ipv6: use RCU in ip6_xmit()

Use RCU in ip6_xmit() in order to use dst_dev_rcu() to prevent
possible UAF.

Fixes: 4a6ce2b6f2ec ("net: introduce a new function dst_dev_put()")
Signed-off-by: Eric Dumazet <[e-mail]>
Reviewed-by: David Ahern <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/ipv6/ip6_output.c

···
 int ip6_xmit(const struct sock *sk, struct sk_buff *skb, struct flowi6 *fl6,
 	     __u32 mark, struct ipv6_txoptions *opt, int tclass, u32 priority)
 {
-	struct net *net = sock_net(sk);
 	const struct ipv6_pinfo *np = inet6_sk(sk);
 	struct in6_addr *first_hop = &fl6->daddr;
 	struct dst_entry *dst = skb_dst(skb);
-	struct net_device *dev = dst_dev(dst);
 	struct inet6_dev *idev = ip6_dst_idev(dst);
 	struct hop_jumbo_hdr *hop_jumbo;
 	int hoplen = sizeof(*hop_jumbo);
+	struct net *net = sock_net(sk);
 	unsigned int head_room;
+	struct net_device *dev;
 	struct ipv6hdr *hdr;
 	u8  proto = fl6->flowi6_proto;
 	int seg_len = skb->len;
-	int hlimit = -1;
+	int ret, hlimit = -1;
 	u32 mtu;
 
+	rcu_read_lock();
+
+	dev = dst_dev_rcu(dst);
 	head_room = sizeof(struct ipv6hdr) + hoplen + LL_RESERVED_SPACE(dev);
 	if (opt)
 		head_room += opt->opt_nflen + opt->opt_flen;
 
 	if (unlikely(head_room > skb_headroom(skb))) {
-		/* Make sure idev stays alive */
-		rcu_read_lock();
+		/* idev stays alive while we hold rcu_read_lock(). */
 		skb = skb_expand_head(skb, head_room);
 		if (!skb) {
 			IP6_INC_STATS(net, idev, IPSTATS_MIB_OUTDISCARDS);
-			rcu_read_unlock();
-			return -ENOBUFS;
+			ret = -ENOBUFS;
+			goto unlock;
 		}
-		rcu_read_unlock();
 	}
 
 	if (opt) {
···
 		 * skb to its handler for processing
 		 */
 		skb = l3mdev_ip6_out((struct sock *)sk, skb);
-		if (unlikely(!skb))
-			return 0;
+		if (unlikely(!skb)) {
+			ret = 0;
+			goto unlock;
+		}
 
 		/* hooks should never assume socket lock is held.
 		 * we promote our socket to non const
 		 */
-		return NF_HOOK(NFPROTO_IPV6, NF_INET_LOCAL_OUT,
-			       net, (struct sock *)sk, skb, NULL, dev,
-			       dst_output);
+		ret = NF_HOOK(NFPROTO_IPV6, NF_INET_LOCAL_OUT,
+			      net, (struct sock *)sk, skb, NULL, dev,
+			      dst_output);
+		goto unlock;
 	}
 
+	ret = -EMSGSIZE;
 	skb->dev = dev;
 	/* ipv6_local_error() does not require socket lock,
 	 * we promote our socket to non const
···
 
 	IP6_INC_STATS(net, idev, IPSTATS_MIB_FRAGFAILS);
 	kfree_skb(skb);
-	return -EMSGSIZE;
+unlock:
+	rcu_read_unlock();
+	return ret;
 }
 EXPORT_SYMBOL(ip6_xmit);