CVE-2026-31606: Linux Kernel Use-After-Free Vulnerability

CVE-2026-31606 Overview

CVE-2026-31606 is a Use After Free vulnerability in the Linux kernel's USB gadget HID function driver (f_hid). The vulnerability occurs when the cdev_init function is called during a bind operation while the character device (cdev) is still in use from a previous binding. This unsafe reinitialization can happen when /dev/hidg* device files remain open during an unbind/bind cycle, leading to kernel instability and potential denial of service conditions.

Critical Impact
Local attackers with low privileges can trigger kernel oops and denial of service by exploiting the unsafe cdev reinitialization during USB gadget rebind operations while device files are held open.

Affected Products

Linux Kernel (multiple versions)
Systems using USB gadget HID functionality (f_hid driver)
Embedded systems and devices implementing USB HID gadgets

Discovery Timeline

2026-04-24 - CVE CVE-2026-31606 published to NVD
2026-04-29 - Last updated in NVD database

Technical Details for CVE-2026-31606

Vulnerability Analysis

The vulnerability resides in the USB gadget HID function driver (drivers/usb/gadget/function/f_hid.c). When a USB gadget configuration is unbound and then rebound, the hidg_bind function calls cdev_init() to initialize the character device structure. However, if a userspace process still holds an open file descriptor to the /dev/hidg* device from the previous bind, references to the old cdev structure still exist.

Reinitializing a cdev structure while it has active references corrupts internal kernel data structures. The cdev structure contains embedded reference counting and list pointers that are invalidated by cdev_init(), leading to kernel oops when the stale references are later accessed or released.

Root Cause

The root cause is improper lifecycle management of the cdev structure in the f_hid driver. The original implementation embedded the cdev structure directly within the f_hidg structure and reused it across bind/unbind cycles. When cdev_init() is called on an in-use cdev, it zeros out critical fields including the embedded kobject reference counter, breaking the kernel's object lifetime management.

The fix replaces the embedded cdev with a dynamically allocated one using cdev_alloc(). This allows a fresh cdev structure to be created for each bind operation while the old structure remains valid until all references are released.

Attack Vector

This vulnerability requires local access to the system. An attacker must have sufficient privileges to interact with the USB gadget subsystem, specifically the ability to:

Open a /dev/hidg* device file
Trigger an unbind/bind cycle of the USB gadget configuration while holding the file descriptor open
Access the device or allow the kernel to clean up the stale reference

The attack results in a kernel oops, causing system instability or a denial of service. While the vulnerability does not directly enable code execution or information disclosure, kernel memory corruption could potentially be leveraged for further exploitation in sophisticated attack scenarios.

The vulnerability is exploited through the following sequence: a process opens /dev/hidg0, the gadget is unbound via configfs or sysfs, the gadget is rebound triggering cdev_init() on the active device structure, and subsequent operations on the held file descriptor or device cleanup trigger the corruption.

Detection Methods for CVE-2026-31606

Indicators of Compromise

Kernel oops or panic messages referencing f_hid, hidg, or cdev functions in the stack trace
System instability when USB gadget configurations are modified while HID devices are in use
Unexpected crashes in embedded systems using USB HID gadget functionality

Detection Strategies

Monitor kernel logs (dmesg, /var/log/kern.log) for oops messages involving the USB gadget subsystem
Implement system call monitoring for suspicious patterns of opening /dev/hidg* followed by configfs manipulation
Deploy kernel live patching solutions to detect and prevent exploitation attempts on vulnerable kernel versions

Monitoring Recommendations

Enable kernel crash dump collection (kdump) to capture forensic data from kernel oops events
Configure audit rules to log access to USB gadget configfs interfaces
Monitor for unusual process behavior involving simultaneous /dev/hidg* access and gadget configuration changes

How to Mitigate CVE-2026-31606

Immediate Actions Required

Update the Linux kernel to a patched version that includes the fix for CVE-2026-31606
Restrict access to USB gadget configuration interfaces to trusted administrators only
Ensure /dev/hidg* device permissions are appropriately restrictive
Consider disabling USB gadget functionality if not required in production environments

Patch Information

The Linux kernel maintainers have released patches addressing this vulnerability. The fix modifies the f_hid driver to use cdev_alloc() for dynamic allocation of the character device structure instead of reinitializing an embedded cdev. This ensures proper reference counting and prevents corruption when devices are rebound while file descriptors remain open.

Patches are available in the stable kernel tree through the following commits:

Workarounds

Ensure all /dev/hidg* file descriptors are closed before modifying USB gadget configurations
Implement administrative procedures to prevent unbind/bind operations while HID gadgets are actively in use
Use cgroups or namespace isolation to limit access to the USB gadget subsystem
Consider building kernels without CONFIG_USB_CONFIGFS_F_HID if HID gadget functionality is not required

bash

# Check if the vulnerable module is loaded
lsmod | grep usb_f_hid

# Restrict access to HID gadget devices
chmod 600 /dev/hidg*
chown root:root /dev/hidg*

# Disable USB gadget HID if not needed (requires recompilation)
# CONFIG_USB_CONFIGFS_F_HID=n