From f7b9580133cf346d77f345d175fa5cb8a591be16 Mon Sep 17 00:00:00 2001
From: Daniel Axtens <dja@axtens.net>
Date: Sat, 15 Aug 2020 02:00:57 +1000
Subject: [PATCH 05/23] docs/grub: Document signing grub under UEFI

Before adding information about how grub is signed with an appended
signature scheme, it's worth adding some information about how it
can currently be signed for UEFI.

Signed-off-by: Daniel Axtens <dja@axtens.net>
---
 docs/grub.texi | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

--- a/docs/grub.texi
+++ b/docs/grub.texi
@@ -6345,6 +6345,7 @@
 * Secure Boot Advanced Targeting::   Embedded information for generation number based revocation
 * Measured Boot::                    Measuring boot components
 * Lockdown::                         Lockdown when booting on a secure setup
+* Signing GRUB itself::              Ensuring the integrity of the GRUB core image
 @end menu
 
 @node Authentication and authorisation
@@ -6423,7 +6424,7 @@
 
 GRUB's @file{core.img} can optionally provide enforcement that all files
 subsequently read from disk are covered by a valid digital signature.
-This document does @strong{not} cover how to ensure that your
+This section does @strong{not} cover how to ensure that your
 platform's firmware (e.g., Coreboot) validates @file{core.img}.
 
 If environment variable @code{check_signatures}
@@ -6586,6 +6587,25 @@
 The @samp{lockdown} variable is set to @samp{y} when the GRUB is locked down.
 Otherwise it does not exit.
 
+@node Signing GRUB itself
+@section Signing GRUB itself
+
+To ensure a complete secure-boot chain, there must be a way for the code that
+loads GRUB to verify the integrity of the core image.
+
+This is ultimately platform-specific and individual platforms can define their
+own mechanisms. However, there are general-purpose mechanisms that can be used
+with GRUB.
+
+@section Signing GRUB for UEFI secure boot
+
+On UEFI platforms, @file{core.img} is a PE binary. Therefore, it can be signed
+with a tool such as @command{pesign} or @command{sbsign}. Refer to the
+suggestions in @pxref{UEFI secure boot and shim} to ensure that the final
+image works under UEFI secure boot and can maintain the secure-boot chain. It
+will also be necessary to enrol the public key used into a relevant firmware
+key database.
+
 @node Platform limitations
 @chapter Platform limitations
 
