The Linux kernel is the beating heart of countless devices—including many you use daily. Despite its robustness, security bugs can slip through, and sometimes, these issues can have serious repercussions. Let’s break down a major Linux kernel vulnerability—CVE-2025-21776—in simple terms, look at how an attacker could exploit it, and show you the fix.

What's the Vulnerability?

CVE-2025-21776 occurs in the Linux kernel’s handling of USB hubs. In short, the Linux USB subsystem didn’t handle certain noncompliant USB devices correctly. If a USB hub device announces *too many* configurations or interfaces (which the USB specification prohibits), this could trick the kernel into a crash—a “general protection fault.” In some rare configurations, this could even lead to code execution.

Here’s a snippet of the crash log reported by Robert Morris, a renowned security researcher

Oops: general protection fault, probably for non-canonical address xcccccccccccccccc
...
RIP: 001:usb_hub_adjust_deviceremovable+x78/x110
...
hub_probe+x7c7/xab
usb_probe_interface+x14b/x350
...

Why Does This Happen?

Normally, the Linux USB hub driver expects a USB hub to comply with the spec: specifically, only one configuration with one interface. But a malicious or buggy device could present itself with multiple interfaces. The kernel might then bind the hub driver to interface 1 (instead of the expected interface ), causing the following line:

usb_hub_to_struct_hub(iface->dev.driver_data)

to dereference a NULL or incorrect pointer. This can result in a kernel panic, a denial of service (crash), or potentially more.

Key Point

> If a USB device presents too many configurations or interfaces, the Linux kernel may crash because internal pointers go awry.

Suppose you have a device with the following USB descriptors

#define FAKE_NUM_CONFIGS 2
#define FAKE_NUM_INTERFACES 2

// Send descriptors to make device non-compliant:
struct usb_device_descriptor fake_device_desc = {
    .bNumConfigurations = FAKE_NUM_CONFIGS,
};

struct usb_config_descriptor fake_config_desc[FAKE_NUM_CONFIGS] = {
    { .bNumInterfaces = FAKE_NUM_INTERFACES, },
    // ... fill out accordingly
};

A user (even non-root) with physical access could connect a specially programmed USB device that returns multiple configurations or interfaces. When the Linux kernel tries to enumerate that device, it may assign the hub driver to the wrong interface. The key function, usb_hub_adjust_deviceremovable(), would then use a bad pointer, causing a crash.

On vulnerable kernels (6.13.-rc3 to before the fix), plugging in such a device reliably triggers a crash.

The solution is simple, but effective

- Ignore USB hub devices that violate the USB spec (have more than one configuration or interface).

Here’s the patch—the fix in code

if (udev->descriptor.bNumConfigurations != 1 ||
    config->desc.bNumInterfaces != 1) {
    dev_warn(dev, "Ignoring non-compliant USB hub device\n");
    return -ENODEV;
}

Mitigating the Issue

- Update your Linux kernel: Make sure your distribution has patched this bug (look for kernel versions after 6.13.-rc3).
- Restrict physical access: Most kernel USB bugs require a local attacker to physically connect a malicious device.
- Consider USBGuard or similar tools: Restrict what types of USB devices can attach to your system.

References

- Linux Kernel Patch – The official fix
- LKML Discussion (thread)
- NVD entry for CVE-2025-21776

Bottom Line

CVE-2025-21776 highlights how even low-level bugs can create significant threats. While the real-world risk may be mitigated by the requirement for physical access, the best defense is always an up-to-date kernel. Stay alert, stay updated—and remember: not all USB sticks should be trusted.


If you found this useful, share it with your sysadmin friends! Stay secure, Linux folks.

Timeline

Published on: 02/27/2025 03:15:18 UTC
Last modified on: 05/04/2025 07:20:56 UTC