CVE-2025-40120

In the Linux kernel, the following vulnerability has been resolved:

net: usb: asix: hold PM usage ref to avoid PM/MDIO + RTNL deadlock

Prevent USB runtime PM (autosuspend) for AX88772* in bind.

usbnet enables runtime PM (autosuspend) by default, so disabling it via the usb_driver flag is ineffective. On AX88772B, autosuspend shows no measurable power saving with current driver (no link partner, admin up/down). The ~0.453 W -> ~0.248 W drop on v6.1 comes from phylib powering the PHY off on admin-down, not from USB autosuspend.

The real hazard is that with runtime PM enabled, ndo_open() (under RTNL) may synchronously trigger autoresume (usb_autopm_get_interface()) into asix_resume() while the USB PM lock is held. Resume paths then invoke phylink/phylib and MDIO, which also expect RTNL, leading to possible deadlocks or PM lock vs MDIO wake issues.

To avoid this, keep the device runtime-PM active by taking a usage reference in ax88772_bind() and dropping it in unbind(). A non-zero PM usage count blocks runtime suspend regardless of userspace policy (.../power/control - pm_runtime_allow/forbid), making this approach robust against sysfs overrides.

Holding a runtime-PM usage ref does not affect system-wide suspend; system sleep/resume callbacks continue to run as before.

This bug in the ASIX USB Ethernet driver is a runtime power-management locking issue. Because usbnet enables autosuspend by default, bringing an AX88772* interface up under RTNL may synchronously trigger a runtime resume while the USB PM lock is held. The resume path then calls phylink/phylib and MDIO helpers which also expect RTNL, creating a possible PM-lock vs RTNL/MDIO deadlock and causing the network device (or system) to hang.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.