CVE-2026-31725: Linux Kernel USB Gadget Vulnerability

CVE-2026-31725 Overview

CVE-2026-31725 affects the Linux kernel's USB gadget Ethernet Control Model (ECM) function driver (f_ecm). The vulnerability stems from improper handling of the net_device lifecycle during USB gadget bind and unbind cycles. When the function unbinds, the parent gadget device is destroyed while the net_device survives, leaving dangling sysfs symlinks under /sys/class/net/usb0. A local authenticated attacker with the ability to trigger gadget bind/unbind operations can cause kernel-level resource inconsistency, leading to availability impact on the affected system.

Critical Impact
Local users can trigger dangling sysfs references in the USB gadget subsystem, resulting in kernel resource corruption and potential denial of service on systems using the f_ecm USB gadget function driver.

Affected Products

Linux Kernel (multiple stable branches)
Linux Kernel 7.0 release candidates (rc1 through rc6)
Systems using the usb: gadget: f_ecm function driver

Discovery Timeline

2026-05-01 - CVE-2026-31725 published to NVD
2026-05-07 - Last updated in NVD database

Technical Details for CVE-2026-31725

Vulnerability Analysis

The vulnerability resides in the USB gadget ECM function driver (f_ecm), which provides Ethernet-over-USB connectivity. The net_device is allocated during function instance creation and registered during the bind phase, with the gadget device set as its sysfs parent.

When the function unbinds, the parent gadget device is destroyed before the net_device is properly reparented or unregistered. This leaves the net_device in an inconsistent state with sysfs symlinks pointing to a destroyed parent device. Listing /sys/class/net/usb0 shows a symlink pointing to a nonexistent path under /sys/devices/platform/.../gadget.0/net/usb0.

This is a kernel resource lifecycle flaw [CWE-NVD-noinfo] affecting the USB gadget subsystem. Exploitation requires local access with privileges to manipulate USB gadget configuration through configfs.

Root Cause

The root cause is missing reparenting logic for the net_device across gadget bind and unbind cycles. The original code assumed the net_device parent would remain valid throughout its lifetime. However, USB gadget functions can unbind while the network device persists, breaking the implicit parent-child relationship in the device model.

Attack Vector

A local attacker with privileges to configure USB gadget functions through configfs can trigger repeated bind and unbind cycles on the ECM function. Each unbind operation leaves dangling sysfs references and corrupts the device hierarchy. Sustained exploitation degrades system stability and can result in availability loss for the USB gadget networking stack.

The upstream fix introduces device_move() calls to reparent the net_device between the gadget device tree and /sys/devices/virtual across bind and unbind cycles. During final unbind, device_move(NULL) moves the net_device to the virtual device tree before the gadget device is destroyed. On rebinding, device_move() reparents the device back under the new gadget. A bound flag maintains compatibility with legacy composite drivers such as multi.c.

Detection Methods for CVE-2026-31725

Indicators of Compromise

Dangling sysfs symlinks under /sys/class/net/usb* pointing to nonexistent gadget paths
Kernel log messages referencing f_ecm bind or unbind failures
Repeated USB gadget configuration changes through configfs from non-administrative contexts
Unexpected net_device registration anomalies in dmesg output

Detection Strategies

Monitor kernel logs for warnings related to device_move, sysfs symlink errors, or f_ecm lifecycle events
Audit configfs operations under /sys/kernel/config/usb_gadget/ to identify unauthorized bind/unbind activity
Track running kernel version against the patched commits in the upstream stable trees
Validate sysfs topology integrity by comparing /sys/class/net/ symlinks against their target paths

Monitoring Recommendations

Enable kernel audit rules covering configfs writes within the USB gadget hierarchy
Collect dmesg and journald output centrally and alert on USB gadget driver anomalies
Restrict access to USB gadget configuration interfaces to trusted administrative accounts only
Track CPE matches against cpe:2.3:o:linux:linux_kernel to identify vulnerable hosts in inventory

How to Mitigate CVE-2026-31725

Immediate Actions Required

Apply the upstream Linux kernel patches from the stable tree once available in your distribution
Restrict access to USB gadget configfs interfaces to root and trusted system services
Disable the f_ecm USB gadget function on systems where USB Ethernet gadget functionality is not required
Inventory all Linux systems running affected kernel versions, including 7.0 release candidates

Patch Information

The vulnerability is resolved in upstream Linux kernel commits. The fix introduces device_move() to properly reparent the net_device across bind and unbind cycles. See the following references for the patch commits: Kernel Git Commit 4e34f3f, Kernel Git Commit 5eaeac2, Kernel Git Commit 9b1e558, and Kernel Git Commit b2cc4fa.

Workarounds

Unload the usb_f_ecm kernel module on systems that do not require USB Ethernet gadget functionality
Remove ECM function configurations from USB gadget configfs setups and substitute with alternative function drivers where feasible
Limit physical and administrative access to embedded devices exposing USB gadget interfaces
Apply mandatory access control policies (SELinux, AppArmor) to restrict configfs write access

bash

# Configuration example: unload the f_ecm module and verify
sudo rmmod usb_f_ecm
lsmod | grep usb_f_ecm

# Restrict access to USB gadget configfs
sudo chmod 700 /sys/kernel/config/usb_gadget/