Subject: [PATCH net v2] net: protect egress device access in the output path with rcu_read_lock

From: Hyunwoo Kim <[email-masked]>
Date: Sun, 31 May 2026 16:06:50 +0900

ip_mc_output(), __ip_local_out(), xfrm4_output(), raw_send_hdrinc(),
xfrm_output_resume() and vrf_output() read the egress network device
from rt->dst.dev / skb_dst(skb)->dev / skb_dst_dev(skb) and then pass
that pointer as the 'out' device argument of NF_HOOK without holding
rcu_read_lock.

dst->dev is a field protected by RCU. When a device is unregistered its
value is replaced with blackhole_netdev, and the previous device is
freed after an RCU grace period. A section that reads dst->dev and uses
that pointer must therefore hold rcu_read_lock. The functions above are
missing this protection, so when the egress device is unregistered
concurrently with transmission, a LOCAL_OUT / POST_ROUTING hook (nft
meta oif, selinux_ip_postroute_compat, etc.) can reference a device
pointer that is no longer valid.

In the leaves above, wrap the section that reads the device and runs
NF_HOOK in rcu_read_lock(), and read the device through the RCU
accessors (skb_dst_dev_rcu() / dst_dev_rcu()). As a result, the RCU
grace period cannot complete while a transmission is in flight, so the
egress device is not freed and the hook always references a valid
device.

Fixes: 4a6ce2b6f2ec ("net: introduce a new function dst_dev_put()")
Signed-off-by: Hyunwoo Kim <[email-masked]>
---
Changes in v2:
- Changed to a net-wide patch that also fixes the issue in
raw_send_hdrinc(), xfrm_output_resume() and vrf_output().
- v1: https://lore.kernel.org/all/ahqIg6vURwYI0LJ5@v4bel/
---
 drivers/net/vrf.c       | 16 +++++++++++-----
 net/ipv4/ip_output.c    | 27 +++++++++++++++++++--------
 net/ipv4/raw.c          |  4 +++-
 net/ipv4/xfrm4_output.c | 13 +++++++++----
 net/xfrm/xfrm_output.c  |  4 +++-
 5 files changed, 45 insertions(+), 19 deletions(-)

diff --git a/drivers/net/vrf.c b/drivers/net/vrf.c
index 46209917ae4d..e9a1dd961805 100644
--- a/drivers/net/vrf.c
+++ b/drivers/net/vrf.c
@@ -833,17 +833,23 @@ static int vrf_finish_output(struct net *net, struct sock *sk, struct sk_buff *s
 
 static int vrf_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 {
-	struct net_device *dev = skb_dst(skb)->dev;
+	struct net_device *dev;
+	int ret;
+
+	rcu_read_lock();
+	dev = skb_dst_dev_rcu(skb);
 
 	IP_UPD_PO_STATS(net, IPSTATS_MIB_OUT, skb->len);
 
 	skb->dev = dev;
 	skb->protocol = htons(ETH_P_IP);
 
-	return NF_HOOK_COND(NFPROTO_IPV4, NF_INET_POST_ROUTING,
-			    net, sk, skb, NULL, dev,
-			    vrf_finish_output,
-			    !(IPCB(skb)->flags & IPSKB_REROUTED));
+	ret = NF_HOOK_COND(NFPROTO_IPV4, NF_INET_POST_ROUTING,
+			   net, sk, skb, NULL, dev,
+			   vrf_finish_output,
+			   !(IPCB(skb)->flags & IPSKB_REROUTED));
+	rcu_read_unlock();
+	return ret;
 }
 
 /* set dst on skb to send packet to us via dev_xmit path. Allows
diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index 5bcd73cbdb41..92a70b7e74a6 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -102,6 +102,7 @@ EXPORT_SYMBOL(ip_send_check);
 int __ip_local_out(struct net *net, struct sock *sk, struct sk_buff *skb)
 {
 	struct iphdr *iph = ip_hdr(skb);
+	int err;
 
 	IP_INC_STATS(net, IPSTATS_MIB_OUTREQUESTS);
 
@@ -117,9 +118,12 @@ int __ip_local_out(struct net *net, struct sock *sk, struct sk_buff *skb)
 
 	skb->protocol = htons(ETH_P_IP);
 
