Require signed agent releases (Refs #64)

agent/install.sh

@@ -33,7 +33,7 @@ command -v systemctl >/dev/null || { echo 'systemd is required.' >&2; exit 1; }
 
 export DEBIAN_FRONTEND=noninteractive
 apt-get update -qq
-apt-get install -y --no-install-recommends ca-certificates curl python3 util-linux
+apt-get install -y --no-install-recommends ca-certificates curl python3 util-linux gpgv
 installation_preflight() {
     if [ "$existing_enrollment" = true ] && [ ! -f /etc/electric-cherry/agent.json ]; then
         echo 'The existing enrollment disappeared during installation. Stop and inspect host deletion before trying again.' >&2
@@ -96,6 +96,11 @@ def trusted_json(path):
 
 trusted_directories(root)
 trusted_directories(config.parent)
+keyring = config.with_name('release-signing.gpg')
+try:
+    trusted_file(keyring)
+except FileNotFoundError:
+    raise SystemExit('Provision the publisher public key at /etc/electric-cherry/release-signing.gpg through a trusted channel before installing. Never obtain the trust key from the controller.') from None
 mountinfo = pathlib.Path('/proc/1/mountinfo')
 if mountinfo.exists():
     for line in mountinfo.read_text().splitlines():
@@ -169,19 +174,43 @@ installation_cleanup() {
 trap installation_cleanup EXIT
 controller="${controller%/}"
 curl --proto '=https' --tlsv1.2 --fail --silent --show-error "$controller/api/agent/release/manifest.json" -o "$temporary_dir/manifest.json"
+curl --proto '=https' --tlsv1.2 --fail --silent --show-error "$controller/api/agent/release/manifest.json.sig" -o "$temporary_dir/manifest.json.sig"
 curl --proto '=https' --tlsv1.2 --fail --silent --show-error "$controller/api/agent/release/ec-agent.py" -o "$temporary_dir/ec-agent.py"
 curl --proto '=https' --tlsv1.2 --fail --silent --show-error "$controller/api/agent/release/ec-agent-update.py" -o "$temporary_dir/ec-agent-update.py"
-python3 - "$temporary_dir/manifest.json" "$temporary_dir/ec-agent.py" "$temporary_dir/ec-agent-update.py" <<'PY'
+python3 - "$temporary_dir/manifest.json" "$temporary_dir/ec-agent.py" "$temporary_dir/ec-agent-update.py" /etc/electric-cherry/release-signing.gpg <<'PY'
 # INSTALLATION_RELEASE: check both downloads before installing or executing either.
-import ast, hashlib, json, pathlib, re, sys
-manifest = json.loads(pathlib.Path(sys.argv[1]).read_text())
+import ast, hashlib, json, pathlib, re, stat, subprocess, sys, tempfile
+manifest_path = pathlib.Path(sys.argv[1])
+manifest_bytes = manifest_path.read_bytes()
+signature = manifest_path.with_name('manifest.json.sig')
+keyring = pathlib.Path(sys.argv[4])
+for path in (*reversed(keyring.parents), keyring):
+    info = path.lstat()
+    expected_type = stat.S_ISREG if path == keyring else stat.S_ISDIR
+    if not expected_type(info.st_mode) or info.st_uid != 0 or info.st_mode & 0o022:
+        raise SystemExit('Release signing key must be a regular root-owned file in protected directories.')
+if len(manifest_bytes) > 16 * 1024 or signature.stat().st_size > 16 * 1024:
+    raise SystemExit('Release signing metadata exceeds its size limit.')
+try:
+    with tempfile.TemporaryDirectory(prefix='gpgv-', dir=manifest_path.parent) as home:
+        verified = subprocess.run(['gpgv', '--homedir', home, '--keyring', str(keyring),
+                                   str(signature), '-'], input=manifest_bytes,
+                                  stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL,
+                                  timeout=30, check=False)
+except (OSError, subprocess.TimeoutExpired):
+    raise SystemExit('Release signature verification requires a working gpgv installation.') from None
+if verified.returncode:
+    raise SystemExit('Release signature verification failed. The installed agent was left unchanged.')
+manifest = json.loads(manifest_bytes)
+if not isinstance(manifest, dict):
+    raise SystemExit('Unsupported agent or updater release manifest.')
 agent = pathlib.Path(sys.argv[2]).read_bytes()
 updater = pathlib.Path(sys.argv[3]).read_bytes()
 if hashlib.sha256(agent).hexdigest() != manifest.get('sha256'):
     raise SystemExit('Agent release checksum verification failed.')
 if hashlib.sha256(updater).hexdigest() != manifest.get('updaterSha256'):
     raise SystemExit('Updater release checksum verification failed.')
-if manifest.get('platform') != 'debian-amd64' or manifest.get('protocolVersion') != 1 or manifest.get('updaterProtocolVersion') != 1:
+if manifest.get('platform') != 'debian-amd64' or type(manifest.get('protocolVersion')) is not int or manifest['protocolVersion'] != 1 or type(manifest.get('updaterProtocolVersion')) is not int or manifest['updaterProtocolVersion'] != 2:
     raise SystemExit('Unsupported agent or updater release manifest.')
 versions = [node.value.value for node in ast.parse(agent).body
             if isinstance(node, ast.Assign) and any(isinstance(target, ast.Name) and target.id == 'VERSION' for target in node.targets)