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:

1. Open a /dev/hidg* device file
2. Trigger an unbind/bind cycle of the USB gadget configuration while holding the file descriptor open
3. 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:

• Kernel commit 5a229016ca3a
• Kernel commit 75ecc46828ec
• Kernel commit 81ebd43cc0d6
• Kernel commit c6c0d13db5d0
• Kernel commit eb6ef6185f20

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

# 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