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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
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
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
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