Overview
CVE-2025-40223 describes a use-after-free vulnerability found within the Linux kernel’s implementation of the Media Oriented Systems Transport (MOST) interface for USB devices. This flaw exists in the hdm_disconnect() function, potentially leading to system instability or, in some scenarios, code execution. This article provides a comprehensive breakdown of the vulnerability, its technical details, potential impact, and available mitigation strategies.
Technical Details
The vulnerability arises from incorrect memory management within the hdm_disconnect() function, which is responsible for handling the disconnection of a MOST interface device. The sequence of events leading to the use-after-free is as follows:
hdm_disconnect()callsmost_deregister_interface().most_deregister_interface()eventually callsdevice_unregister(iface->dev), which unregisters the MOST interface device.- If this unregistration drops the last reference to the device, the device core may immediately call
release_mdev()whilehdm_disconnect()is still executing. - The original code in
hdm_disconnect()freed several memory allocations owned bymdev*before* the `put_device()` calls. Depending on the refcount order, this creates a race condition where `release_mdev()` runs before the put calls are executed. - Consequently, a use-after-free or double-free can occur when
release_mdev()is called, or when unregister paths also performed puts.
The fix addresses this issue by:
- Moving the freeing of
mdev-owned allocations intorelease_mdev(). This ensures that these allocations are freed only once, when the device is truly released. - Removing redundant
put_device()calls inhdm_disconnect(), as they are no longer necessary afterdevice_unregister()andmost_deregister_interface().
This fix directly addresses the KASAN slab-use-after-free reported by syzbot.
/* Vulnerable Code (Simplified) */
void hdm_disconnect(struct hdm *hdm) {
most_deregister_interface(hdm->iface);
kfree(hdm->some_allocation); //Potential use-after-free if release_mdev() runs
put_device(&hdm->iface->dev); //Redundant, and can cause issues
}
/* Fixed Code (Simplified) */
void hdm_disconnect(struct hdm *hdm) {
most_deregister_interface(hdm->iface);
}
void release_mdev(struct device *dev) {
struct hdm *hdm = dev_to_hdm(dev);
kfree(hdm->some_allocation); // Freed here, only when the device is truly released.
}
CVSS Analysis
As of the publication date, the CVE does not have a CVSS score assigned (N/A). The severity is also marked as N/A. However, given the nature of use-after-free vulnerabilities in the kernel, the potential impact could be significant, ranging from denial-of-service to privilege escalation if exploited. The missing CVSS score is likely due to the timing of the report compared to the vulnerability fix.
Possible Impact
A successful exploit of CVE-2025-40223 could lead to the following consequences:
- Kernel Crash: The use-after-free can corrupt kernel memory, resulting in a system crash and denial-of-service.
- Privilege Escalation: In more complex scenarios, an attacker might be able to manipulate the freed memory to gain elevated privileges on the system.
- Information Leakage: The vulnerability could potentially be leveraged to leak sensitive kernel memory, though this is less likely in this specific case.
Mitigation and Patch Steps
The primary mitigation for CVE-2025-40223 is to apply the patches provided by the Linux kernel maintainers. These patches have been incorporated into stable kernel releases. System administrators should ensure that their systems are running a kernel version that includes these fixes. The relevant patches can be found in the references section below.
Specifically, upgrading to a kernel version containing the following commits is recommended:
- https://git.kernel.org/stable/c/33daf469f5294b9d07c4fc98216cace9f4f34cc6
- https://git.kernel.org/stable/c/3a3b8e89c7201c5b3b76ac4a4069d1adde1477d6
- https://git.kernel.org/stable/c/4b1270902609ef0d935ed2faa2ea6d122bd148f5
- https://git.kernel.org/stable/c/578eb18cd111addec94c43f61cd4b4429e454809
- https://git.kernel.org/stable/c/5b5c478f09b1b35e7fe6fc9a1786c9bf6030e831
- https://git.kernel.org/stable/c/72427dc6f87523995f4e6ae35a948bb2992cabce
- https://git.kernel.org/stable/c/f93a84ffb884d761a9d4e869ba29c238711e81f1
