usb: cdc-wdm: close race between read and workqueue

wdm_read() cannot race with itself. However, in
service_outstanding_interrupt() it can race with the
workqueue, which can be triggered by error handling.

Hence we need to make sure that the WDM_RESPONDING
flag is not just only set but tested.

Fixes: afba937e ("USB: CDC WDM driver")
Cc: stable <stable@kernel.org>
Signed-off-by: Oliver Neukum <oneukum@suse.com>
Link: https://lore.kernel.org/r/20240314115132.3907-1-oneukum@suse.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/class/cdc-wdm.c

@@ -485,6 +485,7 @@ static ssize_t wdm_write
 static int service_outstanding_interrupt(struct wdm_device *desc)
 {
 	int rv = 0;
+	int used;
 
 	/* submit read urb only if the device is waiting for it */
 	if (!desc->resp_count || !--desc->resp_count)
@@ -499,7 +500,10 @@ static int service_outstanding_interrupt(struct wdm_device *desc)
 		goto out;
 	}
 
-	set_bit(WDM_RESPONDING, &desc->flags);
+	used = test_and_set_bit(WDM_RESPONDING, &desc->flags);
+	if (used)
+		goto out;
+
 	spin_unlock_irq(&desc->iuspin);
 	rv = usb_submit_urb(desc->response, GFP_KERNEL);
 	spin_lock_irq(&desc->iuspin);