Overview
In this post, we'll be discussing a recently discovered vulnerability in the Linux kernel (CVE-2022-27223) that affects the USB gadget subsystem. This vulnerability presents a potential security risk, as an attacker might be able to trigger out-of-array access through a rogue USB device. We will provide an overview of the vulnerability, dive into the affected code, provide key references, and discuss potential exploit scenarios.

Background

The vulnerability (CVE-2022-27223) lies in the drivers/usb/gadget/udc/udc-xilinx.c file in the Linux kernel before 5.16.12, specifically in the endpoint index handling. Insufficient validation of the endpoint index allows a USB host to potentially manipulate this value, leading to out-of-array access.

In the USB gadget subsystem, endpoints represent communication channels between the USB device and the USB host. The Linux kernel supports various USB gadget drivers for various use cases, such as networking, storage, or serial communication. The Xilinx USB device controller driver, in this case, is being used for endpoint management.

Code Analysis

The vulnerability is located in the xudc_ep_set_halt() function in drivers/usb/gadget/udc/udc-xilinx.c. Here's a snippet of the affected code:

static int xudc_ep_set_halt(struct usb_ep *_ep, int value)
{
    struct xusb_udc_ep *ep = to_xusb_udc_ep(_ep);
    struct xusb_udc *udc = ep->udc;
    u32 epnum = ep->desc->bEndpointAddress & USB_ENDPOINT_NUMBER_MASK;
    ...
    if (value)
        udc->ep[epnum].cr |= USB_EP_HALT;
    else
        udc->ep[epnum].cr &= ~USB_EP_HALT;
    ...
}

The problem with this code is the lack of validation for the epnum variable, which is set using the bEndpointAddress field from the endpoint descriptor. An attacker-controlled USB host could potentially manipulate this value and access an out-of-bound index in the udc->ep array, leading to memory corruption.

Original References

- The original kernel commit that fixes the issue: git.kernel.org
- The CVE entry for the vulnerability: cve.mitre.org
- Linux kernel mailing list discussion: lore.kernel.org

Exploit Details

An attacker would have to first gain control over a USB host connected to the target system to exploit this vulnerability. They could then craft a rogue USB device with a manipulated endpoint index value. By doing that, they can force the kernel to access out-of-array memory in the udc->ep index, potentially leading to:

Denial of service: The kernel might crash due to invalid memory access.

3. Code execution: The attacker could potentially gain arbitrary code execution with kernel privileges if they can corrupt specific memory locations.

It's important to note that successful exploitation requires further understanding and knowledge of kernel internals. Nonetheless, the vulnerability highlights the importance of proper input validation and thorough testing to protect kernel components against potential attacks.

Conclusion

CVE-2022-27223 uncovers a vulnerability in the Linux kernel's USB gadget subsystem, affecting endpoint index handling. The lack of index validation in the xudc_ep_set_halt() function exposes a potential attack vector that might enable out-of-array access and compromise system security. Patching the affected systems with kernel 5.16.12 or later ensures protection against this vulnerability. Furthermore, thoroughly validating user inputs and consistently testing kernel components is essential to maintain overall kernel security.

Timeline

Published on: 03/16/2022 00:15:00 UTC
Last modified on: 07/01/2022 14:15:00 UTC