Re: [PATCH RFC 0/5] mm/gup: Introduce exclusive GUP pinning

Jason Gunthorpe
2024-08-02T11:22:09Z

On Fri, Aug 02, 2024 at 08:26:48AM +0000, Tian, Kevin wrote:
> > From: Jason Gunthorpe <jgg@nvidia.com>
> > Sent: Thursday, June 20, 2024 10:34 PM
> >
> > On Thu, Jun 20, 2024 at 04:14:23PM +0200, David Hildenbrand wrote:
> >
> > > 1) How would the device be able to grab/access "private memory", if not
> > > via the user page tables?
> >
> > The approaches I'm aware of require the secure world to own the IOMMU
> > and generate the IOMMU page tables. So we will not use a GUP approach
> > with VFIO today as the kernel will not have any reason to generate a
> > page table in the first place. Instead we will say "this PCI device
> > translates through the secure world" and walk away.
> >
> > The page table population would have to be done through the KVM path.
>
> Sorry for noting this discussion late. Dave pointed it to me in a related
> thread [1].
>
> I had an impression that above approach fits some trusted IO arch (e.g.
> TDX Connect which has a special secure I/O page table format and
> requires sharing it between IOMMU/KVM) but not all.
>
> e.g. SEV-TIO spec [2] (page 8) describes to have the IOMMU walk the
> existing I/O page tables to get HPA and then verify it through a new
> permission table (RMP) for access control.

It is not possible, you cannot have the unsecure world control the
IOMMU translation and expect a secure guest.

The unsecure world can attack the guest by scrambling the mappings of
its private pages. A RMP does not protect against this.

This is why the secure world controls the CPU's GPA translation
exclusively, same reasoning for iommu.

Jason