[tip: x86/sev] virt: sevguest: Add documentation for SEV-SNP CPUID Enforcement

["tip-bot2 for Michael Roth" <tip-bot2@linutronix.de>]

The following commit has been merged into the x86/sev branch of tip:

Commit-ID:     92a99584d965b930988b28f36d925bd9675828b3
Gitweb:        https://git.kernel.org/tip/92a99584d965b930988b28f36d925bd9675828b3
Author:        Michael Roth <michael.roth@amd.com>
AuthorDate:    Thu, 24 Feb 2022 10:56:25 -06:00
Committer:     Borislav Petkov <bp@suse.de>
CommitterDate: Thu, 07 Apr 2022 16:47:12 +02:00

virt: sevguest: Add documentation for SEV-SNP CPUID Enforcement

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@amd.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20220307213356.2797205-47-brijesh.singh@amd.com
---
 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 625de22..bf593e8 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
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