CVE-2026-31724: Linux Kernel Privilege Escalation Flaw

CVE-2026-31724 Overview

CVE-2026-31724 is a Linux kernel vulnerability in the USB gadget Ethernet Emulation Model (EEM) function driver (f_eem). The flaw stems from improper net_device lifecycle management during gadget bind and unbind cycles. The net_device is allocated during function instance creation and registered with the gadget device as its sysfs parent. When the function unbinds, the parent device is destroyed while the net_device survives, leaving dangling sysfs symlinks under /sys/class/net/usb0. A local authenticated user can trigger this condition to cause a denial of service through kernel state corruption.

Critical Impact

Local low-privileged users can disrupt USB gadget networking subsystems, leading to denial of service through dangling sysfs references and broken power management ordering.

Affected Products

• Linux Kernel (mainline branches affected)
• Linux Kernel 7.0 release candidates rc1 through rc6
• Systems using USB gadget f_eem function driver

Discovery Timeline

• 2026-05-01 - CVE-2026-31724 published to NVD
• 2026-05-07 - Last updated in NVD database

Technical Details for CVE-2026-31724

Vulnerability Analysis

The vulnerability resides in the USB gadget EEM function driver located at drivers/usb/gadget/function/f_eem.c. The driver allocates a net_device instance when the function is created and registers it during the bind phase using the gadget device as its sysfs parent. This parent-child relationship creates a lifecycle dependency that the original code fails to manage correctly.

When the gadget function unbinds, the parent gadget device is destroyed. The net_device object, however, persists across unbind cycles. This mismatch produces dangling sysfs symlinks. The symbolic link at /sys/class/net/usb0 continues pointing to a destroyed device path under /sys/devices/platform/.../gadget.0/net/usb0, which no longer exists in the device tree.

The upstream fix uses device_move() to reparent the net_device between the gadget device tree and /sys/devices/virtual across bind and unbind cycles. During the final unbind, device_move(NULL) migrates the net_device to the virtual device tree before the gadget device is destroyed.

Root Cause

The root cause is improper object lifecycle management between two kernel objects with differing scopes. The net_device outlives its sysfs parent because the function driver does not reparent or unregister the network device before the gadget device is torn down. Legacy composite drivers such as multi.c further complicate the lifecycle by pre-registering and sharing the network device during their bind phase.

Attack Vector

Exploitation requires local access with the ability to interact with USB gadget configuration interfaces, typically through configfs mounted at /sys/kernel/config/usb_gadget/. An attacker repeatedly binds and unbinds the EEM function to corrupt sysfs topology and kernel state. The result is high availability impact through kernel resource corruption, with no confidentiality or integrity impact.

No public proof-of-concept exploit code is currently available for this vulnerability.

Detection Methods for CVE-2026-31724

Indicators of Compromise

• Dangling symlinks under /sys/class/net/ pointing to non-existent gadget.X/net/ paths
• Kernel log messages referencing f_eem errors during USB gadget bind or unbind operations
• Repeated configfs write operations to UDC (USB Device Controller) attributes from non-administrative processes
• Failed ls operations on previously valid /sys/devices/platform/.../gadget.X/net/usbN paths

Detection Strategies

• Monitor kernel ring buffer (dmesg) for warnings related to f_eem, net_device registration failures, or sysfs symlink errors
• Audit Linux kernel versions across the fleet using package management telemetry to identify systems running affected 7.0-rc1 through 7.0-rc6 builds
• Inspect filesystem activity targeting /sys/kernel/config/usb_gadget/ for unusual bind/unbind patterns

Monitoring Recommendations

• Enable kernel auditing (auditd) for write operations against configfs USB gadget paths
• Track process executions invoking modprobe, rmmod, or direct configfs manipulation against gadget functions
• Correlate USB subsystem kernel events with user session activity to detect anomalous local interactions
• Forward kernel telemetry to a centralized analytics platform for retention and historical querying

How to Mitigate CVE-2026-31724

Immediate Actions Required

• Apply the upstream kernel patches referenced in the Linux stable tree commits as soon as vendor builds are available
• Restrict access to USB gadget configfs interfaces to root and trusted system services using filesystem permissions and namespace isolation
• Unload the f_eem module on systems that do not require USB Ethernet Emulation Model functionality

Patch Information

The vulnerability is resolved in the upstream Linux kernel through four commits: 14730506, 4ccdccff, a6b8bce0, and d9270c9a. The fix introduces device_move() calls to correctly reparent the net_device across bind and unbind cycles and adds a bound flag to maintain compatibility with legacy composite drivers.

Workarounds

• Blacklist the f_eem kernel module on systems that do not require USB Ethernet Emulation Model gadget functionality
• Limit physical USB access to the affected systems to reduce the local attack surface
• Constrain configfs mount points using mandatory access control policies such as SELinux or AppArmor

# Blacklist the f_eem module to prevent loading
echo "blacklist usb_f_eem" | sudo tee /etc/modprobe.d/disable-f_eem.conf
sudo update-initramfs -u

# Verify the module is not loaded
lsmod | grep usb_f_eem

# Restrict configfs access to root only
sudo chmod 700 /sys/kernel/config/usb_gadget/