build_kernel_image.sh

#!/bin/bash

# Copyright (c) 2010 The Chromium OS Authors. All rights reserved.
# Use of this source code is governed by a BSD-style license that can be
# found in the LICENSE file.

# Helper script that generates the signed kernel image

SCRIPT_ROOT=$(dirname $(readlink -f "$0"))
. "${SCRIPT_ROOT}/common.sh" || exit 1

# Flags.
DEFINE_string arch "x86" \
  "The boot architecture: arm, x86, or amd64. (Default: x86)"
DEFINE_string to "/tmp/vmlinuz.image" \
  "The path to the kernel image to be created. (Default: /tmp/vmlinuz.image)"
DEFINE_string hd_vblock "/tmp/vmlinuz_hd.vblock" \
  "The path to the installed kernel's vblock (Default: /tmp/vmlinuz_hd.vblock)"
DEFINE_string vmlinuz "vmlinuz" \
  "The path to the kernel (Default: vmlinuz)"
DEFINE_string working_dir "/tmp/vmlinuz.working" \
  "Working directory for in-progress files. (Default: /tmp/vmlinuz.working)"
DEFINE_boolean keep_work ${FLAGS_FALSE} \
  "Keep temporary files (*.keyblock, *.vbpubk). (Default: false)"
DEFINE_string keys_dir "${SRC_ROOT}/platform/vboot_reference/tests/testkeys" \
  "Directory with the RSA signing keys. (Defaults to test keys)"
DEFINE_boolean use_dev_keys ${FLAGS_FALSE} \
  "Use developer keys for signing. (Default: false)"
# Note, to enable verified boot, the caller would manually pass:
# --boot_args='dm="... %U+1 %U+1 ..." \
# --root=/dev/dm-0
DEFINE_string boot_args "noinitrd" \
  "Additional boot arguments to pass to the commandline (Default: noinitrd)"
# If provided, will automatically add verified boot arguments.
DEFINE_string rootfs_image "" \
  "Optional path to the rootfs device or image.(Default: \"\")"
DEFINE_string rootfs_hash "" \
  "Optional path to output the rootfs hash to. (Default: \"\")"
DEFINE_integer verity_error_behavior 3 \
  "Verified boot error behavior [0: I/O errors, 1: reboot, 2: nothing] \
(Default: 3)"
DEFINE_integer verity_max_ios -1 \
  "Optional number of outstanding I/O operations. (Default: -1)"
DEFINE_string verity_hash_alg "sha1" \
  "Cryptographic hash algorithm used for dm-verity. (Default: sha1)"
DEFINE_string verity_salt "" \
  "Salt to use for rootfs hash (Default: \"\")"
DEFINE_boolean enable_rootfs_verification ${FLAGS_TRUE} \
  "Enable kernel-based root fs integrity checking. (Default: true)"
DEFINE_boolean enable_bootcache ${FLAGS_FALSE} \
  "Enable boot cache to accelerate booting. (Default: false)"

# Parse flags
FLAGS "$@" || exit 1
eval set -- "${FLAGS_ARGV}"

# Die on error
switch_to_strict_mode

rootdigest() {
  local digest
  digest=${table#*root_hexdigest=}
  echo ${digest% salt*}
}

salt() {
  local salt
  salt=${table#*salt=}
  echo ${salt%}
}

hashstart() {
  local hash
  hash=${table#*hashstart=}
  echo ${hash% alg*}
}

# Estimate of sectors used by verity
# (num blocks) * 32 (bytes per hash) * 2 (overhead) / 512 (bytes per sector)
veritysize() {
  echo $((root_fs_blocks * 32 * 2 / 512))
}

device_mapper_args=
# Even with a rootfs_image, root= is not changed unless specified.
if [[ -n "${FLAGS_rootfs_image}" && -n "${FLAGS_rootfs_hash}" ]]; then
  # Gets the number of blocks. 4096 byte blocks _are_ expected.
  if [ -f "${FLAGS_rootfs_image}" ]; then
    root_fs_block_sz=4096
    root_fs_sz=$(stat -c '%s' ${FLAGS_rootfs_image})
    root_fs_blocks=$((root_fs_sz / ${root_fs_block_sz}))
  else
    root_fs_blocks=$(sudo dumpe2fs "${FLAGS_rootfs_image}" 2> /dev/null |
                   grep "Block count" |
                   tr -d ' ' |
                   cut -f2 -d:)
    root_fs_block_sz=$(sudo dumpe2fs "${FLAGS_rootfs_image}" 2> /dev/null |
                     grep "Block size" |
                     tr -d ' ' |
                     cut -f2 -d:)
  fi

