[PATCH 3/3] selftests/landlock: Test OverlayFS renames w/o LANDLOCK_ACCESS_FS_MAKE_CHAR

["Günther Noack" <gnoack@google.com>]

Even though OverlayFS uses vfs_rename() with RENAME_WHITEOUT under the
hood, and even though RENAME_WHITEOUT requires
LANDLOCK_ACCESS_FS_MAKE_CHAR, a process that renames files in an OverlayFS
can do so without having the LANDLOCK_ACCESS_FS_MAKE_CHAR right in that
location.  This works because OverlayFS uses the credentials determined at
mount time for the internal vfs_rename() operation.

Signed-off-by: Günther Noack <gnoack@google.com>
---
 security/landlock/fs.c                     | 11 +++++---
 tools/testing/selftests/landlock/fs_test.c | 31 ++++++++++++++++++++++
 2 files changed, 38 insertions(+), 4 deletions(-)

diff --git a/security/landlock/fs.c b/security/landlock/fs.c
index 2b84a229e4d8..9b49f6c3e5da 100644
--- a/security/landlock/fs.c
+++ b/security/landlock/fs.c
@@ -1523,11 +1523,14 @@ static int hook_path_rename(const struct path *const old_dir,
 		int err;
 
 		/*
-		 * This check would better be done together with other path
-		 * walks which are already happening for the normal rename check
-		 * in current_check_refer_path().
+		 * Rename with RENAME_WHITEOUT creates a whiteout object
+		 * (character device file with major=minor=0) in the old
+		 * location, so we check the access right for creating that.
+		 *
+		 * See Documentation/filesystems/overlayfs.rst and renameat2(2).
 		 */
-		err = current_check_access_path(old_dir, LANDLOCK_ACCESS_FS_MAKE_CHAR);
+		err = current_check_access_path(old_dir,
+						LANDLOCK_ACCESS_FS_MAKE_CHAR);
 		if (err)
 			return err;
 	}
diff --git a/tools/testing/selftests/landlock/fs_test.c b/tools/testing/selftests/landlock/fs_test.c
index d867016e3fd3..4cf6fc0bcb71 100644
--- a/tools/testing/selftests/landlock/fs_test.c
+++ b/tools/testing/selftests/landlock/fs_test.c
@@ -6962,6 +6962,37 @@ TEST_F_FORK(layout2_overlay, same_content_different_file)
 	}
 }
 
+TEST_F_FORK(layout2_overlay, rename_in_overlay_without_make_char)
+{
+	struct stat st;
+	const char *merge_fl1_renamed = MERGE_DATA "/fl1_renamed";
+
+	if (self->skip_test)
+		SKIP(return, "overlayfs is not supported (test)");
+
+	enforce_fs(_metadata, LANDLOCK_ACCESS_FS_MAKE_CHAR, NULL);
+
+	/*
+	 * Execute a regular file rename within OverlayFS.
+	 * merge_fl1 originates from lower layer, so this triggers a copy-up
+	 * and creation of a whiteout in the upper layer.
+	 */
+	EXPECT_EQ(0, rename(merge_fl1, merge_fl1_renamed));
+
+	/* Check that the rename worked. */
+	EXPECT_EQ(0, stat(merge_fl1_renamed, &st));
+	EXPECT_EQ(-1, stat(merge_fl1, &st));
+	EXPECT_EQ(ENOENT, errno);
+
+	/*
+	 * Check that the whiteout object on the underlying "upper" filesystem
+	 * exists after the rename.  This is OK because it was done with the
+	 * credentials of the OverlayFS.
+	 */
+	EXPECT_EQ(0, stat(UPPER_DATA "/fl1", &st));
+	EXPECT_TRUE(S_ISCHR(st.st_mode));
+	EXPECT_EQ(0, st.st_rdev);
+}
 
 FIXTURE(layout3_fs)
 {
-- 
2.54.0.rc0.605.g598a273b03-goog