ipv4: add RCU protection to ip4_dst_hoplimit()

[ Upstream commit 469308552ca4560176cfc100e7ca84add1bebd7c ]

ip4_dst_hoplimit() must use RCU protection to make
sure the net structure it reads does not disappear.

Fixes: fa50d974 ("ipv4: Namespaceify ip_default_ttl sysctl knob")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250205155120.1676781-3-edumazet@google.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

include/net/route.h

@@ -362,10 +362,15 @@ static inline int inet_iif(const struct sk_buff *skb)
 static inline int ip4_dst_hoplimit(const struct dst_entry *dst)
 {
 	int hoplimit = dst_metric_raw(dst, RTAX_HOPLIMIT);
-	struct net *net = dev_net(dst->dev);
 
-	if (hoplimit == 0)
+	if (hoplimit == 0) {
+		const struct net *net;
+
+		rcu_read_lock();
+		net = dev_net_rcu(dst->dev);
 		hoplimit = READ_ONCE(net->ipv4.sysctl_ip_default_ttl);
+		rcu_read_unlock();
+	}
 	return hoplimit;
 }