CVE-2025-40249: gpio: cdev: make sure the cdev fd is still active before emitting events

In the Linux kernel, the following vulnerability has been resolved:

gpio: cdev: make sure the cdev fd is still active before emitting events

With the final call to fput() on a file descriptor, the release action may be deferred and scheduled on a work queue. The reference count of that descriptor is still zero and it must not be used. It's possible that a GPIO change, we want to notify the user-space about, happens AFTER the reference count on the file descriptor associated with the character device went down to zero but BEFORE the .release() callback was called from the workqueue and so BEFORE we unregistered from the notifier.

Using the regular getfile() routine in this situation triggers the following warning:

struct file::fcount incremented from zero; use-after-free condition present!

So use the getfileactive() variant that will return NULL on file descriptors that have been or are being released.