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[PATCH v3 03/19] docs/grub: Document signing grub with an appended signa

From: Daniel Axtens
Subject: [PATCH v3 03/19] docs/grub: Document signing grub with an appended signature
Date: Thu, 21 Apr 2022 17:46:58 +1000

Signing grub for firmware that verifies an appended signature is a
bit fiddly. I don't want people to have to figure it out from scratch
so document it here.

Signed-off-by: Daniel Axtens <>
Reviewed-by: Stefan Berger <>
 docs/grub.texi | 42 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 42 insertions(+)

diff --git a/docs/grub.texi b/docs/grub.texi
index fdf2884b814f..a7f5f6cd154c 100644
--- a/docs/grub.texi
+++ b/docs/grub.texi
@@ -6119,6 +6119,48 @@ 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.
+@section Signing GRUB with an appended signature
+The @file{core.img} itself can be signed with a Linux kernel module-style
+appended signature.
+To support IEEE1275 platforms where the boot image is often loaded directly
+from a disk partition rather than from a file system, the @file{core.img}
+can specify the size and location of the appended signature with an ELF
+note added by @command{grub-install}.
+An image can be signed this way using the @command{sign-file} command from
+the Linux kernel:
+# grub.key is your private key and certificate.der is your public key
+# Determine the size of the appended signature. It depends on the signing
+# certificate and the hash algorithm
+touch empty
+sign-file SHA256 grub.key certificate.der empty empty.sig
+SIG_SIZE=`stat -c '%s' empty.sig`
+rm empty empty.sig
+# Build a grub image with $SIG_SIZE reserved for the signature
+grub-install --appended-signature-size $SIG_SIZE --modules="..." ...
+# Replace the reserved size with a signature:
+# cut off the last $SIG_SIZE bytes with truncate's minus modifier
+truncate -s -$SIG_SIZE /boot/grub/powerpc-ieee1275/core.elf core.elf.unsigned
+# sign the trimmed file with an appended signature, restoring the correct size
+sign-file SHA256 grub.key certificate.der core.elf.unsigned core.elf.signed
+# Don't forget to install the signed image as required
+# (e.g. on powerpc-ieee1275, to the PReP partition)
+@end group
+@end example
+As with UEFI secure boot, it is necessary to build in the required modules,
+or sign them separately.
 @node Platform limitations
 @chapter Platform limitations

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