[PATCH v12 46/46] virt: sevguest: Add documentation for SEV-SNP CPUID Enforcement

[Brijesh Singh]

From: Michael Roth <michael.roth@xxxxxxx>

Update the documentation with information regarding SEV-SNP CPUID
Enforcement details and what sort of assurances it provides to guests.

Signed-off-by: Michael Roth <michael.roth@xxxxxxx>
Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
---
 Documentation/virt/coco/sevguest.rst | 29 ++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/Documentation/virt/coco/sevguest.rst b/Documentation/virt/coco/sevguest.rst
index 0f352056572d..48d66e10305b 100644
--- a/Documentation/virt/coco/sevguest.rst
+++ b/Documentation/virt/coco/sevguest.rst
@@ -118,6 +118,35 @@ be updated with the expected value.
 
 See GHCB specification for further detail on how to parse the certificate blob.
 
+3. 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 the 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), and those
+   binaries 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 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 the 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.
+
+
 Reference
 ---------
 
-- 
2.25.1