MTimeProtectedFastFileStorage.php

<?php

namespace Drupal\Component\PhpStorage;

use Drupal\Component\Utility\Crypt;

/**
 * Stores PHP code in files with securely hashed names.
 *
 * The goal of this class is to ensure that if a PHP file is replaced with
 * an untrusted one, it does not get loaded. Since mtime granularity is 1
 * second, we cannot prevent an attack that happens within one second of the
 * initial save(). However, it is very unlikely for an attacker exploiting an
 * upload or file write vulnerability to also know when a legitimate file is
 * being saved, discover its hash, undo its file permissions, and override the
 * file with an upload all within a single second. Being able to accomplish
 * that would indicate a site very likely vulnerable to many other attack
 * vectors.
 *
 * Each file is stored in its own unique containing directory. The hash is based
 * on the virtual file name, the containing directory's mtime, and a
 * cryptographically hard to guess secret string. Thus, even if the hashed file
 * name is discovered and replaced by an untrusted file (e.g., via a
 * move_uploaded_file() invocation by a script that performs insufficient
 * validation), the directory's mtime gets updated in the process, invalidating
 * the hash and preventing the untrusted file from getting loaded.
 *
 * This class does not protect against overwriting a file in-place (e.g. a
 * malicious module that does a file_put_contents()) since this will not change
 * the mtime of the directory. MTimeProtectedFileStorage protects against this
 * at the cost of an additional system call for every load() and exists().
 *
 * The containing directory is created with the same name as the virtual file
 * name (slashes removed) to assist with debugging, since the file itself is
 * stored with a name that's meaningless to humans.
 */
class MTimeProtectedFastFileStorage extends FileStorage {

  /**
   * The secret used in the HMAC.
   *
   * @var string
   */
  protected $secret;

  /**
   * Constructs this MTimeProtectedFastFileStorage object.
   *
   * @param array $configuration
   *   An associated array, containing at least these keys (the rest are
   *   ignored):
   *   - directory: The directory where the files should be stored.
   *   - secret: A cryptographically hard to guess secret string.
   *   -bin. The storage bin. Multiple storage objects can be instantiated with
   *   the same configuration, but for different bins.
   */
  public function __construct(array $configuration) {
    parent::__construct($configuration);
    $this->secret = $configuration['secret'];
  }

  /**
   * {@inheritdoc}
   */
  public function save($name, $data) {
    $this->ensureDirectory($this->directory);

    // Write the file out to a temporary location. Prepend with a '.' to keep it
    // hidden from listings and web servers.
    $temporary_path = $this->tempnam($this->directory, '.');
    if (!$temporary_path || !@file_put_contents($temporary_path, $data)) {
      return FALSE;
    }
    // The file will not be chmod() in the future so this is the final
    // permission.
    chmod($temporary_path, 0444);

    // Determine the exact modification time of the file.
    $mtime = $this->getUncachedMTime($temporary_path);

    // Move the temporary file into the proper directory. Note that POSIX
    // compliant systems as well as modern Windows perform the rename operation
    // atomically, i.e. there is no point at which another process attempting to
    // access the new path will find it missing.
    $directory = $this->getContainingDirectoryFullPath($name);
    $this->ensureDirectory($directory);
    $full_path = $this->getFullPath($name, $directory, $mtime);
    $result = rename($temporary_path, $full_path);

    // Finally reset the modification time of the directory to match the one of
    // the newly created file. In order to prevent the creation of a file if the
    // directory does not exist, ensure that the path terminates with a
    // directory separator.
    //
    // Recall that when subsequently loading the file, the hash is calculated
    // based on the file name, the containing mtime, and a the secret string.
    // Hence updating the mtime here is comparable to pointing a symbolic link
    // at a new target, i.e., the newly created file.
    if ($result) {
      $result &= touch($directory . '/', $mtime);
    }

