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From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
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From: Daniel Axtens <dja@axtens.net>
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Date: Sat, 15 Aug 2020 02:19:36 +1000
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Subject: [PATCH] docs/grub: Document signing grub with an appended signature
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Signing grub for firmware that verifies an appended signature is a
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bit fiddly. I don't want people to have to figure it out from scratch
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so document it here.
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Signed-off-by: Daniel Axtens <dja@axtens.net>
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---
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docs/grub.texi | 42 ++++++++++++++++++++++++++++++++++++++++++
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1 file changed, 42 insertions(+)
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diff --git a/docs/grub.texi b/docs/grub.texi
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index 365d1d6931..afbde7c1f7 100644
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--- a/docs/grub.texi
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+++ b/docs/grub.texi
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@@ -6087,6 +6087,48 @@ image works under UEFI secure boot and can maintain the secure-boot chain. It
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will also be necessary to enrol the public key used into a relevant firmware
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key database.
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+@section Signing GRUB with an appended signature
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+
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+The @file{core.elf} itself can be signed with a Linux kernel module-style
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+appended signature.
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+
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+To support IEEE1275 platforms where the boot image is often loaded directly
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+from a disk partition rather than from a file system, the @file{core.elf}
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+can specify the size and location of the appended signature with an ELF
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+note added by @command{grub-install}.
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+
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+An image can be signed this way using the @command{sign-file} command from
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+the Linux kernel:
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+
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+@example
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+@group
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+# grub.key is your private key and certificate.der is your public key
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+
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+# Determine the size of the appended signature. It depends on the signing
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+# certificate and the hash algorithm
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+touch empty
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+sign-file SHA256 grub.key certificate.der empty empty.sig
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+SIG_SIZE=`stat -c '%s' empty.sig`
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+rm empty empty.sig
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+
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+# Build a grub image with $SIG_SIZE reserved for the signature
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+grub-install --appended-signature-size $SIG_SIZE --modules="..." ...
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+
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+# Replace the reserved size with a signature:
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+# cut off the last $SIG_SIZE bytes with truncate's minus modifier
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+truncate -s -$SIG_SIZE /boot/grub/powerpc-ieee1275/core.elf core.elf.unsigned
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+# sign the trimmed file with an appended signature, restoring the correct size
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+sign-file SHA256 grub.key certificate.der core.elf.unsigned core.elf.signed
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+
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+# Don't forget to install the signed image as required
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+# (e.g. on powerpc-ieee1275, to the PReP partition)
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+@end group
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+@end example
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+
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+As with UEFI secure boot, it is necessary to build in the required modules,
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+or sign them separately.
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+
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+
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@node Platform limitations
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@chapter Platform limitations
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