[PATCH net v5] usbnet: fix EVENT_RX_PAUSED race in usbnet_skb_return()

["Ömer Mete Kaya" <omermetekaya0@gmail.com>]

On SMP, usbnet_skb_return() can observe EVENT_RX_PAUSED as set on CPU0
while usbnet_resume_rx() clears the flag and drains rxq_pause on CPU1.
The skb can then be queued after the drain has completed and remain
stranded until the next MTU change or device stop.

Synchronize the EVENT_RX_PAUSED state transition and paused-queue
dequeue operations with rxq_pause.lock. In usbnet_resume_rx() splice
the paused queue into a local queue under the lock, then drain the
local queue lock-free. In usbnet_skb_return() use a double-check
pattern under rxq_pause.lock on the slow path, keeping the normal
RX fast path lock-free. Use spin_lock_irqsave() to match the lockdep
class shared with other sk_buff_head locks taken from hardirq context
in defer_bh().

Discovered by Sashiko, not triggered, not tested on real hardware.

Fixes: 43daa96b166c ("usbnet: Stop RX Q on MTU change")
Signed-off-by: Ömer Mete Kaya <omermetekaya0@gmail.com>

---
v5: Splice rxq_pause into a local queue under the lock and drain
    lock-free, avoiding lock drop/reacquire per iteration, as
    suggested by Jakub Kicinski; fix reverse xmas tree ordering.

 drivers/net/usb/usbnet.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c
index 84f97f448b2d..97a30dd22c54 100644
--- a/drivers/net/usb/usbnet.c
+++ b/drivers/net/usb/usbnet.c
@@ -329,9 +329,14 @@ void usbnet_skb_return(struct usbnet *dev, struct sk_buff *skb)
 	unsigned long flags;
 	int	status;

-	if (test_bit(EVENT_RX_PAUSED, &dev->flags)) {
-		skb_queue_tail(&dev->rxq_pause, skb);
-		return;
+	if (unlikely(test_bit(EVENT_RX_PAUSED, &dev->flags))) {
+		spin_lock_irqsave(&dev->rxq_pause.lock, flags);
+		if (test_bit(EVENT_RX_PAUSED, &dev->flags)) {
+			__skb_queue_tail(&dev->rxq_pause, skb);
+			spin_unlock_irqrestore(&dev->rxq_pause.lock, flags);
+			return;
+		}
+		spin_unlock_irqrestore(&dev->rxq_pause.lock, flags);
 	}

 	/* only update if unset to allow minidriver rx_fixup override */
@@ -698,13 +703,20 @@ EXPORT_SYMBOL_GPL(usbnet_pause_rx);

 void usbnet_resume_rx(struct usbnet *dev)
 {
+	struct sk_buff_head q;
 	struct sk_buff *skb;
+	unsigned long flags;
 	int num = 0;

+	__skb_queue_head_init(&q);
+
 	local_bh_disable();
+	spin_lock_irqsave(&dev->rxq_pause.lock, flags);
 	clear_bit(EVENT_RX_PAUSED, &dev->flags);
+	skb_queue_splice_init(&dev->rxq_pause, &q);
+	spin_unlock_irqrestore(&dev->rxq_pause.lock, flags);

-	while ((skb = skb_dequeue(&dev->rxq_pause)) != NULL) {
+	while ((skb = __skb_dequeue(&q))) {
 		usbnet_skb_return(dev, skb);
 		num++;
 	}
--
2.56.0