Backport of: From 317b3b587058a05dca95d56dac26568c5b098d33 Mon Sep 17 00:00:00 2001 From: Philip Withnall 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 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