CVE-2022-45888 - Race Condition and Use-After-Free Vulnerability in the Linux Kernel's XillyUSB Driver

A security vulnerability has been discovered in the Linux kernel through version 6..9 and was assigned the identifier CVE-2022-45888. This vulnerability affects the XillyUSB driver (drivers/char/xillybus/xillyusb.c) and involves a race condition and use-after-free issue during the physical removal of a USB device. This post will describe the technical details of the vulnerability, provide code snippets for better understanding, and present available information about potential exploitation techniques and mitigations.

Vulnerability Details

The Linux kernel has a module called XillyUSB which is a part of the Xillybus IP core (a.k.a. FPGA) communication framework. Xillybus makes it easy for developers to design and implement FPGA-based systems with a standard communication interface over USB. The XillyUSB driver is responsible for handling the communication between the FPGA and the Linux kernel.

The vulnerability in question occurs when a physical USB device is being removed. It stems from a race condition in conjunction with a use-after-free error in the xillyusb.c source file. The affected code snippet is as follows:

static void xillyusb_disconnect(struct usb_interface *interface)
{
    struct xillyusb_endpoint *ep;
    struct xillyusb_dev *pdev;

    pdev = usb_get_intfdata(interface);
    ep = pdev->ep;

    kref_put(&pdev->kref, xillyusb_delete);

    usb_put_intfdata(interface, NULL);
}

In the xillyusb_disconnect() function, there is a call to kref_put() which is intended to reduce the reference count of the pdev object and eventually free the associated memory when the count reaches zero. However, the reference count decrement and object deletion are not atomic operations in the Linux kernel, which creates a window for a race condition if a context switch occurs at an inopportune time. More specifically, a race condition could arise if a higher priority process preempts the driver's execution after kref_put() but before usb_put_intfdata().

As a result, another process could potentially use the pdev object after it has already been deleted, leading to a use-after-free error and potential security implications. Attackers could potentially exploit this vulnerability to execute arbitrary code, cause a denial of service, or escalate their privileges on the affected system.

Original References

1. The CVE entry in the NVD database: https://nvd.nist.gov/vuln/detail/CVE-2022-45888

2. The Linux kernel source code repository on GitHub with the vulnerable xillyusb.c file: https://github.com/torvalds/linux/blob/master/drivers/char/xillybus/xillyusb.c

3. Official Xillybus documentation: http://xillybus.com/documentation

Exploit Details

There are currently no known public exploits for this vulnerability. However, exploiting a use-after-free vulnerability typically involves manipulating memory allocation and deallocation routines to reuse the freed memory region in a controlled manner. This can lead to arbitrary code execution or an unauthorized privilege escalation on the affected system. Attackers with sufficient knowledge of the Linux kernel internals and memory management could potentially craft a custom exploit, but this would likely require significant effort and expertise.

Mitigations and Fixes

The Linux kernel developers are aware of this vulnerability and are working on a patch to fix the issue. The patch will likely involve proper synchronization or locking mechanisms to ensure that the reference count decrement and object deletion are atomic operations, eliminating the race condition and the use-after-free issue.

In the meantime, it is recommended to avoid physically removing USB devices while interacting with the XillyUSB driver and related FPGA devices on affected Linux kernel versions, as this has the potential to trigger the vulnerability. Additionally, monitoring for unauthorized access or unusual activity can help identify potential exploitation attempts.

Conclusion

CVE-2022-45888 is a security vulnerability in the Linux kernel's XillyUSB driver (drivers/char/xillybus/xillyusb.c) with potentially severe consequences. It arises from a race condition and a use-after-free issue during the physical removal of a USB device. Linux kernel developers are working to address this vulnerability, and it is essential for users and administrators to be aware of the potential risk and apply any available patches once they become available.

Timeline

Published on: 11/25/2022 04:15:00 UTC
Last modified on: 01/20/2023 20:20:00 UTC