net: treat possible_net_t net pointer as an RCU one and add read_pnet_rcu()

jira VULN-54026
cve-pre CVE-2025-21764
commit-author Jiri Pirko <jiri@nvidia.com>
commit 2034d90

Make the net pointer stored in possible_net_t structure annotated as
an RCU pointer. Change the access helpers to treat it as such.
Introduce read_pnet_rcu() helper to allow caller to dereference
the net pointer under RCU read lock.

	Signed-off-by: Jiri Pirko <jiri@nvidia.com>
	Reviewed-by: Simon Horman <horms@kernel.org>
	Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 2034d90)
	Signed-off-by: Brett Mastbergen <bmastbergen@ciq.com>

Author: bmastbergen

include/net/net_namespace.h

@@ -348,21 +348,30 @@ static inline void put_net_track(struct net *net, netns_tracker *tracker)
 
 typedef struct {
 #ifdef CONFIG_NET_NS
-	struct net *net;
+	struct net __rcu *net;
 #endif
 } possible_net_t;
 
 static inline void write_pnet(possible_net_t *pnet, struct net *net)
 {
 #ifdef CONFIG_NET_NS
-	pnet->net = net;
+	rcu_assign_pointer(pnet->net, net);
 #endif
 }
 
 static inline struct net *read_pnet(const possible_net_t *pnet)
 {
 #ifdef CONFIG_NET_NS
-	return pnet->net;
+	return rcu_dereference_protected(pnet->net, true);
+#else
+	return &init_net;
+#endif
+}
+
+static inline struct net *read_pnet_rcu(possible_net_t *pnet)
+{
+#ifdef CONFIG_NET_NS
+	return rcu_dereference(pnet->net);
 #else
 	return &init_net;
 #endif