[RFC PATCH v3 8/8] tools: gen_pre_load_header.sh: initial import

[Rasmus Villemoes]

On 17/11/2021 18.52, Philippe Reynes wrote:
> This commit adds a script gen_pre_load_header.sh
> that generate the header used by the image pre-load
> stage.
> 
> Signed-off-by: Philippe Reynes <philippe.reynes at softathome.com>
> ---
>  tools/gen_pre_load_header.sh | 174 +++++++++++++++++++++++++++++++++++
>  1 file changed, 174 insertions(+)
>  create mode 100755 tools/gen_pre_load_header.sh
> 
> diff --git a/tools/gen_pre_load_header.sh b/tools/gen_pre_load_header.sh
> new file mode 100755
> index 0000000000..8256fa80ee
> --- /dev/null
> +++ b/tools/gen_pre_load_header.sh
> @@ -0,0 +1,174 @@
> +#!/bin/bash
> +# SPDX-License-Identifier: GPL-2.0+
> +
> +#
> +# default value
> +#
> +size='4096'
> +algo='sha256,rsa2048'
> +padding='pkcs-1.5'
> +key=''
> +verbose='false'
> +input=''
> +output=''
> +
> +usage() {
> +	printf "Usage: $0 -a <algo> -k <key> [-p <padding>] [-s <size>] [-v] -i <input> -o <output>\n"
> +}
> +
> +#
> +# parse arguments
> +#
> +while getopts 'a:hi:k:o:p:s:v' flag; do
> +	case "${flag}" in
> +		a) algo="${OPTARG}" ;;
> +		h) usage
> +		   exit 0 ;;
> +		i) input="${OPTARG}" ;;
> +		k) key="${OPTARG}" ;;
> +		o) output="${OPTARG}" ;;
> +		p) padding="${OPTARG}" ;;
> +		s) size="${OPTARG}" ;;
> +		v) verbose='true' ;;
> +		*) usage
> +		   exit 1 ;;
> +	esac
> +done
> +
> +#
> +# check that mandatory arguments are provided
> +#
> +if [ -z "$key" -o -z "$input" -o -z "$output" ]
> +then
> +	usage
> +	exit 0
> +fi
> +
> +hash=$(echo $algo | cut -d',' -f1)
> +sign=$(echo $algo | cut -d',' -f2)
> +
> +echo "status:"
> +echo "size    = $size"
> +echo "algo    = $algo"
> +echo "hash    = $hash"
> +echo "sign    = $sign"
> +echo "padding = $padding"
> +echo "key     = $key"
> +echo "verbose = $verbose"
> +
> +#
> +# check if input file exist
> +#
> +if [ ! -f "$input" ]
> +then
> +	echo "Error: file '$input' doesn't exist"
> +	exit 1
> +fi
> +
> +#
> +# check if output is not empty
> +#
> +if [ -z "$output" ]
> +then
> +	echo "Error: output is empty"
> +	exit 1
> +fi
> +
> +#
> +# check that size is bigger than 0
> +#
> +if [ $size -le 0 ]
> +then
> +	echo "Error: $size lower than 0"
> +	exit 1
> +fi
> +
> +#
> +# check if the key file exist
> +#
> +if [ ! -f "$key" ]
> +then
> +	echo "Error: file $key doesn't exist\n"
> +	exit 1
> +fi
> +
> +#
> +# check if the hash is valid and supported
> +#
> +print_supported_hash() {
> +	echo "Supported hash:"
> +	echo "- sha1"
> +	echo "- sha256"
> +	echo "- sha384"
> +	echo "- sha512"
> +}
> +
> +case "$hash" in
> +	"sha1") hashOption="-sha1" ;;
> +	"sha256") hashOption="-sha256" ;;
> +	"sha384") hashOption="-sha384" ;;
> +	"sha512") hashOption="-sha512" ;;
> +	*) echo "Error: $hash is an invalid hash"
> +	   print_supported_hash
> +	   exit 1;;
> +esac
> +
> +#
> +# check if the sign is valid and supported
> +#
> +print_supported_sign() {
> +	echo "Supported sign:"
> +	echo "- rsa1024"
> +	echo "- rsa2048"
> +	echo "- rsa4096"
> +}
> +
> +case "$sign" in
> +	"rsa1024") ;;
> +	"rsa2048") ;;
> +	"rsa4096") ;;
> +	*) echo "Error: $sign is an invalid signature type"
> +	   print_supported_sign
> +	   exit 1;;
> +esac
> +
> +#
> +# check if the padding is valid and supported
> +#
> +print_supported_padding() {
> +	echo "Supported padding:"
> +	echo "- pkcs-1.5"
> +	echo "- pss"
> +}
> +
> +case "$padding" in
> +	"pkcs-1.5") optionPadding='' ;;
> +	"pss") optionPadding='-sigopt rsa_padding_mode:pss -sigopt rsa_pss_saltlen:-2' ;;
> +	*) echo "Error: $padding is an invalid padding"
> +	   print_supported_padding
> +	   exit 1;;
> +esac
> +
> +
> +#
> +# generate the sigature
> +#
> +sig=$(openssl dgst $optionHash -sign $key $optionPadding $input | xxd -p)
> +
> +#
> +# generate the header
> +#
> +# 0 = magic
> +# 4 = image size
> +# 8 = signature
> +#
> +h=$(printf "%08x" 0x55425348)
> +i=$(stat --printf="%s" $input)
> +i=$(printf "%08x" $i)
> +
> +echo "$h$i$sig" | xxd -r -p > $output

So this sounds like a completely generic way of prepending a signature
to an arbitrary blob, whether that is a FIT image, a U-Boot script
wrapped in a FIT, some firmware blob or whatnot. So it sounds like it
could be generally useful, and a lot simpler than the complexity
inherent when trying to add signature data within the signed data
structure itself.

So, can we perhaps not tie it to bootm as such? It's not a problem if
bootm learns to recognize 0x55425348 as another image format and then
automatically knows how to locate the "real" image, and/or automatically
verifies it. But I'd really like to be able to

  fatload $loadaddr blabla && \
  verify $loadaddr && \
  source $loadaddr

where fatload can be any random command that gets a bunch of bytes into
memory at a specific location (tftpboot, mmc read, ubi...). Currently,
we simply don't have any sane way to verify a boot script, or random
blobs, AFAICT.

To that end, it would be nice if the header was a little more
self-describing. Something like

0 = magic
4 = header size (including padding)
8 = image size
12 = offset to image signature
16 = flags (currently enforced to 0)
20 = reserved (currently enforced to 0)
24 = signature of first 24 bytes

xx = signature of image

Why do I want the image size signed? Because I'd like to be able to
store the whole thing in a raw partition (or ubi volume, or...), where
there's no concept of "file size" available. So I'd like to be able to
read in some fixed size chunk (24+whatever I expect the signature could
be, so 4096 is certainly enough), and from that compute the whole size I
need to read. But I don't want to blindly trust the "image size" field.
So, for such a case, I'd also like a "verify header $loadaddr"
subcommand (and "verify image $loadaddr", with "verify $loadaddr" being
shorthand for doing both).

And continuing the wishlist, it could be even better if we had

  verify load $loadaddr 'mmc read %l% 0 %s512%'

i.e. we could pass a "parametrized shell command" to verify for it to
use to read in a bunch of bytes to a given address - with %l% being
substituted by the address and %s<optional unit>% by the size to load,
optionally specified in the given unit.

Rasmus