security-misc/usr/bin/permission-hardener

#!/bin/bash

## Copyright (C) 2012 - 2024 ENCRYPTED SUPPORT LP <adrelanos@whonix.org>
## See the file COPYING for copying conditions.

## https://forums.whonix.org/t/disable-suid-binaries/7706
## https://forums.whonix.org/t/re-mount-home-and-other-with-noexec-and-nosuid-among-other-useful-mount-options-for-better-security/7707

## dpkg-statoverride does not support end-of-options ("--").

set -o errexit -o nounset -o pipefail

exit_code=0
store_dir="/var/lib/permission-hardener"
dpkg_admindir_parameter_existing_mode="--admindir ${store_dir}/existing_mode"
dpkg_admindir_parameter_new_mode="--admindir ${store_dir}/new_mode"
delimiter="#permission-hardener-delimiter#"

# shellcheck disable=SC2034
log_level=notice
# shellcheck disable=SC1091
source /usr/libexec/helper-scripts/log_run_die.sh

echo_wrapper_ignore() {
  if test "${1}" = "verbose"; then
    shift
    log notice "Run: $*"
  else
    shift
  fi
  "$@" 2>/dev/null || true
}

echo_wrapper_audit() {
  if test "${1}" = "verbose"; then
    shift
    log notice "Run: $*"
  else
    shift
  fi
  return_code=0
  "$@" ||
    {
      return_code="$?"
      exit_code=203
      log error "Command '$*' failed with exit code '${return_code}'! calling function name: '${FUNCNAME[1]}'" >&2
    }
}

make_store_dir(){
  mkdir --parents "${store_dir}/private"
  mkdir --parents "${store_dir}/existing_mode"
  mkdir --parents "${store_dir}/new_mode"
}

## Some tools may fail on newlines and even variable assignment to array may
## fail if a variable that will be assigned to an array element contains
## characters that are used as delimiters.
block_newlines(){
  local newline_variable newline_value
  newline_variable="${1}"
  newline_value="${2}"
  ## dpkg-statoverride: error: path may not contain newlines
  #if [[ "${newline_value}" == *$'\n'* ]]; then
  if [[ "${newline_value}" != "${newline_value//$'\n'/NEWLINE}" ]]; then
    log warn "Skipping ${newline_variable} that contains newlines: '${newline_value}'" >&2
    return 1
  fi
}

output_stat(){
  local file_name
  file_name="${1}"

  if test -z "${file_name}"; then
    log error "File name is empty. file_name: '${file_name}'" >&2
    return 1
  fi

  block_newlines file "${file_name}"

  declare -a arr
  local file_name_from_stat stat_output stat_output_newlined

  if ! stat_output="$(stat --format="%a${delimiter}%U${delimiter}%G${delimiter}%n${delimiter}" -- "${file_name}")"; then
    log error "Failed to run 'stat' on file: '${file_name}'!" >&2
    return 1
  fi

  if [ "$stat_output" = "" ]; then
    log error "stat_output is empty.
File name: '${file_name}'
Stat output: '${stat_output}'
stat_output_newlined: '${stat_output_newlined}'
line: '${line}'
" >&2
    return 1
  fi

  stat_output_newlined="$(printf '%s\n' "${stat_output//${delimiter}/$'\n'}")"

  if test "${stat_output_newlined}" = ""; then
    log error "stat_output_newlined is empty.
File name: '${file_name}'
Stat output: '${stat_output}'
stat_output_newlined: '${stat_output_newlined}'
line: '${line}'
" >&2
    return 1
  fi

  readarray -t arr <<< "${stat_output_newlined}"

  if test "${#arr[@]}" = 0; then
    log error "Array length is 0.
File name: '${file_name}'
Stat output: '${stat_output}'
stat_output_newlined: '${stat_output_newlined}'
line: '${line}'
" >&2
    return 1
  fi

