[glib/wip/2-58-cve-2021-28153: 4/5] glocalfileoutputstream: Fix CREATE_REPLACE_DESTINATION with symlinks

[Simon McVittie <smcv src gnome org>]

commit 4e64a27f4f170860ddcf835ca6858bda09911a23
Author: Philip Withnall <pwithnall endlessos org>
Date:   Wed Feb 24 17:36:07 2021 +0000

    glocalfileoutputstream: Fix CREATE_REPLACE_DESTINATION with symlinks
    
    The `G_FILE_CREATE_REPLACE_DESTINATION` flag is equivalent to unlinking
    the destination file and re-creating it from scratch. That did
    previously work, but in the process the code would call `open(O_CREAT)`
    on the file. If the file was a dangling symlink, this would create the
    destination file (empty). That’s not an intended side-effect, and has
    security implications if the symlink is controlled by a lower-privileged
    process.
    
    Fix that by not opening the destination file if it’s a symlink, and
    adjusting the rest of the code to cope with
     - the fact that `fd == -1` is not an error iff `is_symlink` is true,
     - and that `original_stat` will contain the `lstat()` results for the
       symlink now, rather than the `stat()` results for its target (again,
       iff `is_symlink` is true).
    
    This means that the target of the dangling symlink is no longer created,
    which was the bug. The symlink itself continues to be replaced (as
    before) with the new file — this is the intended behaviour of
    `g_file_replace()`.
    
    The behaviour for non-symlink cases, or cases where the symlink was not
    dangling, should be unchanged.
    
    Includes a unit test.
    
    Resolves CVE-2021-28153 (glib#2325). Backported to GLib 2.58 by
    Simon McVittie.
    
    Signed-off-by: Philip Withnall <pwithnall endlessos org>
    (cherry-picked from commit 317b3b587058a05dca95d56dac26568c5b098d33)
    [Backport to 2.58.x: replace g_local_file_fstat with fstat]
    [Backport to 2.58.x: replace g_local_file_lstat with lstat]
    [Backport to 2.58.x: replace _g_stat_mode with direct access to st_mode]
    [Backport to 2.58.x: don't call g_test_summary()]
    Signed-off-by: Simon McVittie <smcv collabora com>

 gio/glocalfileoutputstream.c |  49 ++++++++++++++------
 gio/tests/file.c             | 107 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 142 insertions(+), 14 deletions(-)
---
diff --git a/gio/glocalfileoutputstream.c b/gio/glocalfileoutputstream.c
index 8e0e793ff..2be51ae12 100644
--- a/gio/glocalfileoutputstream.c
+++ b/gio/glocalfileoutputstream.c
@@ -779,16 +779,22 @@ handle_overwrite_open (const char    *filename,
       /* Could be a symlink, or it could be a regular ELOOP error,
        * but then the next open will fail too. */
       is_symlink = TRUE;
-      fd = g_open (filename, open_flags, mode);
+      if (!replace_destination_set)
+        fd = g_open (filename, open_flags, mode);
     }
-#else
-  fd = g_open (filename, open_flags, mode);
-  errsv = errno;
+#else  /* if !O_NOFOLLOW */
   /* This is racy, but we do it as soon as possible to minimize the race */
   is_symlink = g_file_test (filename, G_FILE_TEST_IS_SYMLINK);
+
+  if (!is_symlink || !replace_destination_set)
+    {
+      fd = g_open (filename, open_flags, mode);
+      errsv = errno;
+    }
 #endif
 
-  if (fd == -1)
+  if (fd == -1 &&
+      (!is_symlink || !replace_destination_set))
     {
       char *display_name = g_filename_display_name (filename);
       g_set_error (error, G_IO_ERROR,
@@ -802,7 +808,10 @@ handle_overwrite_open (const char    *filename,
 #ifdef G_OS_WIN32
   res = GLIB_PRIVATE_CALL (g_win32_fstat) (fd, &original_stat);
 #else
-  res = fstat (fd, &original_stat);
+  if (!is_symlink)
+    res = fstat (fd, &original_stat);
+  else
+    res = lstat (filename, &original_stat);
 #endif
   errsv = errno;
 
@@ -821,16 +830,27 @@ handle_overwrite_open (const char    *filename,
   if (!S_ISREG (original_stat.st_mode))
     {
       if (S_ISDIR (original_stat.st_mode))
-       g_set_error_literal (error,
-                             G_IO_ERROR,
-                             G_IO_ERROR_IS_DIRECTORY,
-                             _("Target file is a directory"));
-      else
-       g_set_error_literal (error,
+        {
+          g_set_error_literal (error,
+                               G_IO_ERROR,
+                               G_IO_ERROR_IS_DIRECTORY,
+                               _("Target file is a directory"));
+          goto err_out;
+        }
+      else if (!is_symlink ||
+#ifdef S_ISLNK
+               !S_ISLNK (original_stat.st_mode)
+#else
+               FALSE
+#endif
+               )
+        {
+          g_set_error_literal (error,
                              G_IO_ERROR,
                              G_IO_ERROR_NOT_REGULAR_FILE,
                              _("Target file is not a regular file"));
-      goto err_out;
+          goto err_out;
+        }
     }
   
   if (etag != NULL)
@@ -911,7 +931,8 @@ handle_overwrite_open (const char    *filename,
            }
        }
 
