[v8,28/40] KVM: SEV: Add documentation for SEV-SNP CPUID Enforcement

Commit Message

Brijesh Singh Dec. 10, 2021, 3:43 p.m. UTC

From: Michael Roth <[e-mail]>

Update the documentation with SEV-SNP CPUID enforcement.

Signed-off-by: Michael Roth <[e-mail]>
Signed-off-by: Brijesh Singh <[e-mail]>
---
 .../virt/kvm/amd-memory-encryption.rst        | 28 +++++++++++++++++++
 1 file changed, 28 insertions(+)

Comments

Borislav Petkov Jan. 7, 2022, 1:22 p.m. UTC | #1

On Fri, Dec 10, 2021 at 09:43:20AM -0600, Brijesh Singh wrote:
> From: Michael Roth <[e-mail]>
> 
> Update the documentation with SEV-SNP CPUID enforcement.
> 
> Signed-off-by: Michael Roth <[e-mail]>
> Signed-off-by: Brijesh Singh <[e-mail]>
> ---
>  .../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
						 ^
						 the

> +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.

Unknown word [binares] in Documentation.
Suggestions: ['binaries', 'Linares', 'bi nares', 'bi-nares', 'bin ares', 'bin-ares', 'nares']

Also:

s/whose binaries/and those binaries/

> + - Its initial state will be encrypted/pvalidated, so attempts to modify
> +   it during run-time will be result in garbage being written, or #VC

s/be //

> +   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
				      ^
				      the

> +   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
>

Patch

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
 ==========