  existing_mode="${arr[0]}"
  existing_owner="${arr[1]}"
  existing_group="${arr[2]}"
  file_name_from_stat="${arr[3]}"

  if [ ! "$file_name" = "$file_name_from_stat" ]; then
    log error "\
File name is different from file name received from stat:
File name: '${file_name}'
File name from stat: '${file_name_from_stat}'
line: '${line}'
" >&2
    return 1
  fi

  if test -z "${existing_mode}"; then
    log error "Existing mode is empty. Stat output: '${stat_output}', line: '${line}'" >&2
    return 1
  fi
  if test -z "${existing_owner}"; then
    log error "Existing owner is empty. Stat output: '${stat_output}', line: '${line}'" >&2
    return 1
  fi
  if test -z "${existing_group}"; then
    log error "Existing group is empty. Stat output: '${stat_output}', line: '${line}'" >&2
    return 1
  fi
}

sanity_tests() {
  echo_wrapper_audit silent \
    which \
      capsh getcap setcap stat find dpkg-statoverride getent grep 1>/dev/null
}

add_nosuid_statoverride_entry() {
  local fso_to_process
  fso_to_process="${fso}"
  local should_be_counter
  should_be_counter=0
  local counter_actual
  counter_actual=0

  local dummy_line
  while IFS="" read -r -d "" dummy_line; do
    log info "Test would parse line: '${dummy_line}'"
    should_be_counter=$((should_be_counter + 1))
  done < <(find "${fso_to_process}" -perm /u=s,g=s -print0)

  local line
  while IFS="" read -r -d "" file_name; do
    counter_actual=$((counter_actual + 1))

    ## sets:
    ## exiting_mode
    ## existing_owner
    ## existing_group
    output_stat "${file_name}"

    ## -h file      True if file is a symbolic Link.
    ## -u file      True if file has its set-user-id bit set.
    ## -g file      True if file has its set-group-id bit set.

    if test -h "${file_name}"; then
      ## https://forums.whonix.org/t/disable-suid-binaries/7706/14
      log info "Skip symlink: '${file_name}'"
      continue
    fi

    if test -d "${file_name}"; then
      log info "Skip directory: '${file_name}'"
      continue
    fi

    local setuid setgid
    setuid=""
    if test -u "${file_name}"; then
      setuid=true
    fi
    setgid=""
    if test -g "${file_name}"; then
      setgid=true
    fi

    local setuid_or_setgid
    setuid_or_setgid=""
    if test "${setuid}" = "true" || test "${setgid}" = "true"; then
      setuid_or_setgid=true
    fi
    if test -z "${setuid_or_setgid}"; then
      log info "Neither setuid nor setgid. Skipping. file_name: '${file_name}'"
      continue
    fi

    ## Remove suid / gid and execute permission for 'group' and 'others'.
    ## Similar to: chmod og-ugx /path/to/filename
    ## Removing execution permission is useful to make binaries such as 'su'
    ## fail closed rather than fail open if suid was removed from these.
    ## Do not remove read access since no security benefit and easier to
    ## manually undo for users.
    ## Are there suid or sgid binaries which are still useful if suid / sgid
    ## has been removed from these?
    new_mode="744"

    local is_exact_whitelisted
    is_exact_whitelisted=""
    for white_list_entry in "${exact_white_list[@]:-}"; do
      if test -z "${white_list_entry}"; then
        log info "white_list_entry unset. Skipping. file_name: '${file_name}'"
        continue
      fi
      if test "${file_name}" = "${white_list_entry}"; then
        is_exact_whitelisted="true"
        log info "is_exact_whitelisted=true. Skipping. file_name: '${file_name}'"
        ## Stop looping through the whitelist.
        break
      fi
    done

