[PATCH v10 20/22] docs/grub: Document signing GRUB under UEFI

[Sudhakar Kuppusamy <sudhakar@linux.ibm.com>]

From: 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>
Signed-off-by: Sudhakar Kuppusamy <sudhakar@linux.ibm.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
---
 docs/grub.texi | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/docs/grub.texi b/docs/grub.texi
index 15f087f27..72ee8d08c 100644
--- a/docs/grub.texi
+++ b/docs/grub.texi
@@ -8675,6 +8675,7 @@ environment variables and commands are listed in the same order.
 * Measured Boot::                      Measuring boot components
 * Lockdown::                           Lockdown when booting on a secure setup
 * TPM2 key protector::                 Managing disk key with TPM2 key protector
+* Signing GRUB itself::                Ensuring the integrity of the GRUB core image
 @end menu
 
 @node Authentication and authorisation
@@ -8755,7 +8756,7 @@ 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}
@@ -9363,6 +9364,21 @@ which increases the risk of password leakage during the process. Moreover, the
 superuser list must be well maintained, and the password used cannot be
 synchronized with LUKS key rotation.
 
+@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 enroll the public key used into a relevant firmware
+key database.
+
 @node Platform limitations
 @chapter Platform limitations
 
-- 
2.39.5 (Apple Git-154)


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