  info "rootfs is ${root_fs_blocks} blocks of ${root_fs_block_sz} bytes"
  if [[ ${root_fs_block_sz} -ne 4096 ]]; then
    error "Root file system blocks are not 4k!"
  fi

  info "Generating root fs hash tree (salt '${FLAGS_verity_salt}')."
  # Runs as sudo in case the image is a block device.
  # First argument to verity is reserved/unused and MUST be 0
  table=$(sudo verity mode=create \
                      alg=${FLAGS_verity_hash_alg} \
                      payload=${FLAGS_rootfs_image} \
                      payload_blocks=${root_fs_blocks} \
                      hashtree=${FLAGS_rootfs_hash} \
                      salt=${FLAGS_verity_salt})
  if [[ -f "${FLAGS_rootfs_hash}" ]]; then
    sudo chmod a+r "${FLAGS_rootfs_hash}"
  fi
  # Don't claim the root device unless verity is enabled.
  # Doing so will claim /dev/sdDP out from under the system.
  if [[ ${FLAGS_enable_rootfs_verification} -eq ${FLAGS_TRUE} ]]; then
    if [[ ${FLAGS_enable_bootcache} -eq ${FLAGS_TRUE} ]]; then
      base_root='254:0'  # major:minor numbers for /dev/dm-0
    else
      base_root='%U+1'  # kern_guid + 1
    fi
    table=${table//HASH_DEV/${base_root}}
    table=${table//ROOT_DEV/${base_root}}
  fi
  verity_dev="vroot none ro 1,${table}"
  if [[ ${FLAGS_enable_bootcache} -eq ${FLAGS_TRUE} ]]; then
    signature=$(rootdigest)
    cachestart=$(($(hashstart) + $(veritysize)))
    size_limit=512
    max_trace=20000
    max_pages=100000
    bootcache_args="%U+1 ${cachestart} ${signature} ${size_limit}"
    bootcache_args="${bootcache_args} ${max_trace} ${max_pages}"
    bootcache_dev="vboot none ro 1, 0 ${cachestart} bootcache ${bootcache_args}"
    device_mapper_args="dm=\"2 ${bootcache_dev}, ${verity_dev}\""
  else
    device_mapper_args="dm=\"1 ${verity_dev}\""
  fi
  info "device mapper configuration: ${device_mapper_args}"
fi

mkdir -p "${FLAGS_working_dir}"

# Only let dm-verity block if rootfs verification is configured.
# By default, we use a firmware enumerated value, but it isn't reliable for
# production use.  If +%d can be added upstream, then we can use:
#   root_dev=PARTUID=uuid+1
dev_wait=0
root_dev="PARTUUID=%U/PARTNROFF=1"
if [[ ${FLAGS_enable_rootfs_verification} -eq ${FLAGS_TRUE} ]]; then
  dev_wait=1
  if [[ ${FLAGS_enable_bootcache} -eq ${FLAGS_TRUE} ]]; then
    root_dev=/dev/dm-1
  else
    root_dev=/dev/dm-0
  fi
else
  if [[ ${FLAGS_enable_bootcache} -eq ${FLAGS_TRUE} ]]; then
    echo "Having bootcache without verity is not supported"
    exit 2
  fi
fi

cat <<EOF > "${FLAGS_working_dir}/boot.config"
root=${root_dev}
rootwait
ro
dm_verity.error_behavior=${FLAGS_verity_error_behavior}
dm_verity.max_bios=${FLAGS_verity_max_ios}
dm_verity.dev_wait=${dev_wait}
${device_mapper_args}
${FLAGS_boot_args}
vt.global_cursor_default=0
kern_guid=%U
EOF

WORK="${WORK} ${FLAGS_working_dir}/boot.config"
info "Emitted cross-platform boot params to ${FLAGS_working_dir}/boot.config"

