The Linux kernel vulnerability CVE-2026-31741 involves the rz-mtu3-cnt counter interface where writing repeatedly to the sysfs enable file improperly increments or decrements the runtime PM usage count. Writing 0 multiple times causes an underflow, triggering error messages and hardware access with clocks off, while writing 1 multiple times requires corresponding 0 writes to balance the count. Additionally, stopping PWM while it is in progress without proper ownership can occur. The fix prevents toggling the counter multiple times by checking the cached enable state before applying changes. This vulnerability affects multiple Linux kernel versions prior to 6.8.x releases and is addressed in official Ubuntu kernel updates.

The vulnerability can cause runtime PM usage count underflow and improper hardware register access, leading to potential disruption of PWM operations. The CVSS score of 5.5 (medium severity) reflects that the issue does not impact confidentiality or integrity but causes availability impact (denial of service or disruption). There is no indication of privilege escalation or information disclosure from this specific flaw.

A patch is available and included in official Ubuntu kernel updates for affected versions. Users should apply the kernel updates and reboot their systems to ensure the vulnerability is resolved. Due to an ABI change, recompilation and reinstallation of third-party kernel modules may be required. No additional mitigation actions are specified or necessary beyond applying the official fix.