[PATCH net 1/2] net: mctp: usb: fix race between urb completion and rx_retry cancellation

[Jeremy Kerr <jk@codeconstruct.com.au>]

It's possible that sequencing between setting ->stopped and cancelling
the rx_retry work (in ndo_stop) could leave us with an urb queued:

    T1: ndo_stop                  T2: rx_retry_work
    ------------                  ----------------
                                  LD: ->stopped => false
    ST: ->stopped <= true
    usb_kill_urb()
                                  mctp_usb_rx_queue()
                                    usb_submit_urb()
    cancel_delayed_work_sync()

Strenghen the sequencing between the stop (preventing another requeue)
and the cancel by updating both atomically under a new rx lock. After
setting ->rx_stopped, and cancelling pending work, we know that the
requeue cannot occur, so all that's left is killing any pending urb.

Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
---
 drivers/net/mctp/mctp-usb.c | 26 +++++++++++++++++---------
 1 file changed, 17 insertions(+), 9 deletions(-)

diff --git a/drivers/net/mctp/mctp-usb.c b/drivers/net/mctp/mctp-usb.c
index 3b5dff144177..cf6f6a93a451 100644
--- a/drivers/net/mctp/mctp-usb.c
+++ b/drivers/net/mctp/mctp-usb.c
@@ -22,7 +22,6 @@
 struct mctp_usb {
 	struct usb_device *usbdev;
 	struct usb_interface *intf;
-	bool stopped;
 
 	struct net_device *netdev;
 
@@ -32,6 +31,9 @@ struct mctp_usb {
 	struct urb *tx_urb;
 	struct urb *rx_urb;
 
+	/* enforces atomic access to rx_stopped and requeuing the retry work */
+	spinlock_t rx_lock;
+	bool rx_stopped;
 	struct delayed_work rx_retry_work;
 };
 
@@ -122,6 +124,7 @@ static const unsigned long RX_RETRY_DELAY = HZ / 4;
 
 static int mctp_usb_rx_queue(struct mctp_usb *mctp_usb, gfp_t gfp)
 {
+	unsigned long flags;
 	struct sk_buff *skb;
 	int rc;
 
@@ -147,7 +150,10 @@ static int mctp_usb_rx_queue(struct mctp_usb *mctp_usb, gfp_t gfp)
 	return rc;
 
 err_retry:
-	schedule_delayed_work(&mctp_usb->rx_retry_work, RX_RETRY_DELAY);
+	spin_lock_irqsave(&mctp_usb->rx_lock, flags);
+	if (!mctp_usb->rx_stopped)
+		schedule_delayed_work(&mctp_usb->rx_retry_work, RX_RETRY_DELAY);
+	spin_unlock_irqrestore(&mctp_usb->rx_lock, flags);
 	return rc;
 }
 
@@ -248,9 +254,6 @@ static void mctp_usb_rx_retry_work(struct work_struct *work)
 	struct mctp_usb *mctp_usb = container_of(work, struct mctp_usb,
 						 rx_retry_work.work);
 
-	if (READ_ONCE(mctp_usb->stopped))
-		return;
-
 	mctp_usb_rx_queue(mctp_usb, GFP_KERNEL);
 }
 
@@ -258,7 +261,7 @@ static int mctp_usb_open(struct net_device *dev)
 {
 	struct mctp_usb *mctp_usb = netdev_priv(dev);
 
-	WRITE_ONCE(mctp_usb->stopped, false);
+	WRITE_ONCE(mctp_usb->rx_stopped, false);
 
 	netif_start_queue(dev);
 
@@ -268,17 +271,21 @@ static int mctp_usb_open(struct net_device *dev)
 static int mctp_usb_stop(struct net_device *dev)
 {
 	struct mctp_usb *mctp_usb = netdev_priv(dev);
+	unsigned long flags;
 
 	netif_stop_queue(dev);
 
 	/* prevent RX submission retry */
-	WRITE_ONCE(mctp_usb->stopped, true);
+	spin_lock_irqsave(&mctp_usb->rx_lock, flags);
+	mctp_usb->rx_stopped = true;
+	cancel_delayed_work(&mctp_usb->rx_retry_work);
+	spin_unlock_irqrestore(&mctp_usb->rx_lock, flags);
+
+	flush_delayed_work(&mctp_usb->rx_retry_work);
 
 	usb_kill_urb(mctp_usb->rx_urb);
 	usb_kill_urb(mctp_usb->tx_urb);
 
-	cancel_delayed_work_sync(&mctp_usb->rx_retry_work);
-
 	return 0;
 }
 
@@ -331,6 +338,7 @@ static int mctp_usb_probe(struct usb_interface *intf,
 	dev->netdev = netdev;
 	dev->usbdev = interface_to_usbdev(intf);
 	dev->intf = intf;
+	spin_lock_init(&dev->rx_lock);
 	usb_set_intfdata(intf, dev);
 
 	dev->ep_in = ep_in->bEndpointAddress;

-- 
2.47.3