    local is_match_whitelisted
    is_match_whitelisted=""
    for matchwhite_list_entry in "${match_white_list[@]:-}"; do
      if test -z "${matchwhite_list_entry}"; then
        log info "matchwhite_list_entry unset. Skipping. file_name: '${file_name}'"
        continue
      fi
      if echo -- "${file_name}" | grep --quiet --fixed-strings -- "${matchwhite_list_entry}"; then
        is_match_whitelisted="true"
        log info "is_match_whitelisted=true. Skipping. file_name: '${file_name}'"
        ## Stop looping through the match_white_list.
        break
      fi
    done

    local is_disable_whitelisted
    is_disable_whitelisted=""
    for disablematch_list_entry in "${disable_white_list[@]:-}"; do
      if test -z "${disablematch_list_entry}"; then
        log info "disablematch_list_entry unset. Skipping. file_name: '${file_name}'"
        continue
      fi
      if echo -- "${file_name}" | grep --quiet --fixed-strings -- "${disablematch_list_entry}"; then
        is_disable_whitelisted="true"
        log info "is_disable_whitelisted=true. Skipping. file_name: '${file_name}'"
        ## Stop looping through the disablewhitelist.
        break
      fi
    done

    local clean_output_prefix clean_output
    clean_output_prefix="Managing (S|G)UID of line:"
    clean_output="${setuid:+setuid='true'} ${setgid:+setgid='true'} existing_mode='${existing_mode}' new_mode='${new_mode}' file='${file_name}'"
    if test "${whitelists_disable_all:-}" = "true"; then
    log info "${clean_output_prefix} whitelists_disable_all=true ${clean_output}"
    elif test "${is_disable_whitelisted}" = "true"; then
      log info "${clean_output_prefix} is_disable_whitelisted=true ${clean_output}"
    else
      if test "${is_exact_whitelisted}" = "true"; then
        log info "${clean_output_prefix} is_exact_whitelisted=true ${clean_output}"
        continue
      fi
      if test "${is_match_whitelisted}" = "true"; then
        log info "${clean_output_prefix} is_match_whitelisted=true matchwhite_list_entry='${matchwhite_list_entry}' ${clean_output}"
        continue
      fi
    fi

    log notice "${clean_output_prefix} ${clean_output}"

    # shellcheck disable=SC2086
    if dpkg-statoverride ${dpkg_admindir_parameter_existing_mode} --list "${file_name}" >/dev/null; then
      log info "Existing mode already saved previously. Not saving again."
    else
      ## Save existing_mode in separate database.
      ## Not using --update as not intending to enforce existing_mode.
      # shellcheck disable=SC2086
      echo_wrapper_audit silent dpkg-statoverride ${dpkg_admindir_parameter_existing_mode} --add "${existing_owner}" "${existing_group}" "${existing_mode}" "${file_name}"
    fi

    ## No need to check "dpkg-statoverride --list" for existing entries.
    ## If existing_mode was correct already, we would not have reached this
    ## point. Since existing_mode is incorrect, remove from dpkg-statoverride
    ## and re-add.

    ## Remove from real database.
    echo_wrapper_ignore silent dpkg-statoverride --remove "${file_name}"

    ## Remove from separate database.
    # shellcheck disable=SC2086
    echo_wrapper_ignore silent dpkg-statoverride ${dpkg_admindir_parameter_new_mode} --remove "${file_name}"

    ## Add to real database and use --update to make changes on disk.
    echo_wrapper_audit verbose dpkg-statoverride --add --update "${existing_owner}" "${existing_group}" "${new_mode}" "${file_name}"

    ## Not using --update as this is only for recording.
    # shellcheck disable=SC2086
    echo_wrapper_audit silent dpkg-statoverride ${dpkg_admindir_parameter_new_mode} --add "${existing_owner}" "${existing_group}" "${new_mode}" "${file_name}"

    ## /usr/lib will hit ARG_MAX if using bash 'shopt -s globstar' and '/usr/lib/**'.
    ## Using 'find' with '-perm /u=s,g=s' is faster and avoids ARG_MAX.
    ## https://forums.whonix.org/t/disable-suid-binaries/7706/17
  done < <(find "${fso_to_process}" -perm /u=s,g=s -print0)

