MD_STD/MD_JAIL.php

<?PHP
/**
 * Provides class MD_JAIL.
 */
declare(strict_types = 1);

/**
 * A class that, once initialized, forces the programmer to make security instructions explicit.
 * If an object of the class has been created, not specifying security instructions
 * leads to an error.
 * A restriction on basic file operations is not practical in an md context because of
 * the way transations are loaded through MDTlLoader.
 */
final class MD_JAIL {

    const STATUS_NONE      = 0;
    const STATUS_STARTED   = 1;
    const STATUS_SPECIFIED = 2; // Determines that everything is fine.

    /** @var integer */
    private int $_status = self::STATUS_NONE;

    /**
     * @var integer
     * Maximum execution time in seconds.
     */
    public int $max_execution_time;

    /**
     * @var string[]
     * Specifies which paths may be used by this script.
     */
    private array $_open_basedir = [];

    /**
     * @var string
     * Specifies the maximum RAM the script may use.
     */
    public string $memory_limit;

    /**
     * Static function providing an advisory on how to harden the php.ini or
     * .user.ini.
     *
     * @param array{shell_access_whitelist: string[], sys_function_whitelist: string[], file_function_whitelist: string[], file_uploads: bool, allow_url_fopen: bool, max_input_vars: integer, max_input_nesting_level: integer, post_max_size: string, curl: bool} $requested_resources Requested resources.
     *
     * @return string
     */
    public static function check_server_setup(array $requested_resources):string {

        # ini_set("open_basedir", __DIR__ . "/../../");

        $disable_functions = [
            "dl",                            # Loads a PHP extension at runtime
            "opcache_get_status",            # Gets opcache status
            "phpinfo",                       # For obvious reasons
            "parse_ini_file",                # For obvious reasons
            "show_source", "highlight_file", # Aliases; serve the content of a PHP file
            "php_uname",                     # Returns information about the operating system PHP is running on
            "phpcredits",                    # Credits page of PHP authors
            "php_strip_whitespace",          # Return PHP source with stripped comments and whitespace
            "popen", "pclose",               # Similar to fopen, but capable of shell access
            "virtual",                       # Perform an Apache sub-request
        ];

        $disable_aliases = [
            "ini_set"                 => "ini_alter",       # Alias of ini_set(), ini_set is preferred
            "disk_free_space"         => "diskfreespace",   # Alias of disk_free_space()
            "PHP_SAPI"                => "php_sapi_name",   # Alias of constant PHP_SAPI
            "stream_set_write_buffer" => "set_file_buffer", # Alias of stream_set_write_buffer()
        ];

        $shell_access_functions = array_diff(["exec", "passthru", "proc_close", "proc_get_status", "proc_nice", "proc_open", "proc_terminate", "shell_exec", "system"], $requested_resources['shell_access_whitelist']);
        $sys_functions = array_diff(["get_current_user", "getmyuid", "getmygid", "getmypid", "getmyinode", "getlastmod", "getenv", "putenv"], $requested_resources['sys_function_whitelist']);
        $file_functions = array_diff(["chgrp", "chgrp", "lchgrp", "lchown", "link", "linkinfo", "symlink"], $requested_resources['file_function_whitelist']);

        if ($requested_resources['curl'] === false) {
            $disable_functions[] = "curl_init";
            $disable_functions[] = "curl_exec";
            $disable_functions[] = "curl_multi_init";
            $disable_functions[] = "curl_multi_exec";
        }

        $disabledWOAliases = array_merge($disable_functions, $shell_access_functions, $sys_functions, $file_functions);

        $output = '## php.ini' . PHP_EOL . PHP_EOL;
        $output .= PHP_EOL . "php_value[disable_functions] = \"" . implode(", ", array_merge($disabledWOAliases, $disable_aliases)) . "\"";

        if ($requested_resources['file_uploads'] === false) {
            $output .= PHP_EOL . "php_value[file_uploads] = 0";
        }
        if ($requested_resources['allow_url_fopen'] === false) {
            $output .= PHP_EOL . "php_value[allow_url_fopen] = 0";
        }

