CVE-2025-21721 is a medium severity vulnerability in the Linux kernel's nilfs2 filesystem code. It involves improper error handling in the nilfs_prepare_chunk() function, which can fail due to metadata corruption or other reasons. The flaw causes BUG_ON check failures during rename operations and incorrect directory modification time behavior. The issue arises because certain directory manipulation routines assumed nilfs_prepare_chunk() would always succeed, omitting necessary error handling. This has been fixed by adding proper error paths in nilfs_set_link(), nilfs_delete_entry(), and nilfs_rename().