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

[Tian, Kevin]

> From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Sent: Friday, August 2, 2024 7:22 PM
> 
> On Fri, Aug 02, 2024 at 08:26:48AM +0000, Tian, Kevin wrote:
> > > From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> > > 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.
> 

According to [3],

"
  With SNP, when pages are marked as guest-owned in the RMP table,
  they are assigned to a specific guest/ASID, as well as a specific GFN
  with in the guest. Any attempts to map it in the RMP table to a different
  guest/ASID, or a different GFN within a guest/ASID, will result in an RMP
  nested page fault.
"

With that measure in place my impression is that even the CPU's GPA
translation can be controlled by the unsecure world in SEV-SNP.

[3] https://lore.kernel.org/all/20240501085210.2213060-1-michael.roth@xxxxxxx/