virt: Add TDX guest driver

TDX guest driver exposes IOCTL interfaces to service TDX guest
user-specific requests. Currently, it is only used to allow the user to
get the TDREPORT to support TDX attestation.

Details about the TDX attestation process are documented in
Documentation/x86/tdx.rst, and the IOCTL details are documented in
Documentation/virt/coco/tdx-guest.rst.

Operations like getting TDREPORT involves sending a blob of data as
input and getting another blob of data as output. It was considered
to use a sysfs interface for this, but it doesn't fit well into the
standard sysfs model for configuring values. It would be possible to
do read/write on files, but it would need multiple file descriptors,
which would be somewhat messy. IOCTLs seem to be the best fitting
and simplest model for this use case. The AMD sev-guest driver also
uses the IOCTL interface to support attestation.

[Bagas Sanjaya: Ack is for documentation portion]
Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Acked-by: Kai Huang <kai.huang@intel.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Wander Lairson Costa <wander@redhat.com>
Link: https://lore.kernel.org/all/20221116223820.819090-3-sathyanarayanan.kuppuswamy%40linux.intel.com

1 files changed, 43 insertions, 0 deletions

diff --git a/Documentation/x86/tdx.rst b/Documentation/x86/tdx.rst
index b8fa4329e1a5..dc8d9fd2c3f7 100644
--- a/Documentation/x86/tdx.rst
+++ b/Documentation/x86/tdx.rst
@@ -210,6 +210,49 @@ converted to shared on boot.
 For coherent DMA allocation, the DMA buffer gets converted on the
 allocation. Check force_dma_unencrypted() for details.
 
+Attestation
+===========
+
+Attestation is used to verify the TDX guest trustworthiness to other
+entities before provisioning secrets to the guest. For example, a key
+server may want to use attestation to verify that the guest is the
+desired one before releasing the encryption keys to mount the encrypted
+rootfs or a secondary drive.
+
+The TDX module records the state of the TDX guest in various stages of
+the guest boot process using the build time measurement register (MRTD)
+and runtime measurement registers (RTMR). Measurements related to the
+guest initial configuration and firmware image are recorded in the MRTD
+register. Measurements related to initial state, kernel image, firmware
+image, command line options, initrd, ACPI tables, etc are recorded in
+RTMR registers. For more details, as an example, please refer to TDX
+Virtual Firmware design specification, section titled "TD Measurement".
+At TDX guest runtime, the attestation process is used to attest to these
+measurements.
+
+The attestation process consists of two steps: TDREPORT generation and
+Quote generation.
+
+TDX guest uses TDCALL[TDG.MR.REPORT] to get the TDREPORT (TDREPORT_STRUCT)
+from the TDX module. TDREPORT is a fixed-size data structure generated by
+the TDX module which contains guest-specific information (such as build
+and boot measurements), platform security version, and the MAC to protect
+the integrity of the TDREPORT. 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 the 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 it to the Quoting Enclave (QE) to generate the Quote. TDREPORT
+by design can only be verified on the local platform as the MAC key is
+bound to the platform. To support remote verification of the TDREPORT,
+TDX leverages Intel SGX Quoting Enclave to verify the TDREPORT locally
+and convert it to a remotely verifiable Quote. Method of sending TDREPORT
+to QE is implementation specific. Attestation software can choose
+whatever communication channel available (i.e. vsock or TCP/IP) to
+send the TDREPORT to QE and receive the Quote.
+
 References
 ==========