A recently resolved vulnerability (CVE-2025-21877) in the Linux kernel's gl620a USB network driver could let attackers misuse broken or malicious USB devices to trigger kernel warnings, possible system crashes, or undefined USB stack behavior. The root cause involves failing to validate USB endpoint presence during device binding, allowing malformed descriptors to slip through.

This post explains the bug, shows how it could be triggered, presents code, and links you to original references for further reading.

The Vulnerability in Simple Words

When you plug in a USB gadget (like an ethernet dongle), the Linux kernel checks its *endpoints*. Endpoints are "doors" for sending or receiving data. If a driver's code doesn't check these doors carefully, a broken or malicious device can confuse the driver and cause unpredictable behavior. That’s what happened in the genelink_bind function of the gl620a USB network driver.

Potential DoS (Denial of Service) by crashing the system

Reference:
Linux kernel commit fixing the bug (commit 02cfd19d2b53)

Here’s the relevant code snippet from the buggy driver

int genelink_bind(struct usbnet *dev, struct usb_interface *intf)
{
    // ... setup and preparation

    // This was missing:
    // usbnet_get_endpoints(dev, intf);

    // Further code assumes endpoints exist...
}

What’s missing?
The code should check that the required endpoints (doors) actually exist. Without this, a malicious or broken USB gadget can advertise itself as a GL620A device but NOT provide the right endpoints. The driver then tries to use those missing endpoints, causing errors deep in the kernel’s USB core.

How Was It Found?

This bug was discovered by syzkaller/syzbot, a Linux kernel fuzzer. Syzbot plugged in a made-up USB device with a bad descriptor and watched as the driver failed catastrophically.

Example warning

usb 5-1: BOGUS urb xfer, pipe 3 != type 1
WARNING: CPU: 2 PID: 1841 at drivers/usb/core/urb.c:503 usb_submit_urb+xe4b/x173

The bug appeared because the driver assumed endpoints were present and didn’t check.

Realistic Exploitation Scenario

1. An attacker creates a malformed USB device (e.g., using a microcontroller like STM32, Teensy, or Raspberry Pi Pico with custom USB descriptors).

The device claims to be a GL620A adapter but omits or corrupts required endpoints.

3. When the kernel loads the driver, the missing endpoint causes the driver to access the wrong structure or memory, triggering kernel warnings or a crash.

Note: There’s no direct path to code execution, but this can be part of larger kernel fuzzing/exploitation or a local Denial-of-Service attack.

Proof-of-Concept (PoC)

Here’s how you might create a *payload* with tools like LUSB, Facedancer, or GreatFET to simulate such a device.

USB Descriptor for PoC (Python pseudocode for Facedancer)

from usb.core import Device

# Malformed GL620A device: missing bulk endpoint
dev = Device()
dev.idVendor = x066b  # Example GL620A vendor ID
dev.idProduct = x2202 # Example product ID

# Add only control endpoint, skip the usual bulk in/out
dev.add_endpoint(x00, 'control')

# Plug into Linux system and watch for kernel warning or crash

Or, you can use syzkaller’s syz_usb_connect for fuzzing these interface descriptors.

The Fix

The resolution is straightforward: use utility functions to check endpoints early and abort binding if something is missing.

Fixed code

int genelink_bind(struct usbnet *dev, struct usb_interface *intf)
{
    if (!usbnet_get_endpoints(dev, intf))  // <-- This line added!
        return -ENODEV;

    // Now safe to continue: endpoints exist!
    ...
}

Now, if the USB endpoints are wrong or missing, the driver bails out cleanly.

See the fix:
- Patch: usbnet: gl620a: fix endpoint checking in genelink_bind()

Timeline and Credits

- Reported by: syzkaller fuzz testing
- Patched in: Linux mainline (as of commit 02cfd19d2b53)

Fix by: Alexander Levin, Greg Kroah-Hartman, and Linux USB team

- CVE ID: CVE-2025-21877 (pending full public entry)

- Linux Kernel Commit Fix
- Patch on Lore
- Syzbot original report
- How to fuzz USB with syzkaller

Final Thoughts

CVE-2025-21877 is a good example of how even simple validation oversights at the kernel level can lead to system instability and potential security issues. If you use Linux as a desktop, server, or embedded system–keep your system up to date!

If you maintain distributions, enterprise kernels, or ship devices that load USB drivers, be sure you backport this fix to eliminate the issue.

Timeline

Published on: 03/27/2025 15:15:55 UTC
Last modified on: 05/04/2025 07:23:09 UTC