ndisc: extend RCU protection in ndisc_send_skb()

[ Upstream commit ed6ae1f ]

ndisc_send_skb() can be called without RTNL or RCU held.

Acquire rcu_read_lock() earlier, so that we can use dev_net_rcu()
and avoid a potential UAF.

Fixes: 1762f7e ("[NETNS][IPV6] ndisc - make socket control per namespace")
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/20250207135841.1948589-8-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: edumazet

net/ipv6/ndisc.c

@@ -471,23 +471,28 @@ static void ip6_nd_hdr(struct sk_buff *skb,
 void ndisc_send_skb(struct sk_buff *skb, const struct in6_addr *daddr,
 		    const struct in6_addr *saddr)
 {
+	struct icmp6hdr *icmp6h = icmp6_hdr(skb);
 	struct dst_entry *dst = skb_dst(skb);
-	struct net *net = dev_net(skb->dev);
-	struct sock *sk = net->ipv6.ndisc_sk;
 	struct inet6_dev *idev;
+	struct net *net;
+	struct sock *sk;
 	int err;
-	struct icmp6hdr *icmp6h = icmp6_hdr(skb);
 	u8 type;
 
 	type = icmp6h->icmp6_type;
 
+	rcu_read_lock();
+
+	net = dev_net_rcu(skb->dev);
+	sk = net->ipv6.ndisc_sk;
 	if (!dst) {
 		struct flowi6 fl6;
 		int oif = skb->dev->ifindex;
 
 		icmpv6_flow_init(sk, &fl6, type, saddr, daddr, oif);
 		dst = icmp6_dst_alloc(skb->dev, &fl6);
 		if (IS_ERR(dst)) {
+			rcu_read_unlock();
 			kfree_skb(skb);
 			return;
 		}
@@ -502,7 +507,6 @@ void ndisc_send_skb(struct sk_buff *skb, const struct in6_addr *daddr,
 
 	ip6_nd_hdr(skb, saddr, daddr, READ_ONCE(inet6_sk(sk)->hop_limit), skb->len);
 
-	rcu_read_lock();
 	idev = __in6_dev_get(dst->dev);
 	IP6_INC_STATS(net, idev, IPSTATS_MIB_OUTREQUESTS);