[PATCH] USB: cdc-acm: fix racy TIOCMIWAIT implementation

[Johan Hovold]

The wakeup condition needs to be checked after adding the task to the
waitqueue and updating the task state to avoid missing a racing modem
status update or disconnect.

Fixes: 5a6a62bdb925 ("cdc-acm: add TIOCMIWAIT")
Cc: stable@xxxxxxxxxxxxxxx	# 3.14
Cc: Oliver Neukum <oneukum@xxxxxxx>
Signed-off-by: Johan Hovold <johan@xxxxxxxxxx>
---
 drivers/usb/class/cdc-acm.c | 29 +++++++++++++++--------------
 1 file changed, 15 insertions(+), 14 deletions(-)

diff --git a/drivers/usb/class/cdc-acm.c b/drivers/usb/class/cdc-acm.c
index 7bc5329fa3ed..00c250515e83 100644
--- a/drivers/usb/class/cdc-acm.c
+++ b/drivers/usb/class/cdc-acm.c
@@ -1027,7 +1027,10 @@ static int wait_serial_change(struct acm *acm, unsigned long arg)
 	DECLARE_WAITQUEUE(wait, current);
 	struct async_icount old, new;
 
-	do {
+	add_wait_queue(&acm->wioctl, &wait);
+	for (;;) {
+		set_current_state(TASK_INTERRUPTIBLE);
+
 		spin_lock_irq(&acm->read_lock);
 		old = acm->oldcount;
 		new = acm->iocount;
@@ -1044,22 +1047,20 @@ static int wait_serial_change(struct acm *acm, unsigned long arg)
 			old.rng != new.rng)
 			break;
 
-		add_wait_queue(&acm->wioctl, &wait);
-		set_current_state(TASK_INTERRUPTIBLE);
-		schedule();
-		remove_wait_queue(&acm->wioctl, &wait);
 		if (acm->disconnected) {
-			if (arg & TIOCM_CD)
-				break;
-			else
-				rv = -ENODEV;
-		} else {
-			if (signal_pending(current))
-				rv = -ERESTARTSYS;
+			rv = -ENODEV;
+			break;
 		}
-	} while (!rv);
 
-	
+		schedule();
+
+		if (signal_pending(current)) {
+			rv = -ERESTARTSYS;
+			break;
+		}
+	}
+	__set_current_state(TASK_RUNNING);
+	remove_wait_queue(&acm->wioctl, &wait);
 
 	return rv;
 }
-- 
2.55.0