[PATCH 1/3] landlock: Require LANDLOCK_ACCESS_FS_MAKE_CHAR for RENAME_WHITEOUT

["Günther Noack" <gnoack@google.com>]

renameat2(2) with the RENAME_WHITEOUT flag places a whiteout character
device file in the source file location in place of the moved file,
bypassing the LANDLOCK_ACCESS_FS_MAKE_CHAR right.

Fix this by checking for LANDLOCK_ACCESS_FS_MAKE_CHAR if RENAME_WHITEOUT is
passed.

This does not affect normal renames within layered OverlayFS mounts: When
OverlayFS invokes rename with RENAME_WHITEOUT as part of a "normal" rename
operation, it does so in ovl_rename() using the credentials that were set
at the time of mounting the OverlayFS.

Suggested-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Günther Noack <gnoack@google.com>
---
 security/landlock/fs.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/security/landlock/fs.c b/security/landlock/fs.c
index c1ecfe239032..2b84a229e4d8 100644
--- a/security/landlock/fs.c
+++ b/security/landlock/fs.c
@@ -1519,6 +1519,19 @@ static int hook_path_rename(const struct path *const old_dir,
 			    const unsigned int flags)
 {
 	/* old_dir refers to old_dentry->d_parent and new_dir->mnt */
+	if (flags & RENAME_WHITEOUT) {
+		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().
+		 */
+		err = current_check_access_path(old_dir, LANDLOCK_ACCESS_FS_MAKE_CHAR);
+		if (err)
+			return err;
+	}
+
 	return current_check_refer_path(old_dentry, new_dir, new_dentry, true,
 					!!(flags & RENAME_EXCHANGE));
 }
-- 
2.54.0.rc0.605.g598a273b03-goog