最小権限実行とsudoの適切な運用

シェルスクリプトで最小権限が重要な理由

危険な操作の前に実効 UID を確認する

#!/usr/bin/env bash
set -euo pipefail

# Guard: this script must NOT run as root.
if [[ "$EUID" -eq 0 ]]; then
  echo "ERROR: Do not run this script as root. Use a normal user account." >&2
  exit 1
fi

echo "Running as UID $EUID — proceeding safely."

必要な場合にだけ root であることを要求する

#!/usr/bin/env bash
set -euo pipefail

require_root() {
  if [[ "$EUID" -ne 0 ]]; then
    echo "ERROR: $(basename "$0") must be run as root." >&2
    echo "  Try: sudo $(basename "$0") $*" >&2
    exit 1
  fi
}

require_root "$@"

echo "Root confirmed (EUID=0). Starting privileged work..."

sudo を単一のコマンドに限定する

#!/usr/bin/env bash
set -euo pipefail

# BAD: escalate early, do everything as root (avoid this)
# sudo bash -c '
#   cp config.conf /etc/app/config.conf
#   chown app:app /etc/app/config.conf
#   systemctl restart app
# '

# GOOD: escalate only for the commands that require it
LOCAL_CONF="./config.conf"
DEST="/etc/app/config.conf"

# Unprivileged: validate the config before touching anything as root
if ! grep -q '^[[:space:]]*\[main\]' "$LOCAL_CONF"; then
  echo "ERROR: config.conf is missing [main] section" >&2
  exit 1
fi

# Privileged: only these three commands run under sudo
sudo cp "$LOCAL_CONF" "$DEST"
sudo chown app:app "$DEST"
sudo systemctl restart app

echo "Config deployed and service restarted."

厳格な sudoers ルールを記述する

# /etc/sudoers.d/deployer  (edit with: sudo visudo -f /etc/sudoers.d/deployer)
#
# Allow 'deployer' to restart exactly one service — nothing else
deployer ALL=(root) NOPASSWD: /usr/bin/systemctl restart app

# Allow copying a config file to a fixed destination only
deployer ALL=(root) NOPASSWD: /usr/bin/cp /home/deployer/staging/config.conf /etc/app/config.conf

# Allow chown of that specific file only
deployer ALL=(root) NOPASSWD: /usr/bin/chown app\:app /etc/app/config.conf

# NEVER do this — gives full root shell:
# deployer ALL=(ALL) NOPASSWD: ALL

su と runuser で権限を落とす

#!/usr/bin/env bash
# This script is called by systemd as root during pre-deployment
set -euo pipefail

APP_USER="app"
DEPLOY_DIR="/opt/myapp"

# Step 1: privileged — fix ownership of deploy directory
chown -R "${APP_USER}:${APP_USER}" "$DEPLOY_DIR"

# Step 2: drop to app user for the actual migration/startup logic
# runuser is available on most modern Linux systems
runuser -u "$APP_USER" -- bash -c "
  cd $DEPLOY_DIR
  ./bin/migrate.sh
  ./bin/start.sh
"

echo "Deploy complete. Privileged wrapper exiting."

sudo -u で別のユーザーとして単一のコマンドを実行する

#!/usr/bin/env bash
set -euo pipefail

# Scenario: deploy script runs as 'deployer'; DB migrations must run as 'postgres'
# sudoers entry needed:
#   deployer ALL=(postgres) NOPASSWD: /opt/app/bin/run_migrations.sh

DB_MIGRATION_SCRIPT="/opt/app/bin/run_migrations.sh"

if [[ ! -x "$DB_MIGRATION_SCRIPT" ]]; then
  echo "ERROR: migration script not found or not executable: $DB_MIGRATION_SCRIPT" >&2
  exit 1
fi

echo "Running DB migrations as postgres user..."
sudo -u postgres "$DB_MIGRATION_SCRIPT"

echo "Migrations done. Returning to deployer context (EUID=$EUID)."

