Published: 2025-12-04T16:16:18.380
Overview
CVE-2025-40249 addresses a use-after-free vulnerability found in the Linux kernel’s GPIO (General Purpose Input/Output) character device driver. This flaw could potentially lead to system instability or even a security compromise if exploited. The vulnerability occurs when a GPIO change notification is attempted after the file descriptor associated with the character device has been closed but before the release callback is executed. This blog post provides a detailed analysis of the vulnerability, its potential impact, and the necessary mitigation steps.
Technical Details
The vulnerability arises due to a race condition in the handling of file descriptor releases and GPIO change notifications. When a user-space application closes a file descriptor associated with a GPIO character device, the fput() function is called to decrement the reference count of the file object. However, the actual release action (the .release() callback) is often deferred and scheduled on a work queue.
A problem occurs when a GPIO change event happens after the file descriptor’s reference count reaches zero but before the .release() callback has been executed and the driver has unregistered from the notifier. In this scenario, the driver attempts to access the file descriptor using get_file(), which, because the reference count is already zero, triggers a warning indicating a use-after-free condition.
The fix implemented in the kernel replaces get_file() with get_file_active(). The get_file_active() function specifically checks if the file descriptor is still active (i.e., not being released) and returns NULL if it is not, preventing the use-after-free condition.
CVSS Analysis
Due to the complex nature of exploiting this race condition and the fact that it primarily results in a denial-of-service or system instability, a formal CVSS score has not been assigned. However, its potential for system-level impact makes it a significant vulnerability that warrants patching.
Possible Impact
While a direct remote code execution exploit might be challenging to achieve, the use-after-free vulnerability could lead to:
- Kernel Panic: The use-after-free condition can corrupt kernel memory, leading to a system crash.
- Denial of Service (DoS): By repeatedly triggering the vulnerable code path, an attacker could potentially cause the system to become unresponsive.
- Information Leakage: In some scenarios, the corrupted memory could potentially leak sensitive kernel information.
Mitigation or Patch Steps
The recommended mitigation is to upgrade to a Linux kernel version that includes the fix for CVE-2025-40249. The fix has been backported to stable kernel branches. Specifically, ensure your kernel includes the following commit:
- gpio: cdev: make sure the cdev fd is still active before emitting events
- Revert “gpio: cdev: make sure the cdev fd is still active before emitting events”
Applying the available patch to vulnerable systems resolves the described vulnerability.
References
Kernel Commit (Fix): https://git.kernel.org/stable/c/d4cd0902c156b2ca60fdda8cd8b5bcb4b0e9ed64
Kernel Commit (Revert): https://git.kernel.org/stable/c/dccc6daa8afa0f64c432e4c867f275747e3415e1
