Re: [PATCH v4 1/3] x86/tdx: Add TDX Guest attestation interface driver

[Sathyanarayanan Kuppuswamy]

On 4/26/22 9:28 PM, Kai Huang wrote:
> > How about the following summary? It includes important notes mentioned
> > by you and some more driver info.
>
> Yes fine to me, except minor comments below:
>
> > x86/tdx: Add TDX Guest attestation interface driver
> >
> > In TDX guest, attestation is used to verify the trustworthiness of a TD
> > to other entities before provisioning secrets to the TD.
> >
> > One usage example is, when a TD guest uses encrypted drive and if the
> > decryption keys required to access the drive are stored in a secure 3rd
> > party key server, the key server can use attestation to verify TD's
> > trustworthiness and release the decryption keys to the TD.
> >
> > The attestation process consists of two steps: TDREPORT generation and
> > Quote generation.
> >
> > TDREPORT (TDREPORT_STRUCT) is a fixed-size data structure generated by
> > the TDX module which contains TD-specific information (such as TD
> > measurements), platform security version, and the MAC to protect the
> > integrity of the TDREPORT. The TD kernel uses TDCALL[TDG.MR.REPORT] to
> > get the TDREPORT from the TDX module. A user-provided 64-Byte
> > REPORTDATA is used as input and included in the TDREPORT. Typically it
> > can be some nonce provided by attestation service so the TDREPORT can
> > be verified uniquely. More details about TDREPORT can be found in
> > Intel TDX Module specification, section titled "TDG.MR.REPORT Leaf".
> >
> > After getting the TDREPORT, the second step of the attestation process
> > is to send the TDREPORT to Quoting Enclave (QE) or Quote Generation
> > Service (QGS) to generate the Quote. However, the method of sending the
> > TDREPORT to QE/QGS, communication channel used and data format used is
> > specific to the implementation of QE/QGS.
> >
> > A typical implementation is, TD userspace attestation software gets the
> > TDREPORT from TD kernel, sends it to QE/QGS, and QE/QGS returns the
> > Quote. TD attestation software can use any available communication
> > channel to talk to QE/QGS, such as using vsock and tcp/ip.
> >
> > To support the case that those communication channels are not directly
> > available to the TD, TDX also defines TDVMCALL
> > (TDG.VP.VMCALL<GetQuote>) to allow TD to ask VMM to help with sending
> > the TDREPORT and receiving the Quote. This support is documented in the
> > GHCI spec section titled "5.4 TD attestation".
>
> I intentionally omitted to mention TDG.VP.VMCALL<GetQuote> as I personally
> believe there are still couple issues around GetQuote that we haven't discussed
> thoroughly (timeout, etc).  I am still considering whether we should change GHCI
> to use TDG.VP.VMCALL<Service> defined in GHCI 1.5 for attestation.  And the name
> of TDVMCALL doesn't actually matter here, so I think we don't need to mention
> GetQuote here but just say we have TDVMCALLs for that.

Ok.

> > Implement a basic attestation driver to allow TD userspace to get the
> > TDREPORT, which is sent to QE by the attestation software to generate
> > a Quote for remote verification. Add a wrapper function
> > (tdx_mcall_tdreport()) to get the TDREPORT from the TDX Module. This
> > API will be used by the interface driver to request for TDREPORT.
>
> I don't think you need to mention tdx_mcall_tdreport().

Ok. Will remove it.

> > Also note that explicit access permissions are not enforced in this
> > driver because the quote and measurements are not a secret. However
> > the access permissions of the device node can be used to set any
> > desired access policy. The udev default is usually root access
> > only.

--
Sathyanarayanan Kuppuswamy
Linux Kernel Developer