Backport of:

From 317b3b587058a05dca95d56dac26568c5b098d33 Mon Sep 17 00:00:00 2001
From: Philip Withnall <pwithnall@endlessos.org>
Date: Wed, 24 Feb 2021 17:35:40 +0000
Subject: [PATCH] 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 |  70 ++++++++++++++++-------
 gio/tests/file.c             | 108 +++++++++++++++++++++++++++++++++++
 2 files changed, 158 insertions(+), 20 deletions(-)

diff --git a/gio/glocalfileoutputstream.c b/gio/glocalfileoutputstream.c
index a3dd62172..553fcbbae 100644
--- a/gio/glocalfileoutputstream.c
+++ b/gio/glocalfileoutputstream.c
@@ -874,16 +874,22 @@ 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 (!(flags & G_FILE_CREATE_REPLACE_DESTINATION))
+        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 || !(flags & G_FILE_CREATE_REPLACE_DESTINATION))
+    {
+      fd = g_open (filename, open_flags, mode);
+      errsv = errno;
+    }
 #endif
 
-  if (fd == -1)
+  if (fd == -1 &&
+      (!is_symlink || !(flags & G_FILE_CREATE_REPLACE_DESTINATION)))
     {
       char *display_name = g_filename_display_name (filename);
       g_set_error (error, G_IO_ERROR,
@@ -893,13 +899,25 @@ handle_overwrite_open (const char    *filename,
       g_free (display_name);
       return -1;
     }
-  
+
+  if (!is_symlink)
+    {
 #ifdef G_OS_WIN32
-  res = GLIB_PRIVATE_CALL (g_win32_fstat) (fd, &original_stat);
+      res = GLIB_PRIVATE_CALL (g_win32_fstat) (fd, &original_stat);
 #else
-  res = fstat (fd, &original_stat);
+      res = fstat (fd, &original_stat);
 #endif
-  errsv = errno;
+      errsv = errno;
+    }
+  else
+    {
+#ifdef G_OS_WIN32
+      res = GLIB_PRIVATE_CALL (g_win32_lstat_utf8) (filename, &original_stat);
+#else
+      res = g_lstat (filename, &original_stat);
+#endif
+      errsv = errno;
+    }
 
   if (res != 0)
     {
@@ -916,16 +934,27 @@ 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)
@@ -1006,7 +1035,8 @@ 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 efb2eaadd..bc55f3af4 100644
--- a/gio/tests/file.c
+++ b/gio/tests/file.c
@@ -804,6 +804,113 @@ 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");
+  g_test_summary ("Test that G_FILE_CREATE_REPLACE_DESTINATION doesn’t follow symlinks");
+
+  /* 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,
@@ -1754,6 +1861,7 @@ 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);
-- 
2.30.1