[PATCH] net: airoha: Fix skb->priority underflow in airoha_dev_select_queue()

["Wayen.Yan" <win847@gmail.com>]

In airoha_dev_select_queue(), the expression:

  queue = (skb->priority - 1) % AIROHA_NUM_QOS_QUEUES;

implicitly converts to unsigned arithmetic: when skb->priority is 0
(the default for unclassified traffic), (0u - 1u) wraps to UINT_MAX,
and UINT_MAX % 8 = 7, routing default best-effort packets to the
highest-priority QoS queue. This causes QoS inversion where the
majority of traffic on a PON gateway starves actual high-priority
flows (VoIP, gaming, etc.).

Fix by guarding the subtraction: when priority is 0, map to queue 0
(lowest priority), otherwise apply the original (priority - 1) % 8
mapping.

Fixes: 2b288b81560b ("net: airoha: Introduce ndo_select_queue callback")
Signed-off-by: Wayen <win847@gmail.com>
---
 drivers/net/ethernet/airoha/airoha_eth.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/airoha/airoha_eth.c b/drivers/net/ethernet/airoha/airoha_eth.c
index 31cdb11cd7..d476ef83c3 100644
--- a/drivers/net/ethernet/airoha/airoha_eth.c
+++ b/drivers/net/ethernet/airoha/airoha_eth.c
@@ -1933,7 +1933,7 @@ static u16 airoha_dev_select_queue(struct net_device *dev, struct sk_buff *skb,
 	 */
 	channel = netdev_uses_dsa(dev) ? skb_get_queue_mapping(skb) : port->id;
 	channel = channel % AIROHA_NUM_QOS_CHANNELS;
-	queue = (skb->priority - 1) % AIROHA_NUM_QOS_QUEUES; /* QoS queue */
+	queue = skb->priority ? (skb->priority - 1) % AIROHA_NUM_QOS_QUEUES : 0;
 	queue = channel * AIROHA_NUM_QOS_QUEUES + queue;
 
 	return queue < dev->num_tx_queues ? queue : 0;
-- 
2.51.0