[PATCH 2/3] docs/grub: Document signing grub under UEFI

[Daniel Axtens <dja@axtens.net>]

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(-)

diff --git a/docs/grub.texi b/docs/grub.texi
index 1ce9993a53fc..35da48456d9e 100644
--- a/docs/grub.texi
+++ b/docs/grub.texi
@@ -5736,6 +5736,7 @@ environment variables and commands are listed in the same order.
 * Using digital signatures::         Booting digitally signed code
 * UEFI secure boot and shim::        Booting digitally signed PE files
 * Measured Boot::                    Measuring boot components
+* Signing GRUB itself::              Ensuring the integrity of the GRUB core image
 @end menu
 
 @node Authentication and authorisation
@@ -5814,7 +5815,7 @@ commands.
 
 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}
@@ -5950,6 +5951,25 @@ into @file{core.img} in order to avoid a potential gap in measurement between
 
 Measured boot is currently only supported on EFI platforms.
 
+@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
 
-- 
2.25.1