  ## Sanity test.
  if test ! "${should_be_counter}" = "${counter_actual}"; then
    log info "File (parsed/wanted): '${fso_to_process}': (${counter_actual}/${should_be_counter})"
    log error "Expected number of files to be parsed was not met." >&2
    exit_code=202
  fi
}

set_file_perms() {
  log info "START parsing config file: '${config_file}'"

  local line
  while read -r line || test -n "${line}"; do
    if test -z "${line}"; then
      true "DEBUG: line is empty. Skipping."
      continue
    fi

    if [[ "${line}" =~ ^\s*# ]]; then
      continue
    fi

    if ! [[ "${line}" =~ [0-9a-zA-Z/] ]]; then
      exit_code=200
      log error "Line contains invalid characters: '${line}'" >&2
      ## Safer to exit with error in this case.
      ## https://forums.whonix.org/t/disable-suid-binaries/7706/59
      exit "${exit_code}"
    fi

    if test "${line}" = 'whitelists_disable_all=true'; then
      whitelists_disable_all=true
      log info "whitelists_disable_all=true"
      continue
    fi

    #global fso
    local mode_from_config owner_from_config group_from_config capability_from_config
    if ! read -r fso mode_from_config owner_from_config group_from_config capability_from_config <<<"${line}"; then
      exit_code=201
      log error "Cannot parse line: '${line}'" >&2
      ## Debugging.
      du -hs /tmp || true
      echo -- "test -w /tmp: '$(test -w /tmp)'" >&2 || true
      ## Safer to exit with error in this case.
      ## https://forums.whonix.org/t/disable-suid-binaries/7706/59
      exit "${exit_code}"
    fi

    log info "Parsing line: fso='${fso}' mode_from_config='${mode_from_config}' owner_from_config='${owner_from_config}' group_from_config='${group_from_config}' capability_from_config='${capability_from_config}'"

    ## Debugging.
    #echo "line: '${line}'"
    #echo "fso: '${fso}'"
    #echo "mode_from_config: '${mode_from_config}'"
    #echo "owner_from_config: '${owner_from_config}'"

    local fso_without_trailing_slash
    fso_without_trailing_slash="${fso%/}"

    declare -g disable_white_list exact_white_list match_white_list
    case "${mode_from_config}" in
      disablewhitelist)
        disable_white_list+=("${fso}")
        continue
        ;;
      exactwhitelist)
        exact_white_list+=("${fso}")
        continue
        ;;
      matchwhitelist)
        match_white_list+=("${fso}")
        continue
        ;;
    esac

    if test ! -e "${fso}"; then
      log info "File does not exist: '${fso}'"
      continue
    fi

    ## Use dpkg-statoverride so permissions are not reset during upgrades.

    if test "${mode_from_config}" = "nosuid"; then
      ## If mode_from_config is "nosuid" the config does not set owner and
      ## group. Therefore do not enforce owner/group check.
      add_nosuid_statoverride_entry
    else
      local string_length_of_mode_from_config
      string_length_of_mode_from_config="${#mode_from_config}"
      if test "${string_length_of_mode_from_config}" -gt "4"; then
        log error "Invalid mode: '${mode_from_config}'" >&2
        continue
      fi
      if test "${string_length_of_mode_from_config}" -lt "3"; then
        log error "Invalid mode: '${mode_from_config}'" >&2
        continue
      fi

      if ! grep --quiet --fixed-strings -- "${owner_from_config}:" "${store_dir}/private/passwd"; then
        log error "Owner from config does not exist: '${owner_from_config}'" >&2
        continue
      fi

      if ! grep --quiet --fixed-strings -- "${group_from_config}:" "${store_dir}/private/group"; then
        log error "Group from config does not exist: '${group_from_config}'" >&2
        continue
      fi

      local mode_for_grep
      mode_for_grep="${mode_from_config}"
      first_character_of_mode_from_config="${mode_from_config::1}"
      if test "${first_character_of_mode_from_config}" = "0"; then
        ## Remove leading '0'.
        mode_for_grep="${mode_from_config:1}"
      fi

      file_name="${fso_without_trailing_slash}"

