In June 2024, a significant vulnerability was discovered (and fixed) in the Linux kernel's "User Mode Linux" (UML) subsystem, specifically affecting the release process of vector devices. Catalogued as CVE-2024-53181, this bug could lead to a system crash or "kernel panic" when a vector device is removed. Understanding this vulnerability helps inform both system administrators and curious users about the impact, the patch, and how such kernel bugs arise.

This article breaks down the original problem, demonstrates the relevant code, discusses the underlying risks, and explains how it was fixed. We'll also show how a malicious actor could exploit the bug and provide all the original references for you to read further.

What is the Vector Device and UML?

User Mode Linux (UML) is a port of the Linux kernel that allows Linux to run as a user process under another Linux host system. It's mainly used for testing, kernel development, and research.

A vector device is a virtual peripheral managed by UML for testing or simulation purposes—think of it as a software emulation of hardware devices.

Core Details of the Vulnerability

When a vector device in UML is removed (unregistered), the kernel calls a function — vector_device_release() — to clean up the device. The error happened because this function tried to access memory (drvdata) that had already been freed or was unavailable, resulting in a crash. The correct way is to use the kernel's standard container_of() macro to retrieve the parent structure safely.

The result: A potential malicious user or buggy system process could force a kernel panic and denial of service (DoS) by triggering this clean-up in the wrong context.

Here's part of the crash output you might see

RIP: 0033:vector_device_release+xf/x50
...
Kernel panic - not syncing: Segfault with no mm
CPU:  UID:  PID: 16 Comm: kworker/:1 Not tainted 6.12.-rc6-g59b723cd2adb #1
Workqueue: events mc_work_proc
...
Call Trace:
 [<60028f61>] ? vector_device_release+x/x50
 [<60276fcd>] device_release+x70/xba
...

Here’s a simplified version of the buggy function (before the fix)

static void vector_device_release(struct device *dev)
{
    struct vector_device *vdev = dev_get_drvdata(dev); // <--- Problem!
    kfree(vdev);
}

Why is this wrong?
The dev_get_drvdata(dev) assumes that drvdata is always set and available. However, by the time the release function is called, drvdata may have been cleared or the pointer invalidated, resulting in a use-after-free and crash.

The Patch

The fix changes the function to use container_of(), a kernel macro that safely retrieves the parent structure from its member pointer:

static void vector_device_release(struct device *dev)
{
    struct vector_device *vdev = container_of(dev, struct vector_device, dev);
    kfree(vdev);
}

Now, even if drvdata is gone, we can get the correct pointer to free safely.

How Could This Be Exploited?

While this isn't a direct privilege escalation, the bug can be exploited for a Denial of Service (DoS):

Triggering Removal: The device is then removed (e.g., via rmmod or a management command).

3. Crash Kernel: The kernel, using the buggy code, attempts to run the release function, dereferences an invalid pointer, and crashes—panicking the whole UML instance.

Example PoC (Proof of Concept)

# Inside a UML instance (as root or a privileged user)
echo vectorX > /sys/bus/platform/devices/remove

You might have to adjust based on the device naming, or trigger device removal through any UML management tool. Crashing is unintentional for most, but possible. If the victim runs UML instances in production (e.g., for sandboxing or CI), this DoS could matter a lot.

Who Is Affected?

- Linux kernels with UML enabled before this patch (merged pre-6.13)
- Any system that creates/removes UML vector devices.

Mitigation:
Upgrade to a kernel version that includes the fix (6.13+ or the relevant backported patch).

References

- Linux Kernel Patch: "um: vector: Do not use drvdata in release"
- UML Kernel Source
- CVE Record *(record may take some days to appear)*

Conclusion

CVE-2024-53181 is a strong example of how even small mistakes in kernel release routines can cause critical stability or security issues. If you run UML or work on kernel internals, make sure your systems are updated—especially with virtualization or sandboxing setups. For attackers, this was a target for easy denial-of-service on UML guests. For defenders, it's an issue that's now already solved—but worth learning from to spot similar kernel mistakes in the future.

Timeline

Published on: 12/27/2024 14:15:25 UTC
Last modified on: 05/04/2025 09:55:06 UTC