On 10/2/25 8:31 AM, Sudhakar Kuppusamy wrote:
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 <[email protected]>
Signed-off-by: Sudhakar Kuppusamy <[email protected]>
Reviewed-by: Avnish Chouhan <[email protected]>
Reviewed-by: Daniel Kiper <[email protected]>
---
docs/grub.texi | 94 ++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 94 insertions(+)
diff --git a/docs/grub.texi b/docs/grub.texi
index 358975fc8..b87a32ea9 100644
--- a/docs/grub.texi
+++ b/docs/grub.texi
@@ -9611,6 +9611,100 @@ 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.elf} itself can be signed with a Linux kernel module-style
+appended signature (@pxref{Using appended signatures}).
+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.elf}
+can specify the size and location of the appended signature with an ELF
+Note added by @command{grub-install} or @command{grub-mkimage}.
+An image can be signed this way using the @command{sign-file} command from
+the Linux kernel:
+
+@itemize
+@item Signing a GRUB image using a single signer key. The grub.key is your
+private key used for GRUB signing, grub.der is a corresponding public key
+(certificate) used for GRUB signature verification, and the kernel.der is
+your public key (certificate) used for kernel signature verification.
+@example
+@group
+# Determine the size of the appended signature. It depends on the
+# signing key and the hash algorithm.
+#
+# Signing the /dev/null with an appended signature.
Nit: Remove the 'the' before /dev/null.
+
+sign-file SHA256 grub.key grub.der /dev/null ./empty.sig
+
+# Build a GRUB image for the signature.
+
+grub-mkimage -O powerpc-ieee1275 -o core.elf.unsigned -x kernel.der \
+ -p /grub --appended-signature-size $(stat -c '%s' ./empty.sig) \
+ --modules="appendedsig ..." ...
+
+# Remove the empty file.
I think the file has not 0 bytes (empty) but has a real signature, so
say 'Remove the signature file.'
+
+rm ./empty.sig
+
+# Signing a GRUB image with an appended signature.
+
+sign-file SHA256 grub.key grub.der core.elf.unsigned core.elf.signed
+
+@end group
+@end example
+@item Signing a GRUB image using more than one signer key. The grub1.key and
+grub2.key are private keys used for GRUB signing, grub1.der and grub2.der
+are corresponding public keys (certificates) used for GRUB signature
verification.
+The kernel1.der and kernel2.der are your public keys (certificates) used for
+kernel signature verification.
+@example
+@group
+# Generate a raw signature for /dev/null signing using OpenSSL.
# Generate a signature by signing /dev/null.
+
+openssl cms -sign -binary -nocerts -in /dev/null -signer \
+ grub1.der -inkey grub1.key -signer grub2.der -inkey grub2.key \
+ -out ./empty.p7s -outform DER -noattr -md sha256
+
+# Signing the /dev/null with an appended signature.
Nit: Remove the 'the' before /dev/null.
You are not signing 'with' an appended signature but you want to 'get'
and appended signature I suppose.
# To be able to determine the size of an appended signature, sign an
# empty file (/dev/null) to which a signature will be appended to.
+
+sign-file -s ./empty.p7s sha256 /dev/null /dev/null ./empty.sig
+
+# Build a GRUB image for the signature.
+
+grub-mkimage -O powerpc-ieee1275 -o core.elf.unsigned -x kernel1.der \
+ kernel2.der -p /grub --appended-signature-size $(stat -c '%s' ./empty.sig) \
+ --modules="appendedsig ..." ...
+
+# Remove the empty files.
Remove the signature files.
+
+rm ./empty.sig ./empty.p7s
+
+# Generate a raw signature for GRUB image signing using OpenSSL.
+
+openssl cms -sign -binary -nocerts -in core.elf.unsigned -signer \
+ grub1.der -inkey grub1.key -signer grub2.der -inkey grub2.key \
+ -out core.p7s -outform DER -noattr -md sha256
+
+# Signing a GRUB image with an appended signature.
# Sign a GRUB image to get an image file with an appended signature.
+
+sign-file -s core.p7s sha256 /dev/null core.elf.unsigned core.elf.signed
+
+@end group
+@end example
+@item Don't forget to install the signed image as required
+(e.g. on powerpc-ieee1275, to the PReP partition).
+@example
+@group
+# Install signed GRUB image to the PReP partition on powerpc-ieee1275
+
+dd if=core.elf.signed of=/dev/sda1
+
+@end group
+@end example
+@end itemize
+
+As with UEFI secure boot, it is necessary to build-in the required modules,
+or sign them if they are not part of the GRUB image.
> +> @node Platform limitations
@chapter Platform limitations
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