constellation/image/measured-boot/precalculate_pcr_8.sh

#!/usr/bin/env bash
# Copyright (c) Edgeless Systems GmbH
#
# SPDX-License-Identifier: AGPL-3.0-only

# This script is used to precalculate the PCR[8] value for a Constellation OS image.
# PCR[8] contains the hash of the kernel command line and is measured by systemd-boot.
# This value is deprecated and will be moved to PCR[12] in the future.
# This script may produce wrong results after 2023 (when the kernel command line is only measured in PCR[12]).
# Usage: precalculate_pcr_8.sh <path to image> <path to output file>

set -euo pipefail
source "$(dirname "$0")/measure_util.sh"

get_cmdline_from_uki () {
    local uki="$1"
    local output="$2"
    objcopy -O binary --only-section=.cmdline "${uki}" "${output}"
}

cmdline_measure () {
    local path="$1"
    local tmp=$(mktemp)
    # convert to utf-16le and add a null terminator
    iconv -f utf-8 -t utf-16le "${path}" -o "${tmp}"
    truncate -s +2 "${tmp}"
    sha256sum "${tmp}" | cut -d " " -f 1
    rm "${tmp}"
}

write_output () {
    local out="$1"
    cat > "${out}" <<EOF
{
    "pcr8": "${expected_pcr_8}",
    "cmdline": "${cmdline}"
}
EOF
}

IMAGE="$1"
OUT="$2"
CSP="$3"

DIR=$(mktempdir)
trap 'cleanup "${DIR}"' EXIT

extract "${IMAGE}" "/efi/EFI/Linux" "${DIR}/uki"
sudo chown -R "$USER:$USER" "${DIR}/uki"
cp ${DIR}/uki/*.efi "${DIR}/03-uki.efi"
get_cmdline_from_uki "${DIR}/03-uki.efi" "${DIR}/cmdline"
cmdline=$(cat "${DIR}/cmdline")

cmdline_hash=$(cmdline_measure "${DIR}/cmdline")
cleanup "${DIR}"

expected_pcr_8=0000000000000000000000000000000000000000000000000000000000000000
expected_pcr_8=$(pcr_extend "${expected_pcr_8}" "${cmdline_hash}" "sha256sum")
if [ "${CSP}" == "azure" ]; then
    # Azure displays the boot menu
    # triggering an extra measurement of the kernel command line.
    expected_pcr_8=$(pcr_extend "${expected_pcr_8}" "${cmdline_hash}" "sha256sum")
fi

echo "Kernel commandline:            ${cmdline}"
echo "Kernel Commandline measurement ${cmdline_hash}"
echo ""
echo "Expected PCR[8]:               ${expected_pcr_8}"
echo ""

write_output "${OUT}"