[PATCH v7 33/45] KVM: SEV: Add documentation for SEV-SNP CPUID Enforcement

Brijesh Singh
2021-11-10T22:09:41Z

From: Michael Roth <michael.roth@amd.com>

Update the documentation with SEV-SNP CPUID enforcement.

Signed-off-by: Michael Roth <michael.roth@amd.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
---
.../virt/kvm/amd-memory-encryption.rst | 28 +++++++++++++++++++
1 file changed, 28 insertions(+)

diff --git a/Documentation/virt/kvm/amd-memory-encryption.rst b/Documentation/virt/kvm/amd-memory-encryption.rst
index 5c081c8c7164..aa8292fa579a 100644
--- a/Documentation/virt/kvm/amd-memory-encryption.rst
+++ b/Documentation/virt/kvm/amd-memory-encryption.rst
@@ -427,6 +427,34 @@ issued by the hypervisor to make the guest ready for execution.

Returns: 0 on success, -negative on error

+SEV-SNP CPUID Enforcement
+=========================
+
+SEV-SNP guests can access a special page that contains a table of CPUID values
+that have been validated by the PSP as part of SNP_LAUNCH_UPDATE firmware
+command. It provides the following assurances regarding the validity of CPUID
+values:
+
+ - Its address is obtained via bootloader/firmware (via CC blob), whose
+ binares will be measured as part of the SEV-SNP attestation report.
+ - Its initial state will be encrypted/pvalidated, so attempts to modify
+ it during run-time will be result in garbage being written, or #VC
+ exceptions being generated due to changes in validation state if the
+ hypervisor tries to swap the backing page.
+ - Attempts to bypass PSP checks by hypervisor by using a normal page, or a
+ non-CPUID encrypted page will change the measurement provided by the
+ SEV-SNP attestation report.
+ - The CPUID page contents are *not* measured, but attempts to modify the
+ expected contents of a CPUID page as part of guest initialization will be
+ gated by the PSP CPUID enforcement policy checks performed on the page
+ during SNP_LAUNCH_UPDATE, and noticeable later if the guest owner
+ implements their own checks of the CPUID values.
+
+It is important to note that this last assurance is only useful if the kernel
+has taken care to make use of the SEV-SNP CPUID throughout all stages of boot.
+Otherwise guest owner attestation provides no assurance that the kernel wasn't
+fed incorrect values at some point during boot.
+
References
==========

--
2.25.1