Re: Linux guest kernel threat model for Confidential Computing

Samuel Ortiz
2023-01-27T07:02:21Z

On Thu, Jan 26, 2023 at 04:07:29PM +0000, Jonathan Cameron wrote:
> On Thu, 26 Jan 2023 14:15:05 +0100
> Samuel Ortiz <sameo@rivosinc.com> wrote:
>
> > On Thu, Jan 26, 2023 at 10:58:47AM +0000, Jonathan Cameron wrote:
> > > On Thu, 26 Jan 2023 10:24:32 +0100
> > > Samuel Ortiz <sameo@rivosinc.com> wrote:
> > >
> > > > Hi Lukas,
> > > >
> > > > On Wed, Jan 25, 2023 at 11:03 PM 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
> > > >
> > > > Nice, thanks a lot for that.
> > > >
> > > >
> > > >
> > > > > 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.
> > > > >
> > > > > Trusted root certificates to validate device certificates can be
> > > > > installed into a kernel keyring using the familiar keyctl(1) utility,
> > > > > but platform-specific roots of trust (such as a HSM) could be
> > > > > supported as well.
> > > > >
> > > >
> > > > This may have been discussed at LPC, but are there any plans to also
> > > > support confidential computing flows where the host kernel is not part
> > > > of the TCB and would not be trusted for validating the device cert chain
> > > > nor for running the SPDM challenge?
> > >
> > > There are lots of possible models for this. One simple option if the assigned
> > > VF supports it is a CMA instance per VF. That will let the guest
> > > do full attestation including measurement of whether the device is
> > > appropriately locked down so the hypervisor can't mess with
> > > configuration that affects the guest (without a reset anyway and that
> > > is guest visible).
> >
> > So the VF would be directly assigned to the guest, and the guest kernel
> > would create a CMA instance for the VF, and do the SPDM authentication
> > (based on a guest provided trusted root certificate). I think one
> > security concern with that approach is assigning the VF to the
> > (potentially confidential) guest address space without the guest being
> > able to attest of the device trustworthiness first. That's what TDISP is
> > aiming at fixing (establish a secure SPDM between the confidential guest
> > and the device, lock the device from the guest, attest and then enable
> > DMA).
>
> Agreed, TDISP is more comprehensive, but also much more complex with
> more moving parts that we don't really have yet.
>
> Depending on your IOMMU design (+ related stuff) and interaction with
> the secure guest, you might be able to block any rogue DMA until
> after attestation / lock down checks even if the Hypervisor was letting
> it through.

Provided that the guest or, in the TDX and AP-TEE cases, the TSM have
protected access to the IOMMU, yes. But then the implementation becomes
platform specific.

Cheers,
Samuel.