netpoll: hold rcu read lock in __netpoll_send_skb()

JIRA: https://issues.redhat.com/browse/RHEL-87474

Upstream commit:
commit 505ead7
Author: Breno Leitao <leitao@debian.org>
Date:   Thu Mar 6 05:16:18 2025 -0800

    netpoll: hold rcu read lock in __netpoll_send_skb()

    The function __netpoll_send_skb() is being invoked without holding the
    RCU read lock. This oversight triggers a warning message when
    CONFIG_PROVE_RCU_LIST is enabled:

            net/core/netpoll.c:330 suspicious rcu_dereference_check() usage!

             netpoll_send_skb
             netpoll_send_udp
             write_ext_msg
             console_flush_all
             console_unlock
             vprintk_emit

    To prevent npinfo from disappearing unexpectedly, ensure that
    __netpoll_send_skb() is protected with the RCU read lock.

    Fixes: 2899656 ("netpoll: take rcu_read_lock_bh() in netpoll_send_skb_on_dev()")
    Signed-off-by: Breno Leitao <leitao@debian.org>
    Reviewed-by: Simon Horman <horms@kernel.org>
    Link: https://patch.msgid.link/20250306-netpoll_rcu_v2-v2-1-bc4f5c51742a@debian.org
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: Paolo Abeni

net/core/netpoll.c

@@ -309,6 +309,7 @@ static int netpoll_owner_active(struct net_device *dev)
 static netdev_tx_t __netpoll_send_skb(struct netpoll *np, struct sk_buff *skb)
 {
 	netdev_tx_t status = NETDEV_TX_BUSY;
+	netdev_tx_t ret = NET_XMIT_DROP;
 	struct net_device *dev;
 	unsigned long tries;
 	/* It is up to the caller to keep npinfo alive. */
@@ -317,11 +318,12 @@ static netdev_tx_t __netpoll_send_skb(struct netpoll *np, struct sk_buff *skb)
 	lockdep_assert_irqs_disabled();
 
 	dev = np->dev;
+	rcu_read_lock();
 	npinfo = rcu_dereference_bh(dev->npinfo);
 
 	if (!npinfo || !netif_running(dev) || !netif_device_present(dev)) {
 		dev_kfree_skb_irq(skb);
-		return NET_XMIT_DROP;
+		goto out;
 	}
 
 	/* don't get messages out of order, and no recursion */
@@ -360,7 +362,10 @@ static netdev_tx_t __netpoll_send_skb(struct netpoll *np, struct sk_buff *skb)
 		skb_queue_tail(&npinfo->txq, skb);
 		schedule_delayed_work(&npinfo->tx_work,0);
 	}
-	return NETDEV_TX_OK;
+	ret = NETDEV_TX_OK;
+out:
+	rcu_read_unlock();
+	return ret;
 }
 
 netdev_tx_t netpoll_send_skb(struct netpoll *np, struct sk_buff *skb)