Documentation: syfs-class-firmware-attributes: Lenovo Certificate support

Certificate based authentication is available as an alternative to
password based authentication.

The WMI commands are cryptographically signed using a separate
signing server and will be verified by the BIOS before being
accepted.

This commit details the fields that are needed to support that
implementation. At present the changes are intended for Lenovo
platforms, but have been designed to keep them as flexible as possible
for future implementations from other vendors.
Signed-off-by: Mark Pearson <markpearson@lenovo.com>
Link: https://lore.kernel.org/r/20220317214008.3459-1-markpearson@lenovo.comReviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>

Documentation/ABI/testing/sysfs-class-firmware-attributes

@@ -246,6 +246,51 @@ Description:
 					that is being referenced (e.g hdd0, hdd1 etc)
 					This attribute defaults to device 0.
 
+		certificate:
+		signature:
+		save_signature:
+					These attributes are used for certificate based authentication. This is
+					used in conjunction with a signing server as an alternative to password
+					based authentication.
+					The user writes to the attribute(s) with a BASE64 encoded string obtained
+					from the signing server.
+					The attributes can be displayed to check the stored value.
+
+					Some usage examples:
+					Installing a certificate to enable feature:
+						echo <supervisor password > authentication/Admin/current_password
+						echo <signed certificate> > authentication/Admin/certificate
+
+					Updating the installed certificate:
+						echo <signature> > authentication/Admin/signature
+						echo <signed certificate> > authentication/Admin/certificate
+
+					Removing the installed certificate:
+						echo <signature> > authentication/Admin/signature
+						echo '' > authentication/Admin/certificate
+
+					Changing a BIOS setting:
+						echo <signature> > authentication/Admin/signature
+						echo <save signature> > authentication/Admin/save_signature
+						echo Enable > attribute/PasswordBeep/current_value
+
+					You cannot enable certificate authentication if a supervisor password
+					has not been set.
+					Clearing the certificate results in no bios-admin authentication method
+					being configured allowing anyone to make changes.
+					After any of these operations the system must reboot for the changes to
+					take effect.
+
+		certificate_thumbprint:
+					Read only attribute used to display the MD5, SHA1 and SHA256 thumbprints
+					for the certificate installed in the BIOS.
+
+		certificate_to_password:
+					Write only attribute used to switch from certificate based authentication
+					back to password based.
+					Usage:
+						echo <signature> > authentication/Admin/signature
+						echo <password> > authentication/Admin/certificate_to_password
 
 
 What:		/sys/class/firmware-attributes/*/attributes/pending_reboot