[PATCH net v2] usbnet: fix EVENT_RX_PAUSED race in usbnet_skb_return()

[PATCH net v2] usbnet: fix EVENT_RX_PAUSED race in usbnet_skb_return()

[Ömer Mete Kaya]

usbnet_resume_rx() disables local BHs before clearing EVENT_RX_PAUSED,
but this does not synchronize with usbnet_skb_return() running on
another CPU. On SMP, usbnet_skb_return() on CPU0 can pass the test_bit()
check while CPU1 runs usbnet_resume_rx(), clears the flag and drains the
queue, leaving the skb stranded in rxq_pause until the next MTU change
or device stop.

Fix with a double-check pattern: move clear_bit() in usbnet_resume_rx()
under rxq_pause.lock; in usbnet_skb_return() take the lock and recheck
the flag only on the slow path (unlikely paused), keeping the normal RX
fast path lock-free.

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

---
v2: Squash usbnet_resume_rx() fix into same patch; previous version
    only changed usbnet_skb_return() and was missing the clear_bit()
    change.

 drivers/net/usb/usbnet.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c
index 84f97f448b2d..5eba2cef8bac 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 */
@@ -701,8 +706,9 @@ void usbnet_resume_rx(struct usbnet *dev)
 	struct sk_buff *skb;
 	int num = 0;

-	local_bh_disable();
+	spin_lock_bh(&dev->rxq_pause.lock);
 	clear_bit(EVENT_RX_PAUSED, &dev->flags);
+	spin_unlock_bh(&dev->rxq_pause.lock);

 	while ((skb = skb_dequeue(&dev->rxq_pause)) != NULL) {
 		usbnet_skb_return(dev, skb);
@@ -710,7 +716,6 @@ void usbnet_resume_rx(struct usbnet *dev)
 	}

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

 	netif_dbg(dev, rx_status, dev->net,
 		  "paused rx queue disabled, %d skbs requeued\n", num);
--
2.55.0

Re: [PATCH net v2] usbnet: fix EVENT_RX_PAUSED race in usbnet_skb_return()

[netdev-bot+sinfo]

Hi!

This is an automated message. This series looks like a fix, but its
commit messages seem to be missing some information:

 - How the issue was discovered, e.g. hit in production, hit during
   development, syzbot report, manual code inspection, LLM or static
   analysis tool scan.

 - Whether the issue was actually triggered, or is only theoretical
   (e.g. found by code inspection). If it was triggered please include
   the symptoms, like the stack trace or error messages.

Please do not repost the series just to address the above. Instead,
reply to this email with the missing information, so that reviewers
can take it into account. If the series needs another revision for
other reasons, please include the information in the commit messages
then.

The evaluation is done by an LLM so it may be wrong, if you think
that is the case please reply and explain.