      ## sets:
      ## exiting_mode
      ## existing_owner
      ## existing_group
      output_stat "${file_name}"

      ## Check there is an entry for the fso.
      ##
      ## example: dpkg-statoverride --list | grep /home
      ## output:
      ## root root 755 /home
      ##
      ## dpkg-statoverride does not show leading '0'.
      local dpkg_statoverride_list_output=""
      local dpkg_statoverride_list_exit_code=0
      dpkg_statoverride_list_output="$(dpkg-statoverride --list "${fso_without_trailing_slash}")" || {
        dpkg_statoverride_list_exit_code=$?
        true
      }

      if test "${dpkg_statoverride_list_exit_code}" = "0"; then
        local grep_line
        grep_line="${owner_from_config} ${group_from_config} ${mode_for_grep} ${fso_without_trailing_slash}"
        if echo -- "${dpkg_statoverride_list_output}" | grep --quiet --fixed-strings -- "${grep_line}"; then
          log info "The owner/group/mode matches fso entry. No further action required."
        else
          log info "The owner/group/mode does not match fso entry, updating entry."
          ## fso_without_trailing_slash instead of fso to prevent
          ## "dpkg-statoverride: warning: stripping trailing /"

          # shellcheck disable=SC2086
          if dpkg-statoverride ${dpkg_admindir_parameter_existing_mode} --list "${fso_without_trailing_slash}" >/dev/null; then
            log info "Existing mode already saved previously. Not saving again."
          else
            ## Save existing_mode in separate database.
            ## Not using --update as not intending to enforce existing_mode.
            # shellcheck disable=SC2086
            echo_wrapper_audit silent dpkg-statoverride ${dpkg_admindir_parameter_existing_mode} --add "${existing_owner}" "${existing_group}" "${existing_mode}" "${fso_without_trailing_slash}"
          fi

          # shellcheck disable=SC2086
          echo_wrapper_ignore silent dpkg-statoverride ${dpkg_admindir_parameter_new_mode} --remove "${fso_without_trailing_slash}"

          ## Remove from and add to real database.
          echo_wrapper_ignore silent dpkg-statoverride --remove "${fso_without_trailing_slash}"
          echo_wrapper_audit verbose dpkg-statoverride --add --update "${owner_from_config}" "${group_from_config}" "${mode_from_config}" "${fso_without_trailing_slash}"

          ## Save in separate database.
          ## Not using --update as this is only for saving.
          # shellcheck disable=SC2086
          echo_wrapper_audit silent dpkg-statoverride ${dpkg_admindir_parameter_new_mode} --add "${owner_from_config}" "${group_from_config}" "${mode_from_config}" "${fso_without_trailing_slash}"
        fi
      else
        log info "There is no fso entry, adding one."

        # shellcheck disable=SC2086
        if dpkg-statoverride ${dpkg_admindir_parameter_existing_mode} --list "${fso_without_trailing_slash}" >/dev/null; then
          log info "Existing mode already saved previously. Not saving again."
        else
          ## Save existing_mode in separate database.
          ## Not using --update as not intending to enforce existing_mode.
          # shellcheck disable=SC2086
          echo_wrapper_audit silent dpkg-statoverride ${dpkg_admindir_parameter_existing_mode} --add "${existing_owner}" "${existing_group}" "${existing_mode}" "${fso_without_trailing_slash}"
        fi

        ## Add to real database.
        echo_wrapper_audit verbose dpkg-statoverride --add --update "${owner_from_config}" "${group_from_config}" "${mode_from_config}" "${fso_without_trailing_slash}"