-	return nf_hook(NFPROTO_IPV4, NF_INET_LOCAL_OUT,
-		       net, sk, skb, NULL, skb_dst_dev(skb),
-		       dst_output);
+	rcu_read_lock();
+	err = nf_hook(NFPROTO_IPV4, NF_INET_LOCAL_OUT,
+		      net, sk, skb, NULL, skb_dst_dev_rcu(skb),
+		      dst_output);
+	rcu_read_unlock();
+	return err;
 }
 
 int ip_local_out(struct net *net, struct sock *sk, struct sk_buff *skb)
@@ -368,7 +372,11 @@ static int ip_mc_finish_output(struct net *net, struct sock *sk,
 int ip_mc_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 {
 	struct rtable *rt = skb_rtable(skb);
-	struct net_device *dev = rt->dst.dev;
+	struct net_device *dev;
+	int ret;
+
+	rcu_read_lock();
+	dev = dst_dev_rcu(&rt->dst);
 
 	/*
 	 *	If the indicated interface is up and running, send the packet.
@@ -407,6 +415,7 @@ int ip_mc_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 
 		if (ip_hdr(skb)->ttl == 0) {
 			kfree_skb(skb);
+			rcu_read_unlock();
 			return 0;
 		}
 	}
@@ -419,10 +428,12 @@ int ip_mc_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 				ip_mc_finish_output);
 	}
 
-	return NF_HOOK_COND(NFPROTO_IPV4, NF_INET_POST_ROUTING,
-			    net, sk, skb, NULL, skb->dev,
-			    ip_finish_output,
-			    !(IPCB(skb)->flags & IPSKB_REROUTED));
+	ret = NF_HOOK_COND(NFPROTO_IPV4, NF_INET_POST_ROUTING,
+			   net, sk, skb, NULL, skb->dev,
+			   ip_finish_output,
+			   !(IPCB(skb)->flags & IPSKB_REROUTED));
+	rcu_read_unlock();
+	return ret;
 }
 
 int ip_output(struct net *net, struct sock *sk, struct sk_buff *skb)
diff --git a/net/ipv4/raw.c b/net/ipv4/raw.c
index 68e88cb3e55c..555e62eacdc6 100644
--- a/net/ipv4/raw.c
+++ b/net/ipv4/raw.c
@@ -410,9 +410,11 @@ static int raw_send_hdrinc(struct sock *sk, struct flowi4 *fl4,
 				skb_transport_header(skb))->type);
 	}
 
+	rcu_read_lock();
 	err = NF_HOOK(NFPROTO_IPV4, NF_INET_LOCAL_OUT,
-		      net, sk, skb, NULL, rt->dst.dev,
+		      net, sk, skb, NULL, skb_dst_dev_rcu(skb),
 		      dst_output);
+	rcu_read_unlock();
 	if (err > 0)
 		err = net_xmit_errno(err);
 	if (err)
diff --git a/net/ipv4/xfrm4_output.c b/net/ipv4/xfrm4_output.c
index 0ae67d537499..d5dc084777a3 100644
--- a/net/ipv4/xfrm4_output.c
+++ b/net/ipv4/xfrm4_output.c
@@ -30,10 +30,15 @@ static int __xfrm4_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 
 int xfrm4_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 {
-	return NF_HOOK_COND(NFPROTO_IPV4, NF_INET_POST_ROUTING,
-			    net, sk, skb, skb->dev, skb_dst_dev(skb),
-			    __xfrm4_output,
-			    !(IPCB(skb)->flags & IPSKB_REROUTED));
+	int ret;
+
+	rcu_read_lock();
+	ret = NF_HOOK_COND(NFPROTO_IPV4, NF_INET_POST_ROUTING,
+			   net, sk, skb, skb->dev, skb_dst_dev_rcu(skb),
+			   __xfrm4_output,
+			   !(IPCB(skb)->flags & IPSKB_REROUTED));
+	rcu_read_unlock();
+	return ret;
 }
 
 void xfrm4_local_error(struct sk_buff *skb, u32 mtu)
diff --git a/net/xfrm/xfrm_output.c b/net/xfrm/xfrm_output.c
index cc35c2fcbbe0..2b69ac55c8a5 100644
--- a/net/xfrm/xfrm_output.c
+++ b/net/xfrm/xfrm_output.c
@@ -604,9 +604,11 @@ int xfrm_output_resume(struct sock *sk, struct sk_buff *skb, int err)
 		if (!skb_dst(skb)->xfrm)
 			return dst_output(net, sk, skb);
 
+		rcu_read_lock();
 		err = nf_hook(skb_dst(skb)->ops->family,
 			      NF_INET_POST_ROUTING, net, sk, skb,
-			      NULL, skb_dst(skb)->dev, xfrm_output2);
+			      NULL, skb_dst_dev_rcu(skb), xfrm_output2);
+		rcu_read_unlock();
 		if (unlikely(err != 1))
 			goto out;
 	}
-- 
2.43.0