From: Daniel Axtens <d...@axtens.net>
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 <d...@axtens.net>
Signed-off-by: Sudhakar Kuppusamy <sudha...@linux.ibm.com>
Reviewed-by: Stefan Berger <stef...@linux.ibm.com>
Reviewed-by: Avnish Chouhan <avn...@linux.ibm.com>
---
docs/grub.texi | 32 ++++++++++++++++++++++++++++++++
1 file changed, 32 insertions(+)
diff --git a/docs/grub.texi b/docs/grub.texi
index 0e877e026..19bbe9b2b 100644
--- a/docs/grub.texi
+++ b/docs/grub.texi
@@ -9262,6 +9262,38 @@ 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.
+@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:
+@example
+@group
+# 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
--
2.43.5
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