環境変数を利用した権限昇格を防ぐ

#!/usr/bin/env bash
set -euo pipefail

# BAD: relies on $PATH — attacker who controls PATH can hijack 'cp'
# sudo cp config.conf /etc/app/

# GOOD: absolute paths for every command called under elevated context
SUDO_BIN="/usr/bin/sudo"
CP_BIN="/usr/bin/cp"
CHOWN_BIN="/usr/bin/chown"
SYSTEMCTL_BIN="/usr/bin/systemctl"

"$SUDO_BIN" "$CP_BIN" ./config.conf /etc/app/config.conf
"$SUDO_BIN" "$CHOWN_BIN" app:app /etc/app/config.conf
"$SUDO_BIN" "$SYSTEMCTL_BIN" restart app

echo "Deployed with hardened absolute-path invocations."

コマンド引数の検証で sudo を厳格に制限する

deployer ALL=(root) NOPASSWD: /opt/scripts/manage.sh

#!/usr/bin/env bash
# /opt/scripts/manage.sh — called via sudo; must validate its own args
set -euo pipefail

# Allowlist of valid actions
declare -A ALLOWED_ACTIONS=(
  [restart]=1
  [status]=1
  [reload]=1
)

ACTION="${1:-}"

if [[ -z "$ACTION" ]]; then
  echo "Usage: $(basename "$0") <restart|status|reload>" >&2
  exit 1
fi

if [[ -z "${ALLOWED_ACTIONS[$ACTION]:-}" ]]; then
  echo "ERROR: Unknown action '${ACTION}'. Allowed: ${!ALLOWED_ACTIONS[*]}" >&2
  exit 2
fi

/usr/bin/systemctl "$ACTION" app
echo "Action '$ACTION' executed successfully."

一時的な特権昇格とクリーンアップ用トラップ

#!/usr/bin/env bash
set -euo pipefail

# Must run as root for this demo
if [[ "$EUID" -ne 0 ]]; then
  echo "Run as root" >&2; exit 1
fi

TMP_SECRET=""

cleanup() {
  local exit_code=$?
  if [[ -n "$TMP_SECRET" && -f "$TMP_SECRET" ]]; then
    # Overwrite before deletion to reduce forensic recovery risk
    shred -u "$TMP_SECRET" 2>/dev/null || rm -f "$TMP_SECRET"
    echo "[cleanup] Removed privileged temp file." >&2
  fi
  exit "$exit_code"
}

trap cleanup EXIT INT TERM

# Create a root-owned temp file for a short-lived secret
TMP_SECRET="$(mktemp /tmp/deploy_secret.XXXXXXXX)"
chmod 600 "$TMP_SECRET"

# Simulate fetching a secret into the temp file
echo "super-secret-token" > "$TMP_SECRET"

# Use the secret (e.g., pass to a sub-command via file descriptor)
/usr/bin/some-privileged-tool --key-file "$TMP_SECRET"

echo "Privileged operation complete."

特権操作の監査とログ記録

#!/usr/bin/env bash
set -euo pipefail

AUDIT_LOG="/var/log/app_deploy_audit.log"

log_privileged_action() {
  local action="$1"
  local reason="${2:-unspecified}"
  local ts
  ts="$(date -u '+%Y-%m-%dT%H:%M:%SZ')"
  printf '{"ts":"%s","user":"%s","euid":%d,"action":"%s","reason":"%s"}\n' \
    "$ts" "${SUDO_USER:-$USER}" "$EUID" "$action" "$reason" \
    | sudo tee -a "$AUDIT_LOG" > /dev/null
}

# Each privileged step is logged before execution
log_privileged_action "cp_config" "deploy release v2.4.1"
sudo /usr/bin/cp ./config.conf /etc/app/config.conf

log_privileged_action "chown_config" "ensure app user owns config"
sudo /usr/bin/chown app:app /etc/app/config.conf

log_privileged_action "restart_service" "activate new config"
sudo /usr/bin/systemctl restart app

echo "Deployment complete. Audit entries written to $AUDIT_LOG"

理解度チェック：sudo の適用範囲

まとめ：最小権限の実行と sudo の適切な運用