CVE-2025-21631 is a use-after-free (UAF) vulnerability in the Linux kernel's BFQ (Budget Fair Queueing) block I/O scheduler. The flaw occurs after the bfq_split_bfqq() function, leading to a KASAN-detected slab-use-after-free error in bfq_init_rq. This vulnerability affects Linux kernel version 6.6.0 and allows local attackers with limited privileges to cause memory corruption, potentially resulting in system crashes or escalation of privileges. The issue has been fixed in a later kernel update.