Check safety of save_as earlier if possible

The check in the writer still serves as a safety net.

pelican/contents.py

@@ -17,8 +17,9 @@ from pelican import signals
 from pelican.settings import DEFAULT_CONFIG
 from pelican.utils import (SafeDatetime, deprecated_attribute, memoized,
                            path_to_url, posixize_path,
-                           python_2_unicode_compatible, set_date_tzinfo,
-                           slugify, strftime, truncate_html_words)
+                           python_2_unicode_compatible, sanitised_join,
+                           set_date_tzinfo, slugify, strftime,
+                           truncate_html_words)
 
 # Import these so that they're avalaible when you import from pelican.contents.
 from pelican.urlwrappers import (Author, Category, Tag, URLWrapper)  # NOQA
@@ -161,6 +162,22 @@ class Content(object):
             if not hasattr(self, prop):
                 raise NameError(prop)
 
+    def valid_save_as(self):
+        """Return true if save_as doesn't write outside output path, false
+        otherwise."""
+        try:
+            output_path = self.settings["OUTPUT_PATH"]
+        except KeyError:
+            # we cannot check
+            return True
+
+        try:
+            sanitised_join(output_path, self.save_as)
+        except RuntimeError:  # outside output_dir
+            return False
+
+        return True
+
     @property
     def url_format(self):
         """Returns the URL, formatted with the proper values"""
@@ -470,9 +487,20 @@ class Static(Page):
 def is_valid_content(content, f):
     try:
         content.check_properties()
-        return True
     except NameError as e:
         logger.error(
             "Skipping %s: could not find information about '%s'",
             f, six.text_type(e))
         return False
+
+    if not content.valid_save_as():
+        logger.error(
+            "Skipping %s: file %r would be written outside output path",
+            f,
+            content.save_as,
+        )
+        # Note: future code might want to use a result variable instead, to
+        # allow showing multiple error messages at once.
+        return False
+
+    return True

pelican/tests/test_contents.py

@@ -497,6 +497,30 @@ class TestArticle(TestPage):
         article = Article(**article_kwargs)
         self.assertEqual(article.url, 'fedora.qa/this-week-in-fedora-qa/')
 
+    def test_valid_save_as_detects_breakout(self):
+        settings = get_settings()
+        article_kwargs = self._copy_page_kwargs()
+        article_kwargs['metadata']['slug'] = '../foo'
+        article_kwargs['settings'] = settings
+        article = Article(**article_kwargs)
+        self.assertFalse(article.valid_save_as())
+
+    def test_valid_save_as_detects_breakout_to_root(self):
+        settings = get_settings()
+        article_kwargs = self._copy_page_kwargs()
+        article_kwargs['metadata']['slug'] = '/foo'
+        article_kwargs['settings'] = settings
+        article = Article(**article_kwargs)
+        self.assertFalse(article.valid_save_as())
+
+    def test_valid_save_as_passes_valid(self):
+        settings = get_settings()
+        article_kwargs = self._copy_page_kwargs()
+        article_kwargs['metadata']['slug'] = 'foo'
+        article_kwargs['settings'] = settings
+        article = Article(**article_kwargs)
+        self.assertTrue(article.valid_save_as())
+
 
 class TestStatic(LoggedTestCase):
 

pelican/tests/test_utils.py

@@ -11,6 +11,8 @@ from tempfile import mkdtemp
 
 import pytz
 
+import six
+
 from pelican import utils
 from pelican.generators import TemplatePagesGenerator
 from pelican.settings import read_settings
@@ -666,3 +668,34 @@ class TestDateFormatter(unittest.TestCase):
         with utils.pelican_open(output_path) as output_file:
             self.assertEqual(output_file,
                              utils.strftime(self.date, 'date = %A, %d %B %Y'))
+
+
+class TestSanitisedJoin(unittest.TestCase):
+    def test_detect_parent_breakout(self):
+        with six.assertRaisesRegex(
+                self,
+                RuntimeError,
+                "Attempted to break out of output directory to /foo/test"):
+            utils.sanitised_join(
+                "/foo/bar",
+                "../test"
+            )
+
+    def test_detect_root_breakout(self):
+        with six.assertRaisesRegex(
+                self,
+                RuntimeError,
+                "Attempted to break out of output directory to /test"):
+            utils.sanitised_join(
+                "/foo/bar",
+                "/test"
+            )
+
+    def test_pass_deep_subpaths(self):
+        self.assertEqual(
+            utils.sanitised_join(
+                "/foo/bar",
+                "test"
+            ),
+            os.path.join("/foo/bar", "test")
+        )

pelican/utils.py

@@ -36,6 +36,18 @@ except ImportError:
 logger = logging.getLogger(__name__)
 
 
+def sanitised_join(base_directory, *parts):
+    joined = os.path.abspath(os.path.join(base_directory, *parts))
+    if not joined.startswith(os.path.abspath(base_directory)):
+        raise RuntimeError(
+            "Attempted to break out of output directory to {}".format(
+                joined
+            )
+        )
+
+    return joined
+
+
 def strftime(date, date_format):
     '''
     Replacement for built-in strftime

pelican/writers.py

@@ -13,7 +13,7 @@ import six
 from pelican import signals
 from pelican.paginator import Paginator
 from pelican.utils import (get_relative_path, is_selected_for_writing,
-                           path_to_url, set_date_tzinfo)
+                           path_to_url, sanitised_join, set_date_tzinfo)
 
 if not six.PY3:
     from codecs import open
@@ -21,18 +21,6 @@ if not six.PY3:
 logger = logging.getLogger(__name__)
 
 
-def _sanitised_join(base_directory, *parts):
-    joined = os.path.abspath(os.path.join(base_directory, *parts))
-    if not joined.startswith(base_directory):
-        raise RuntimeError(
-            "attempt to break out of output directory to {}".format(
-                joined
-            )
-        )
-
-    return joined
-
-
 class Writer(object):
 
     def __init__(self, output_path, settings=None):
@@ -135,7 +123,7 @@ class Writer(object):
             self._add_item_to_the_feed(feed, elements[i])
 
         if path:
-            complete_path = _sanitised_join(self.output_path, path)
+            complete_path = sanitised_join(self.output_path, path)
 
             try:
                 os.makedirs(os.path.dirname(complete_path))
@@ -182,7 +170,7 @@ class Writer(object):
             if localcontext['localsiteurl']:
                 context['localsiteurl'] = localcontext['localsiteurl']
             output = template.render(localcontext)
-            path = _sanitised_join(output_path, name)
+            path = sanitised_join(output_path, name)
 
             try:
                 os.makedirs(os.path.dirname(path))