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 get_file() routine in this situation triggers the
following warning:

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

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

Fixes: 40b7c49 ("gpio: cdev: put emitting the line state events on a workqueue")
Reported-by: Alexander Sverdlin <alexander.sverdlin@siemens.com>
Closes: https://lore.kernel.org/all/5d605f7fc99456804911403102a4fe999a14cc85.camel@siemens.com/
Tested-by: Alexander Sverdlin <alexander.sverdlin@siemens.com>
Link: https://lore.kernel.org/r/20251117-gpio-cdev-get-file-v1-1-28a16b5985b8@linaro.org
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

Author: Bartosz Golaszewski

drivers/gpio/gpiolib-cdev.c

@@ -2548,10 +2548,17 @@ static int lineinfo_changed_notify(struct notifier_block *nb,
 		container_of(nb, struct gpio_chardev_data, lineinfo_changed_nb);
 	struct lineinfo_changed_ctx *ctx;
 	struct gpio_desc *desc = data;
+	struct file *fp;
 
 	if (!test_bit(gpio_chip_hwgpio(desc), cdev->watched_lines))
 		return NOTIFY_DONE;
 
+	/* Keep the file descriptor alive for the duration of the notification. */
+	fp = get_file_active(&cdev->fp);
+	if (!fp)
+		/* Chardev file descriptor was or is being released. */
+		return NOTIFY_DONE;
+
 	/*
 	 * If this is called from atomic context (for instance: with a spinlock
 	 * taken by the atomic notifier chain), any sleeping calls must be done
@@ -2575,8 +2582,6 @@ static int lineinfo_changed_notify(struct notifier_block *nb,
 	/* Keep the GPIO device alive until we emit the event. */
 	ctx->gdev = gpio_device_get(desc->gdev);
 	ctx->cdev = cdev;
-	/* Keep the file descriptor alive too. */
-	get_file(ctx->cdev->fp);
 
 	INIT_WORK(&ctx->work, lineinfo_changed_func);
 	queue_work(ctx->gdev->line_state_wq, &ctx->work);