diff --git a/docs/kbase/launch_security_sev.rst b/docs/kbase/launch_security_sev.rst index 2734832487..7f692af748 100644 --- a/docs/kbase/launch_security_sev.rst +++ b/docs/kbase/launch_security_sev.rst @@ -206,6 +206,20 @@ libvirt to the correct OVMF binary. ... +If intending to attest the boot measurement, it is required to use a +firmware binary that is stateless, as persistent NVRAM can undermine +the trust of the secure guest. This is achieved by telling libvirt +that a stateless binary is required + +:: + + ... + + hvm + + + ... + Memory ------ @@ -373,6 +387,97 @@ running: # dmesg | grep -i sev AMD Secure Encrypted Virtualization (SEV) active +Guest attestation for SEV/SEV-ES from a trusted host +==================================================== + +Before a confidential guest is used, it may be desirable to attest the boot +measurement. To be trustworthy the attestation process needs to be performed +from a machine that is already trusted. This would typically be a physical +machine that the guest owner controls, or could be a previously launched +confidential guest that has already itself been attested. Most notably, it is +**not** possible to securely attest a guest from the hypervisor host itself, +as the goal of the attestation process is to detect whether the hypervisor is +malicious. + +Performing an attestation requires that the ```` element is +configured with a guest owner Diffie-Hellman (DH) certificate, and a session +data blob. These must be unique for every guest launch attempt. Any reuse will +open avenues of attack for the hypervisor to fake the measurement. Unique data +can be generated using the `sevctl `_ tool. + +First of all the Platform Diffie-Hellman key (PDH) for the hypervisor host +needs to be obtained. The PDH is used to negotiate a master secret between +the SEV firmware and external entities. + +The admin of the hypervisor can extract the PDH using:: + + $ sevctl export --full ${hostname}.pdh + +Upon receiving the PDH associated with the hypervisor, the guest owner should +validate its integrity:: + + $ sevctl verify --sev ${hostname}.pdh + PDH EP384 D256 008cec87d6bd9df67a35e7d6057a933463cd8a02440f60c5df150821b5662ee0 + ⬑ PEK EP384 E256 431ba88424378200d58b6fb5db9657268c599b1be25f8047ac2e2981eff667e6 + •⬑ OCA EP384 E256 b4f1d0a2144186d1aa9c63f19039834e729f508000aa05a76ba044f8e1419765 + ⬑ CEK EP384 E256 22c27ee3c1c33287db24d3c06869a5ae933eb44148fdb70838019e267077c6b8 + ⬑ ASK R4096 R384 d8cd9d1798c311c96e009a91552f17b4ddc4886a064ec933697734965b9ab29db803c79604e2725658f0861bfaf09ad4 + •⬑ ARK R4096 R384 3d2c1157c29ef7bd4207fc0c8b08db080e579ceba267f8c93bec8dce73f5a5e2e60d959ac37ea82176c1a0c61ae203ed + + • = self signed, ⬑ = signs, •̷ = invalid self sign, ⬑̸ = invalid signs + +Assuming this is successful, it is now possible to generate a unique launch +data for the guest boot attempt:: + + $ sevctl session --name ${myvmname} ${hostname}.pdh ${policy} + +This will generate four files + + * ``${myvmname}_tik.bin`` + * ``${myvmname}_tek.bin`` + * ``${myvmname}_godh.bin`` + * ``${myvmname}_session.bin`` + +The ``tik.bin`` and ``tek.bin`` files will be needed to perform the boot +attestation, and must be kept somewhere secure, away from the hypervisor +host. + +The ``godh.bin`` file contents should be copied into the ```` field +in the ```` configuration, while the ``session.bin`` file +contents should be copied into the ```` field. + +When launching the guest, it should be set to remain in the paused state with +no vCPUs running:: + + $ virsh start --paused ${myvmname} + +With it launched, it is possible to query the launch measurement:: + + $ virsh domlaunchsecinfo ${myvmname} + sev-measurement: LMnv8i8N2QejezMPkscShF0cyPYCslgUoCxGWRqQuyt0Q0aUjVkH/T6NcmkwZkWp + sev-api-major : 0 + sev-api-minor : 24 + sev-build-id : 15 + sev-policy : 3 + +The techiques required to validate the measurement reported are beyond the +scope of this document. Fortunately, libvirt provides a tool that can be used +to perform this validation:: + + $ virt-qemu-sev-validate \ + --measurement LMnv8i8N2QejezMPkscShF0cyPYCslgUoCxGWRqQuyt0Q0aUjVkH/T6NcmkwZkWp + --api-major 0 + --api-minor 24 + --build-id 15 + --policy 3 + --tik ${myvmname}_tik.bin + --tek ${myvmname}_tek.bin + OK: Looks good to me + +The `man page <../manpages/virt-qemu-sev-validate.html>`__ for +``virt-qemu-sev-validate`` outlines a great many other ways to invoke this +tool. + Limitations ===========