-      g_close (fd, NULL);
+      if (fd >= 0)
+        g_close (fd, NULL);
       *temp_filename = tmp_filename;
       return tmpfd;
     }
diff --git a/gio/tests/file.c b/gio/tests/file.c
index 9c2af641f..1bdbe19b4 100644
--- a/gio/tests/file.c
+++ b/gio/tests/file.c
@@ -787,6 +787,112 @@ test_replace_cancel (void)
   g_object_unref (tmpdir);
 }
 
+static void
+test_replace_symlink (void)
+{
+#ifdef G_OS_UNIX
+  gchar *tmpdir_path = NULL;
+  GFile *tmpdir = NULL, *source_file = NULL, *target_file = NULL;
+  GFileOutputStream *stream = NULL;
+  const gchar *new_contents = "this is a test message which should be written to source and not target";
+  gsize n_written;
+  GFileEnumerator *enumerator = NULL;
+  GFileInfo *info = NULL;
+  gchar *contents = NULL;
+  gsize length = 0;
+  GError *local_error = NULL;
+
+  g_test_bug ("https://gitlab.gnome.org/GNOME/glib/-/issues/2325";);
+
+  /* Create a fresh, empty working directory. */
+  tmpdir_path = g_dir_make_tmp ("g_file_replace_symlink_XXXXXX", &local_error);
+  g_assert_no_error (local_error);
+  tmpdir = g_file_new_for_path (tmpdir_path);
+
+  g_test_message ("Using temporary directory %s", tmpdir_path);
+  g_free (tmpdir_path);
+
+  /* Create symlink `source` which points to `target`. */
+  source_file = g_file_get_child (tmpdir, "source");
+  target_file = g_file_get_child (tmpdir, "target");
+  g_file_make_symbolic_link (source_file, "target", NULL, &local_error);
+  g_assert_no_error (local_error);
+
+  /* Ensure that `target` doesn’t exist */
+  g_assert_false (g_file_query_exists (target_file, NULL));
+
+  /* Replace the `source` symlink with a regular file using
+   * %G_FILE_CREATE_REPLACE_DESTINATION, which should replace it *without*
+   * following the symlink */
+  stream = g_file_replace (source_file, NULL, FALSE  /* no backup */,
+                           G_FILE_CREATE_REPLACE_DESTINATION, NULL, &local_error);
+  g_assert_no_error (local_error);
+
+  g_output_stream_write_all (G_OUTPUT_STREAM (stream), new_contents, strlen (new_contents),
+                             &n_written, NULL, &local_error);
+  g_assert_no_error (local_error);
+  g_assert_cmpint (n_written, ==, strlen (new_contents));
+
+  g_output_stream_close (G_OUTPUT_STREAM (stream), NULL, &local_error);
+  g_assert_no_error (local_error);
+
+  g_clear_object (&stream);
+
+  /* At this point, there should still only be one file: `source`. It should
+   * now be a regular file. `target` should not exist. */
+  enumerator = g_file_enumerate_children (tmpdir,
+                                          G_FILE_ATTRIBUTE_STANDARD_NAME ","
+                                          G_FILE_ATTRIBUTE_STANDARD_TYPE,
+                                          G_FILE_QUERY_INFO_NOFOLLOW_SYMLINKS, NULL, &local_error);
+  g_assert_no_error (local_error);
+
+  info = g_file_enumerator_next_file (enumerator, NULL, &local_error);
+  g_assert_no_error (local_error);
+  g_assert_nonnull (info);
+
+  g_assert_cmpstr (g_file_info_get_name (info), ==, "source");
+  g_assert_cmpint (g_file_info_get_file_type (info), ==, G_FILE_TYPE_REGULAR);
+
+  g_clear_object (&info);
+
+  info = g_file_enumerator_next_file (enumerator, NULL, &local_error);
+  g_assert_no_error (local_error);
+  g_assert_null (info);
+
+  g_file_enumerator_close (enumerator, NULL, &local_error);
+  g_assert_no_error (local_error);
+  g_clear_object (&enumerator);
+
+  /* Double-check that `target` doesn’t exist */
+  g_assert_false (g_file_query_exists (target_file, NULL));
+
+  /* Check the content of `source`. */
+  g_file_load_contents (source_file,
+                        NULL,
+                        &contents,
+                        &length,
+                        NULL,
+                        &local_error);
+  g_assert_no_error (local_error);
+  g_assert_cmpstr (contents, ==, new_contents);
+  g_assert_cmpuint (length, ==, strlen (new_contents));
+  g_free (contents);
+
+  /* Tidy up. */
+  g_file_delete (source_file, NULL, &local_error);
+  g_assert_no_error (local_error);
+
+  g_file_delete (tmpdir, NULL, &local_error);
+  g_assert_no_error (local_error);
+
+  g_clear_object (&target_file);
+  g_clear_object (&source_file);
+  g_clear_object (&tmpdir);
+#else  /* if !G_OS_UNIX */
+  g_test_skip ("Symlink replacement tests can only be run on Unix")
+#endif
+}
+
 static void
 on_file_deleted (GObject      *object,
                 GAsyncResult *result,
@@ -1182,6 +1288,7 @@ main (int argc, char *argv[])
   g_test_add_data_func ("/file/async-create-delete/4096", GINT_TO_POINTER (4096), test_create_delete);
   g_test_add_func ("/file/replace-load", test_replace_load);
   g_test_add_func ("/file/replace-cancel", test_replace_cancel);
+  g_test_add_func ("/file/replace-symlink", test_replace_symlink);
   g_test_add_func ("/file/async-delete", test_async_delete);
 #ifdef G_OS_UNIX
   g_test_add_func ("/file/copy-preserve-mode", test_copy_preserve_mode);