Re: [PATCH v2 06/22] docs/grub: Document signing grub with an appended signature

[Stefan Berger <stefanb@linux.ibm.com>]

On 6/30/21 4:40 AM, Daniel Axtens 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 <dja@axtens.net>
> ---
>   docs/grub.texi | 42 ++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 42 insertions(+)
>
> diff --git a/docs/grub.texi b/docs/grub.texi
> index 2ffc3b417312..bed565371460 100644
> --- a/docs/grub.texi
> +++ b/docs/grub.texi
> @@ -6055,6 +6055,48 @@ 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:
> +
> +@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
>   


Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>