net: add dev_net_rcu() helper

jira VULN-54026
cve-pre CVE-2025-21764
commit-author Eric Dumazet <edumazet@google.com>
commit 482ad2a

dev->nd_net can change, readers should either
use rcu_read_lock() or RTNL.

We currently use a generic helper, dev_net() with
no debugging support. We probably have many hidden bugs.

Add dev_net_rcu() helper for callers using rcu_read_lock()
protection.

	Signed-off-by: Eric Dumazet <edumazet@google.com>
	Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250205155120.1676781-2-edumazet@google.com
	Signed-off-by: Jakub Kicinski <kuba@kernel.org>
(cherry picked from commit 482ad2a)
	Signed-off-by: Brett Mastbergen <bmastbergen@ciq.com>

Author: bmastbergen

include/linux/netdevice.h

@@ -2470,6 +2470,12 @@ struct net *dev_net(const struct net_device *dev)
 	return read_pnet(&dev->nd_net);
 }
 
+static inline
+struct net *dev_net_rcu(const struct net_device *dev)
+{
+	return read_pnet_rcu(&dev->nd_net);
+}
+
 static inline
 void dev_net_set(struct net_device *dev, struct net *net)
 {

include/net/net_namespace.h

@@ -368,7 +368,7 @@ static inline struct net *read_pnet(const possible_net_t *pnet)
 #endif
 }
 
-static inline struct net *read_pnet_rcu(possible_net_t *pnet)
+static inline struct net *read_pnet_rcu(const possible_net_t *pnet)
 {
 #ifdef CONFIG_NET_NS
 	return rcu_dereference(pnet->net);