        ## Save in separate database.
        ## Not using --update as this is only for saving.
        # shellcheck disable=SC2086
        echo_wrapper_audit silent dpkg-statoverride ${dpkg_admindir_parameter_new_mode} --add "${owner_from_config}" "${group_from_config}" "${mode_from_config}" "${fso_without_trailing_slash}"
      fi
    fi
    if test -z "${capability_from_config}"; then
      log info "capability_from_config is empty. Skipping. file_name: '${file_name}'"
      continue
    fi

    if test "${capability_from_config}" = "none"; then
      ## https://forums.whonix.org/t/disable-suid-binaries/7706/45
      ## sudo setcap -r /bin/ping 2>/dev/null
      ## Failed to set capabilities on file '/bin/ping' (No data available)
      ## The value of the capability argument is not permitted for a file. Or
      ## the file is not a regular (non-symlink) file
      ## Therefore use echo_wrapper_ignore.
      echo_wrapper_ignore verbose setcap -r -- "${fso}"
      getcap_output="$(getcap -- "${fso}")"
      if test -n "${getcap_output}"; then
        exit_code=205
        log error "Removing capabilities failed. File: '${fso}'" >&2
        continue
      fi
    else
      if ! capsh --print | grep --fixed-strings -- "Bounding set" | grep --quiet -- "${capability_from_config}"; then
        log error "Capability from config does not exist: '${capability_from_config}'" >&2
        continue
      fi

      ## feature request: dpkg-statoverride: support for capabilities
      ## https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=502580
      echo_wrapper_audit verbose setcap "${capability_from_config}+ep" -- "${fso}"
    fi

  done <"${config_file}"
  log info "END parsing config file: '${config_file}'"
}

parse_config_folder() {
  touch "${store_dir}/private/passwd"
  chmod og-rwx "${store_dir}/private/passwd"
  touch "${store_dir}/private/group"
  chmod og-rwx "${store_dir}/private/group"

  local passwd_file_contents_temp
  ## Query contents of password and group databases only once and buffer them
  ##
  ## If we don't buffer we sometimes get incorrect results when checking for
  ## entries using 'if getent passwd | grep --quiet -- '^root:'; ...' since
  ## 'grep' exits after the first match in this case causing 'getent' to
  ## receive SIGPIPE, which then fails the pipeline since 'set -o pipefail' is
  ## set for this script.
  passwd_file_contents_temp="$(getent passwd)"
  echo -- "${passwd_file_contents_temp}" | tee -- "${store_dir}/private/passwd" >/dev/null
  group_file_contents_temp="$(getent group)"
  echo -- "${group_file_contents_temp}" | tee -- "${store_dir}/private/group" >/dev/null

  #passwd_file_contents="$(cat "${store_dir}/private/passwd")"
  #group_file_contents="$(cat "${store_dir}/private/group")"

  shopt -s nullglob
  for config_file in \
    /etc/permission-hardener.d/*.conf \
    /usr/local/etc/permission-hardener.d/*.conf \
    /etc/permission-hardening.d/*.conf \
    /usr/local/etc/permission-hardening.d/*.conf
  do
    set_file_perms

  done
}

apply() {
  check_root
  make_store_dir
  sanity_tests
  parse_config_folder

  log notice "\
To compare the current and previous permission modes, install 'meld' (or preferred diff tool) for comparison of file mode changes:
    sudo apt install --no-install-recommends meld
    meld ${store_dir}/existing_mode/statoverride ${store_dir}/new_mode/statoverride"
}

spare() {
  check_root
  make_store_dir

  remove_file="${1}"
  exit_code=0
  dpkg_admindir_parameter_existing_mode="--admindir ${store_dir}/existing_mode"
  dpkg_admindir_parameter_new_mode="--admindir ${store_dir}/new_mode"

  if test ! -f "${store_dir}/existing_mode/statoverride"; then
    true "DEBUG: Stat file does not exist, hardening was not applied before."
    return 0
  fi

  local line
  while read -r line; do
    ## example line:
    ## root root 4755 /usr/lib/eject/dmcrypt-get-device

