
> /fuad