# Add common boot options first.
cat <<EOF | cat - "${FLAGS_working_dir}/boot.config" \
  > "${FLAGS_working_dir}/config.txt"
loglevel=7
console=
init=/sbin/init
cros_secure
EOF

if [[ "${FLAGS_arch}" = "x86" || "${FLAGS_arch}" = "amd64" ]]; then
  # Legacy BIOS will use the kernel in the rootfs (via syslinux), as will
  # standard EFI BIOS (via grub, from the EFI System Partition). Chrome OS
  # BIOS will use a separate signed kernel partition, which we'll create now.
  cat <<EOF >> "${FLAGS_working_dir}/config.txt"
add_efi_memmap
boot=local
noresume
noswap
i915.modeset=1
tpm_tis.force=1
tpm_tis.interrupts=0
nmi_watchdog=panic,lapic
EOF
  WORK="${WORK} ${FLAGS_working_dir}/config.txt"

  bootloader_path="/lib64/bootstub/bootstub.efi"
  kernel_image="${FLAGS_vmlinuz}"
elif [[ "${FLAGS_arch}" = "arm" ]]; then
  WORK="${WORK} ${FLAGS_working_dir}/config.txt"

  # arm does not need/have a bootloader in kernel partition
  dd if="/dev/zero" of="${FLAGS_working_dir}/bootloader.bin" bs=512 count=1
  WORK="${WORK} ${FLAGS_working_dir}/bootloader.bin"

  bootloader_path="${FLAGS_working_dir}/bootloader.bin"
  kernel_image="${FLAGS_vmlinuz/vmlinuz/vmlinux.uimg}"
else
  error "Unknown arch: ${FLAGS_arch}"
fi

# We sign the image with the recovery_key, because this is what goes onto the
# USB key. We can only boot from the USB drive in recovery mode.
# For dev install shim, we need to use the installer keyblock instead of
# the recovery keyblock because of the difference in flags.
if [ ${FLAGS_use_dev_keys} -eq ${FLAGS_TRUE} ]; then
  USB_KEYBLOCK=installer_kernel.keyblock
  info "DEBUG: use dev install signing key"
else
  USB_KEYBLOCK=recovery_kernel.keyblock
  info "DEBUG: use recovery signing key"
fi

# Create and sign the kernel blob
vbutil_kernel \
  --pack "${FLAGS_to}" \
  --keyblock "${FLAGS_keys_dir}/${USB_KEYBLOCK}" \
  --signprivate "${FLAGS_keys_dir}/recovery_kernel_data_key.vbprivk" \
  --version 1 \
  --config "${FLAGS_working_dir}/config.txt" \
  --bootloader "${bootloader_path}" \
  --vmlinuz "${kernel_image}" \
  --arch "${FLAGS_arch}"

# And verify it.
vbutil_kernel \
  --verify "${FLAGS_to}" \
  --signpubkey "${FLAGS_keys_dir}/recovery_key.vbpubk"


# Now we re-sign the same image using the normal keys. This is the kernel
# image that is put on the hard disk by the installer. Note: To save space on
# the USB image, we're only emitting the new verfication block, and the
# installer just replaces that part of the hard disk's kernel partition.
vbutil_kernel \
  --repack "${FLAGS_hd_vblock}" \
  --vblockonly \
  --keyblock "${FLAGS_keys_dir}/kernel.keyblock" \
  --signprivate "${FLAGS_keys_dir}/kernel_data_key.vbprivk" \
  --oldblob "${FLAGS_to}"


# To verify it, we have to replace the vblock from the original image.
tempfile=$(mktemp)
trap "rm -f $tempfile" EXIT
cat "${FLAGS_hd_vblock}" > $tempfile
dd if="${FLAGS_to}" bs=65536 skip=1 >> $tempfile

vbutil_kernel \
  --verify $tempfile \
  --signpubkey "${FLAGS_keys_dir}/kernel_subkey.vbpubk"

rm -f $tempfile
trap - EXIT

set +e  # cleanup failure is a-ok

if [[ ${FLAGS_keep_work} -eq ${FLAGS_FALSE} ]]; then
  info "Cleaning up temporary files: ${WORK}"
  rm ${WORK}
  rmdir ${FLAGS_working_dir}
fi

info "Kernel partition image emitted: ${FLAGS_to}"

if [[ -f ${FLAGS_rootfs_hash} ]]; then
  info "Root filesystem hash emitted: ${FLAGS_rootfs_hash}"
fi