vrf: use RCU protection in l3mdev_l3_out()

JIRA: https://issues.redhat.com/browse/RHEL-81542
Upstream Status: linux.git
CVE: CVE-2025-21791

commit 6d0ce46
Author: Eric Dumazet <edumazet@google.com>
Date:   Fri Feb 7 13:58:38 2025 +0000

    vrf: use RCU protection in l3mdev_l3_out()

    l3mdev_l3_out() can be called without RCU being held:

    raw_sendmsg()
     ip_push_pending_frames()
      ip_send_skb()
       ip_local_out()
        __ip_local_out()
         l3mdev_ip_out()

    Add rcu_read_lock() / rcu_read_unlock() pair to avoid
    a potential UAF.

    Fixes: a8e3e1a ("net: l3mdev: Add hook to output path")
    Signed-off-by: Eric Dumazet <edumazet@google.com>
    Reviewed-by: David Ahern <dsahern@kernel.org>
    Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
    Link: https://patch.msgid.link/20250207135841.1948589-7-edumazet@google.com
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Signed-off-by: Guillaume Nault <gnault@redhat.com>

Author: Guillaume Nault

include/net/l3mdev.h

@@ -209,10 +209,12 @@ struct sk_buff *l3mdev_l3_out(struct sock *sk, struct sk_buff *skb, u16 proto)
 	if (netif_is_l3_slave(dev)) {
 		struct net_device *master;
 
+		rcu_read_lock();
 		master = netdev_master_upper_dev_get_rcu(dev);
 		if (master && master->l3mdev_ops->l3mdev_l3_out)
 			skb = master->l3mdev_ops->l3mdev_l3_out(master, sk,
 								skb, proto);
+		rcu_read_unlock();
 	}
 
 	return skb;