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67 lines
2.4 KiB
67 lines
2.4 KiB
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 365d1d6931b..afbde7c1f7b 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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