CVE-2025-21902 addresses a vulnerability in the Linux kernel's ACPI backend for UCSI (USB Type-C Connector System Software Interface). The issue involves improper handling of register synchronization between the embedded controller and the ACPI opregion, which can cause spurious kernel warnings and potential instability when polling the CCI register with notifications disabled. The vulnerability is mitigated by introducing a new ->poll_cci() method that forces synchronization safely under these conditions.