[PATCH net v3] usbnet: fix EVENT_RX_PAUSED race in usbnet_skb_return()

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

On SMP, usbnet_skb_return() can pass the test_bit(EVENT_RX_PAUSED) check
on CPU0 while usbnet_resume_rx() clears the flag and drains rxq_pause on
CPU1, leaving the skb stranded until the next MTU change or device stop.

Protect the flag check and enqueue with a double-check pattern under
rxq_pause.lock on the slow path, keeping the normal RX fast path
lock-free. Wrap clear_bit() in usbnet_resume_rx() under the same lock.

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

---
v3: Use spin_lock() inside existing local_bh_disable() section in
    usbnet_resume_rx() instead of spin_lock_bh(); move local_bh_enable()
    before queue_work() to restore BH context correctly.

 drivers/net/usb/usbnet.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c
index 84f97f448b2d..d138d353fd98 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_bh(&dev->rxq_pause.lock);
+		if (test_bit(EVENT_RX_PAUSED, &dev->flags)) {
+			__skb_queue_tail(&dev->rxq_pause, skb);
+			spin_unlock_bh(&dev->rxq_pause.lock);
+			return;
+		}
+		spin_unlock_bh(&dev->rxq_pause.lock);
 	}

 	/* only update if unset to allow minidriver rx_fixup override */
@@ -702,15 +707,16 @@ void usbnet_resume_rx(struct usbnet *dev)
 	int num = 0;

 	local_bh_disable();
+	spin_lock(&dev->rxq_pause.lock);
 	clear_bit(EVENT_RX_PAUSED, &dev->flags);
-
+	spin_unlock(&dev->rxq_pause.lock);
 	while ((skb = skb_dequeue(&dev->rxq_pause)) != NULL) {
 		usbnet_skb_return(dev, skb);
 		num++;
 	}

-	queue_work(system_bh_wq, &dev->bh_work);
 	local_bh_enable();
+	queue_work(system_bh_wq, &dev->bh_work);

 	netif_dbg(dev, rx_status, dev->net,
 		  "paused rx queue disabled, %d skbs requeued\n", num);
--
2.56.0