From 6c10e8ce6905e8fcc3466eb8af707b5d0d3bdb85 Mon Sep 17 00:00:00 2001

From: Philip Withnall <pwithnall@endlessos.org>

Date: Wed, 24 Feb 2021 17:36:07 +0000

Subject: [PATCH 2/3] glocalfileoutputstream: Fix CREATE_REPLACE_DESTINATION

 with symlinks

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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.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Fixes: #2325

---

 gio/glocalfileoutputstream.c |  63 ++++++++++++++-------

 gio/tests/file.c             | 107 ++++++++++++++++++++++++++++++++++-

 2 files changed, 149 insertions(+), 21 deletions(-)

diff --git a/gio/glocalfileoutputstream.c b/gio/glocalfileoutputstream.c

index 6a70b2a04..4a7766f68 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,

@@ -800,10 +806,17 @@ 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);

+#error This patch has not been ported to Windows, sorry

 #endif

+

+  if (!is_symlink)

+    {

+      res = fstat (fd, &original_stat);

+    }

+  else

+    {

+      res = lstat (filename, &original_stat);

+    }

   errsv = errno;

   if (res != 0)

@@ -821,16 +834,27 @@ 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_IO_ERROR,

-                             G_IO_ERROR_NOT_REGULAR_FILE,

-                             _("Target file is not a regular file"));

-      goto err_out;

+        {

+          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;

+        }

     }

   if (etag != NULL)

@@ -911,7 +935,8 @@ 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 98eeb85d4..44db6e295 100644

--- a/gio/tests/file.c

+++ b/gio/tests/file.c

@@ -671,8 +671,6 @@ test_replace_cancel (void)

   guint count;

   GError *error = NULL;

-  g_test_bug ("629301");

-

   path = g_dir_make_tmp ("g_file_replace_cancel_XXXXXX", &error);

   g_assert_no_error (error);

   tmpdir = g_file_new_for_path (path);

@@ -779,6 +777,110 @@ 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;

+

+  /* 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,

@@ -1170,6 +1272,7 @@ 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);

-- 

2.31.1