um: Add winch to winch_handlers before registering winch IRQ

Registering a winch IRQ is racy, an interrupt may occur before the winch is
added to the winch_handlers list.

If that happens, register_winch_irq() adds to that list a winch that is
scheduled to be (or has already been) freed, causing a panic later in
winch_cleanup().

Avoid the race by adding the winch to the winch_handlers list before
registering the IRQ, and rolling back if um_request_irq() fails.

Fixes: 42a359e3 ("uml: SIGIO support cleanup")
Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/drivers/line.c

@@ -676,23 +676,25 @@ void register_winch_irq(int fd, int tty_fd, int pid, struct tty_port *port,
 		goto cleanup;
 	}
 
-	*winch = ((struct winch) { .list  	= LIST_HEAD_INIT(winch->list),
-				   .fd  	= fd,
+	*winch = ((struct winch) { .fd  	= fd,
 				   .tty_fd 	= tty_fd,
 				   .pid  	= pid,
 				   .port 	= port,
 				   .stack	= stack });
 
+	spin_lock(&winch_handler_lock);
+	list_add(&winch->list, &winch_handlers);
+	spin_unlock(&winch_handler_lock);
+
 	if (um_request_irq(WINCH_IRQ, fd, IRQ_READ, winch_interrupt,
 			   IRQF_SHARED, "winch", winch) < 0) {
 		printk(KERN_ERR "register_winch_irq - failed to register "
 		       "IRQ\n");
-		goto out_free;
-	}
-
 		spin_lock(&winch_handler_lock);
-	list_add(&winch->list, &winch_handlers);
+		list_del(&winch->list);
 		spin_unlock(&winch_handler_lock);
+		goto out_free;
+	}
 
 	return;