Re: Linux guest kernel threat model for Confidential Computing

[Jonathan Cameron]

On Thu, 26 Jan 2023 14:15:05 +0100
Samuel Ortiz <sameo@xxxxxxxxxxxx> 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@xxxxxxxxxxxx> wrote:
> >   
> > > Hi Lukas,
> > > 
> > > On Wed, Jan 25, 2023 at 11:03 PM Lukas Wunner <lukas@xxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxx) 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.

> 
> > Whether anyone builds that option isn't yet clear
> > though. If they do, Lukas' work should work there as well as for the
> > host OS. (Note I'm not a security expert so may be missing something!)
> > 
> > For extra fun, why should the device trust the host? Mutual authentication
> > fun (there are usecases where that matters)
> > 
> > There are way more complex options supported in PCIe TDISP (Tee Device
> > security interface protocols). Anyone have an visibility of open solutions
> > that make use of that? May be too new.  
> 
> It's still a PCI ECN, so quite new indeed.
> FWIW the rust spdm crate [1] implements the TDISP state machine.

Cool. thanks for the reference.
> 
> Cheers,
> Samuel.
> 
> [1] https://github.com/jyao1/rust-spdm
> >