Re: Linux guest kernel threat model for Confidential Computing

Samuel Ortiz
2023-01-26T13:32:14Z

On Thu, Jan 26, 2023 at 10:48:50AM +0000, Dr. David Alan Gilbert wrote:
> * Lukas Wunner (lukas@wunner.de) wrote:
> > [cc += Jonathan Cameron, linux-pci]
> >
> > On Wed, Jan 25, 2023 at 02:57:40PM +0000, Dr. David Alan Gilbert wrote:
> > > Greg Kroah-Hartman (gregkh@linuxfoundation.org) wrote:
> > > > Great, so why not have hardware attestation also for your devices you
> > > > wish to talk to? Why not use that as well? Then you don't have to
> > > > worry about anything in the guest.
> > >
> > > There were some talks at Plumbers where PCIe is working on adding that;
> > > it's not there yet though. I think that's PCIe 'Integrity and Data
> > > Encryption' (IDE - sigh), and PCIe 'Security Prtocol and Data Model' -
> > > SPDM. I don't know much of the detail of those, just that they're far
> > > enough off that people aren't depending on them yet.
> >
> > CMA/SPDM (PCIe r6.0 sec 6.31) is in active development on this branch:
> >
> > https://github.com/l1k/linux/commits/doe
>
> Thanks for the pointer - I'll go and hunt down that spec.
>
> > It will allow for authentication of PCIe devices. Goal is to submit
> > this quarter (Q1). Afterwards we'll look into retrieving measurements
> > via CMA/SPDM and bringing up IDE encryption.
> >
> > It's a kernel-native implementation which uses the existing crypto and
> > keys infrastructure and is wired into the appropriate places in the
> > PCI core to authenticate devices on enumeration and reauthenticate
> > when CMA/SPDM state is lost (after resume from D3cold, after a
> > Secondary Bus Reset and after a DPC-induced Hot Reset).
> >
> > The device authentication service afforded here is generic.
> > It is up to users and vendors to decide how to employ it,
> > be it for "confidential computing" or something else.
>
> As Samuel asks about who is doing the challenge; but I guess there are
> also things like what happens when the host controls intermediate
> switches

You'd want to protect that through IDE selective streams.

> and BAR access and when only VFs are passed to guests.

TDISP aims at addressing that afaiu. Once the VF (aka TDI) is locked,
any changes to its BAR(s) or any PF MMIO that would affect the VF would
get the VF back to unlocked (and let the guest reject it).

Cheers,
Samuel.