netdev - [PATCH net-next 4/8] ipv6: use RCU in ip6

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Message-ID: <20250828195823.3958522-5-edumazet@google.com>
Date: Thu, 28 Aug 2025 19:58:19 +0000
From: Eric Dumazet <edumazet@...gle.com>
To: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Neal Cardwell <ncardwell@...gle.com>
Cc: Simon Horman <horms@...nel.org>, Kuniyuki Iwashima <kuniyu@...gle.com>, 
	David Ahern <dsahern@...nel.org>, netdev@...r.kernel.org, eric.dumazet@...il.com, 
	Eric Dumazet <edumazet@...gle.com>
Subject: [PATCH net-next 4/8] ipv6: use RCU in ip6_output()

Use RCU in ip6_output() in order to use dst_dev_rcu() to prevent
possible UAF.

We can remove rcu_read_lock()/rcu_read_unlock() pairs
from ip6_finish_output2().

Fixes: 4a6ce2b6f2ec ("net: introduce a new function dst_dev_put()")
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 net/ipv6/ip6_output.c | 29 +++++++++++++++--------------
 1 file changed, 15 insertions(+), 14 deletions(-)

diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index e234640433d6b30d3c13d8367dbe7270ddb2c9d7..9d64c13bab5eacb4cc05c78cccd86a7aeb36d37e 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -60,7 +60,7 @@
 static int ip6_finish_output2(struct net *net, struct sock *sk, struct sk_buff *skb)
 {
 	struct dst_entry *dst = skb_dst(skb);
-	struct net_device *dev = dst_dev(dst);
+	struct net_device *dev = dst_dev_rcu(dst);
 	struct inet6_dev *idev = ip6_dst_idev(dst);
 	unsigned int hh_len = LL_RESERVED_SPACE(dev);
 	const struct in6_addr *daddr, *nexthop;
@@ -70,15 +70,12 @@ static int ip6_finish_output2(struct net *net, struct sock *sk, struct sk_buff *
 
 	/* Be paranoid, rather than too clever. */
 	if (unlikely(hh_len > skb_headroom(skb)) && dev->header_ops) {
-		/* Make sure idev stays alive */
-		rcu_read_lock();
+		/* idev stays alive because we hold rcu_read_lock(). */
 		skb = skb_expand_head(skb, hh_len);
 		if (!skb) {
 			IP6_INC_STATS(net, idev, IPSTATS_MIB_OUTDISCARDS);
-			rcu_read_unlock();
 			return -ENOMEM;
 		}
-		rcu_read_unlock();
 	}
 
 	hdr = ipv6_hdr(skb);
@@ -123,7 +120,6 @@ static int ip6_finish_output2(struct net *net, struct sock *sk, struct sk_buff *
 
 	IP6_UPD_PO_STATS(net, idev, IPSTATS_MIB_OUT, skb->len);
 
-	rcu_read_lock();
 	nexthop = rt6_nexthop(dst_rt6_info(dst), daddr);
 	neigh = __ipv6_neigh_lookup_noref(dev, nexthop);
 
@@ -131,7 +127,6 @@ static int ip6_finish_output2(struct net *net, struct sock *sk, struct sk_buff *
 		if (unlikely(!neigh))
 			neigh = __neigh_create(&nd_tbl, nexthop, dev, false);
 		if (IS_ERR(neigh)) {
-			rcu_read_unlock();
 			IP6_INC_STATS(net, idev, IPSTATS_MIB_OUTNOROUTES);
 			kfree_skb_reason(skb, SKB_DROP_REASON_NEIGH_CREATEFAIL);
 			return -EINVAL;
@@ -139,7 +134,6 @@ static int ip6_finish_output2(struct net *net, struct sock *sk, struct sk_buff *
 	}
 	sock_confirm_neigh(skb, neigh);
 	ret = neigh_output(neigh, skb, false);
-	rcu_read_unlock();
 	return ret;
 }
 
@@ -233,22 +227,29 @@ static int ip6_finish_output(struct net *net, struct sock *sk, struct sk_buff *s
 int ip6_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 {
 	struct dst_entry *dst = skb_dst(skb);
-	struct net_device *dev = dst_dev(dst), *indev = skb->dev;
-	struct inet6_dev *idev = ip6_dst_idev(dst);
+	struct net_device *dev, *indev = skb->dev;
+	struct inet6_dev *idev;
+	int ret;
 
 	skb->protocol = htons(ETH_P_IPV6);
+	rcu_read_lock();
+	dev = dst_dev_rcu(dst);
+	idev = ip6_dst_idev(dst);
 	skb->dev = dev;
 
 	if (unlikely(!idev || READ_ONCE(idev->cnf.disable_ipv6))) {
 		IP6_INC_STATS(net, idev, IPSTATS_MIB_OUTDISCARDS);
+		rcu_read_unlock();
 		kfree_skb_reason(skb, SKB_DROP_REASON_IPV6DISABLED);
 		return 0;
 	}
 
-	return NF_HOOK_COND(NFPROTO_IPV6, NF_INET_POST_ROUTING,
-			    net, sk, skb, indev, dev,
-			    ip6_finish_output,
-			    !(IP6CB(skb)->flags & IP6SKB_REROUTED));
+	ret = NF_HOOK_COND(NFPROTO_IPV6, NF_INET_POST_ROUTING,
+			   net, sk, skb, indev, dev,
+			   ip6_finish_output,
+			   !(IP6CB(skb)->flags & IP6SKB_REROUTED));
+	rcu_read_unlock();
+	return ret;
 }
 EXPORT_SYMBOL(ip6_output);
 
-- 
2.51.0.318.gd7df087d1a-goog