    return (bool) $result;
  }

  /**
   * Gets the full path where the file is or should be stored.
   *
   * This function creates a file path that includes a unique containing
   * directory for the file and a file name that is a hash of the virtual file
   * name, a cryptographic secret, and the containing directory mtime. If the
   * file is overridden by an insecure upload script, the directory mtime gets
   * modified, invalidating the file, thus protecting against untrusted code
   * getting executed.
   *
   * @param string $name
   *   The virtual file name. Can be a relative path.
   * @param string $directory
   *   (optional) The directory containing the file. If not passed, this is
   *   retrieved by calling getContainingDirectoryFullPath().
   * @param int $directory_mtime
   *   (optional) The mtime of $directory. Can be passed to avoid an extra
   *   filesystem call when the mtime of the directory is already known.
   *
   * @return string
   *   The full path where the file is or should be stored.
   */
  public function getFullPath($name, &$directory = NULL, &$directory_mtime = NULL) {
    if (!isset($directory)) {
      $directory = $this->getContainingDirectoryFullPath($name);
    }
    if (!isset($directory_mtime)) {
      $directory_mtime = file_exists($directory) ? filemtime($directory) : 0;
    }
    return $directory . '/' . Crypt::hmacBase64($name, $this->secret . $directory_mtime) . '.php';
  }

  /**
   * {@inheritdoc}
   */
  public function delete($name) {
    $path = $this->getContainingDirectoryFullPath($name);
    if (file_exists($path)) {
      return $this->unlink($path);
    }
    return FALSE;
  }

  /**
   * {@inheritdoc}
   */
  public function garbageCollection() {
    $flags = \FilesystemIterator::CURRENT_AS_FILEINFO;
    $flags += \FilesystemIterator::SKIP_DOTS;

    foreach ($this->listAll() as $name) {
      $directory = $this->getContainingDirectoryFullPath($name);
      try {
        $dir_iterator = new \FilesystemIterator($directory, $flags);
      }
      catch (\UnexpectedValueException) {
        // FilesystemIterator throws an UnexpectedValueException if the
        // specified path is not a directory, or if it is not accessible.
        continue;
      }

      $directory_unlink = TRUE;
      $directory_mtime = filemtime($directory);
      foreach ($dir_iterator as $fileinfo) {
        if ($directory_mtime > $fileinfo->getMTime()) {
          // Ensure the folder is writable.
          @chmod($directory, 0777);
          @unlink($fileinfo->getPathName());
        }
        else {
          // The directory still contains valid files.
          $directory_unlink = FALSE;
        }
      }

      if ($directory_unlink) {
        $this->unlink($name);
      }
    }
  }

  /**
   * Gets the full path of the file storage directory's parent.
   *
   * @param string $name
   *   The virtual file name. Can be a relative path.
   *
   * @return string
   *   The full path of the containing directory where the file is or should be
   *   stored.
   */
  protected function getContainingDirectoryFullPath($name) {
    // Remove the .php file extension from the directory name.
    // Within a single directory, a subdirectory cannot have the same name as a
    // file. Thus, when switching between MTimeProtectedFastFileStorage and
    // FileStorage, the subdirectory or the file cannot be created in case the
    // other file type exists already.
    if (str_ends_with($name, '.php')) {
      $name = substr($name, 0, -4);
    }
    return $this->directory . '/' . str_replace('/', '#', $name);
  }

  /**
   * Clears PHP's stat cache and returns the directory's mtime.
   */
  protected function getUncachedMTime($directory) {
    clearstatcache(TRUE, $directory);
    return filemtime($directory);
  }

  /**
   * A brute force tempnam implementation supporting streams.
   *
   * @param string $directory
   *   The directory where the temporary filename will be created.
   * @param string $prefix
   *   The prefix of the generated temporary filename.
   *
   * @return string
   *   Returns the new temporary filename (with path), or FALSE on failure.
   */
  protected function tempnam($directory, $prefix) {
    do {
      $path = $directory . '/' . $prefix . Crypt::randomBytesBase64(20);
    } while (file_exists($path));
    return $path;
  }

}