USB: cdc-acm: fix write and resume race

Fix race between write() and resume() due to improper locking that could
lead to writes being reordered.

Resume must be done atomically and susp_count be protected by the
write_lock in order to prevent racing with write(). This could otherwise
lead to writes being reordered if write() grabs the write_lock after
susp_count is decremented, but before the delayed urb is submitted.

Fixes: 11ea859d64b6 ("USB: additional power savings for cdc-acm devices
that support remote wakeup")

Cc: <stable@vger.kernel.org>	# v2.6.27
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/class/cdc-acm.c

@@ -1541,27 +1541,20 @@ static int acm_resume(struct usb_interface *intf)
 	struct acm *acm = usb_get_intfdata(intf);
 	struct acm_wb *wb;
 	int rv = 0;
-	int cnt;
 
 	spin_lock_irq(&acm->read_lock);
-	acm->susp_count -= 1;
-	cnt = acm->susp_count;
-	spin_unlock_irq(&acm->read_lock);
+	spin_lock(&acm->write_lock);
 
-	if (cnt)
-		return 0;
+	if (--acm->susp_count)
+		goto out;
 
 	if (test_bit(ASYNCB_INITIALIZED, &acm->port.flags)) {
-		rv = usb_submit_urb(acm->ctrlurb, GFP_NOIO);
+		rv = usb_submit_urb(acm->ctrlurb, GFP_ATOMIC);
 
-		spin_lock_irq(&acm->write_lock);
 		if (acm->delayed_wb) {
 			wb = acm->delayed_wb;
 			acm->delayed_wb = NULL;
-			spin_unlock_irq(&acm->write_lock);
 			acm_start_wb(acm, wb);
-		} else {
-			spin_unlock_irq(&acm->write_lock);
 		}
 
 		/*
@@ -1569,12 +1562,14 @@ static int acm_resume(struct usb_interface *intf)
 		 * do the write path at all cost
 		 */
 		if (rv < 0)
-			goto err_out;
+			goto out;
 
-		rv = acm_submit_read_urbs(acm, GFP_NOIO);
+		rv = acm_submit_read_urbs(acm, GFP_ATOMIC);
 	}
+out:
+	spin_unlock(&acm->write_lock);
+	spin_unlock_irq(&acm->read_lock);
 
-err_out:
 	return rv;
 }