    local owner group mode file_name
    if ! read -r owner group mode file_name <<< "${line}"; then
      exit_code=201
      log error "Cannot parse line: '${line}'" >&2
      continue
    fi
    log info "Parsing line: owner='${owner}' group='${group}' mode='${mode}' file_name='${file_name}'"

    if test "${remove_file}" = "all"; then
      verbose=""
      remove_one=false
    else
      if test "${remove_file}" = "${file_name}"; then
        verbose="--verbose"
        remove_one=true
        echo -- "${remove_one}" | tee -- "${store_dir}/remove_one" >/dev/null
      else
        echo -- "false" | tee -- "${store_dir}/remove_one" >/dev/null
        continue
      fi
    fi

    if test "${remove_one}" = "true"; then
      set -o xtrace
    fi

    if test -e "${file_name}"; then
      # shellcheck disable=SC2086
      chown ${verbose} "${owner}:${group}" "${file_name}" || exit_code=202
      ## chmod need to be run after chown since chown removes suid.
      ## https://unix.stackexchange.com/questions/53665/chown-removes-setuid-bit-bug-or-feature
      # shellcheck disable=SC2086
      chmod ${verbose} "${mode}" "${file_name}" || exit_code=203
    else
      log info "File does not exist: '${file_name}'"
    fi

    dpkg-statoverride --remove "${file_name}" &>/dev/null || true
    # shellcheck disable=SC2086
    dpkg-statoverride ${dpkg_admindir_parameter_existing_mode} --remove "${file_name}" &>/dev/null || true
    # shellcheck disable=SC2086
    dpkg-statoverride ${dpkg_admindir_parameter_new_mode} --remove "${file_name}" &>/dev/null || true

    if test "${remove_one}" = "true"; then
      set +o xtrace
      break
    fi

  done < "${store_dir}/existing_mode/statoverride"

  if test ! "${remove_file}" = "all"; then
    if test "$(cat "${store_dir}/remove_one")" = "false"; then
      log info "No file was removed.

  File '${remove_file}' has not been removed from SUID Disabler and Permission Hardener during this invocation. This is expected if already done earlier.

  This program expects the full path to the file. Example:
    $0 disable /usr/bin/newgrp  # absolute path: works
    $0 disable newgrp           # relative path: does not work

  To remove all:
    $0 disable all

  This change might not be permanent. For full instructions, see:
    https://www.kicksecure.com/wiki/SUID_Disabler_and_Permission_Hardener

  To view list of changed by SUID Disabler and Permission Hardener:
    https://www.kicksecure.com/wiki/SUID_Disabler_and_Permission_Hardener#View_List_of_Permissions_Changed_by_SUID_Disabler_and_Permission_Hardener

  For re-enabling any specific SUID binary:
    https://www.kicksecure.com/wiki/SUID_Disabler_and_Permission_Hardener#Re-Enable_Specific_SUID_Binaries

  For completely disabling SUID Disabler and Permission Hardener:
    https://www.kicksecure.com/wiki/SUID_Disabler_and_Permission_Hardener#Disable_SUID_Disabler_and_Permission_Hardener"
    fi
  fi
}

check_root(){
  if test "$(id -u)" != "0"; then
    log error "Not running as root, aborting."
    exit 1
  fi
}

usage(){
  echo -- "Usage: ${0##*/} enable
       ${0##*/} disable [FILE|all]

Examples:
  ${0##*/} enable
  ${0##*/} disable all
  ${0##*/} disable /usr/bin/newgrp" >&2
  exit "${1}"
}

case "${1:-}" in
  enable) shift; apply "$@";;
  disable)
    shift
    case "${1:-}" in
      "") usage 1;;
      *) spare "${1}";;
    esac
    ;;
  -h|--help) usage 0;;
  *) usage 1;;
esac

if test "${exit_code}" != "0"; then
  log error "Exiting with non-zero exit code: '${exit_code}'" >&2
fi

exit "${exit_code}"