        $output .= PHP_EOL . PHP_EOL . '## .user.ini' . PHP_EOL;

        if ($requested_resources['file_uploads'] === false) {
            $output .= PHP_EOL . "upload_max_filesize = 1";
        }
        if ($requested_resources['max_input_vars'] != ini_get("max_input_vars")) {
            $output .= PHP_EOL . "max_input_vars = " . $requested_resources['max_input_vars'];
        }
        if ($requested_resources['max_input_nesting_level'] != ini_get("max_input_nesting_level")) {
            $output .= PHP_EOL . "max_input_nesting_level = " . $requested_resources['max_input_nesting_level'];
        }
        if ($requested_resources['post_max_size'] != ini_get("post_max_size")) {
            $output .= PHP_EOL . "post_max_size = " . $requested_resources['post_max_size'];
        }

        $output .= PHP_EOL . PHP_EOL . '## PHPStan Directives' . PHP_EOL . PHP_EOL . "    disallowedFunctionCalls:" . PHP_EOL;
        foreach ($disable_aliases as $to_keep => $disabled) {

            $output .= '
    -
        function: \'' . $disabled . '()\'
        message: \'use ' . $to_keep . ' instead\'';

        }
        foreach ($disabledWOAliases as $disabled) {

            $output .= '
    - function: \'' . $disabled . '()\'';

        }

        return $output;

    }

    /**
     * Registers an additional accessible directory for open_basedir.
     *
     * @param string $dir Directory to register.
     *
     * @return void
     */
    public function register_accessible_dir(string $dir):void {

        $this->_open_basedir[] = $dir;

    }

    /**
     * Applies the memory limit setting.
     *
     * @return void
     */
    private function _apply_memory_limit():void {

        if (!isset($this->memory_limit)) {
            throw new MDJailSecurityOptionNotSetException("It has not been specified, which memory limit the script should hold. Set MD_JAIL->memory_limit = string.");
        }
        if (ini_set("memory_limit", $this->memory_limit) === false) {
            throw new Exception('Failed to change memory_limit to ' . $this->memory_limit);
        }

    }

    /**
     * Applies the maximum execution time setting.
     *
     * @return void
     */
    private function _apply_time_limit():void {

        if (!isset($this->max_execution_time)) {
            throw new MDJailSecurityOptionNotSetException("It has not been specified, which maximum execution time the script should hold. Set MD_JAIL->max_execution_time = integer.");
        }
        if (set_time_limit($this->max_execution_time) === false) {
            throw new Exception('Failed to change max_execution_time to ' . $this->max_execution_time);
        }

    }

    /**
     * Applies basedir restrictions.
     *
     * @return void
     */
    private function _apply_basedir_restriction():void {

        if (empty($this->_open_basedir)) {
            throw new MDJailSecurityOptionNotSetException("It has not been specified, which memory limit the script should hold. Set MD_JAIL->open_basedir = string.");
        }
        if (ini_set("open_basedir", implode(':', $this->_open_basedir)) === false) {
            throw new Exception('Failed to set open_basedir restrictions');
        }

    }

    /**
     * Enforces security options previously set.
     *
     * @return void
     */
    public function enforce():void {

        // Special instructions on CLI, so as to not disturb PHPUnit
        if (PHP_SAPI === 'cli') {

            if (!isset($this->memory_limit)) {
                throw new MDJailSecurityOptionNotSetException("It has not been specified, which memory limit the script should hold. Set MD_JAIL->memory_limit = string.");
            }
            if (!isset($this->max_execution_time)) {
                throw new MDJailSecurityOptionNotSetException("It has not been specified, which maximum execution time the script should hold. Set MD_JAIL->max_execution_time = integer.");
            }

            $this->_status = self::STATUS_SPECIFIED;
            $this->__destruct();
            return;
        }

        $this->_apply_memory_limit();
        $this->_apply_time_limit();

        // Set accessible file paths
        $this->_apply_basedir_restriction();

        $this->_status = self::STATUS_SPECIFIED;
        $this->__destruct();

    }

    /**
     * Setup function. Registers a shutdown function that throws an error
     * if the security specifications have not been made.
     *
     * @return void
     */
    public function __construct() {

        $this->_status = self::STATUS_STARTED;

    }

    /**
     * Destructor. Throws an exception if the settings have not been set.
     */
    public function __destruct() {

        if ($this->_status !== self::STATUS_SPECIFIED) {

            echo "Security specifications need to be set.";

            if (!isset($this->memory_limit)) {
                echo "Set memory limit";
            }

            if (!isset($this->max_execution_time)) {
                echo "Set max_execution_time";
            }

            if (empty($this->_open_basedir)) {
                echo "Set open_basedir";
            }

            throw new MDJailSecurityOptionNotSetException("Security specifications need to be set.");
        }

    }
}