[PATCH net v2 1/1] ip6_tunnel: snapshot encap in xmit

From: Zixuan Chai <[email protected]>

ip6_tnl_changelink() can update encapsulation parameters while the
netdevice is transmitting packets. ip6_tnl_xmit() can calculate packet
headroom with t->encap_hlen and later build an encapsulation header from
the live t->encap. A concurrent update can change the encapsulation
header between these accesses and make skb_push() underflow the skb head.

Take a local snapshot of t->encap before calculating the encapsulation
header length. Use that same snapshot for headroom accounting, metadata
validation, and build_header(). This keeps all encapsulation decisions
for an skb consistent even if changelink updates the live configuration.

Fixes: b3a27b519b22 ("ip6_tunnel: Add support for fou/gue encapsulation")
Cc: [email protected]
Reported-by: Vega <[email protected]>
Assisted-by: Codex:gpt-5.4
Signed-off-by: Zixuan Chai <[email protected]>
Signed-off-by: Ren Wei <[email protected]>
---
 include/net/ip6_tunnel.h | 10 +++++-----
 net/ipv6/ip6_tunnel.c    | 21 ++++++++++++++++-----
 2 files changed, 21 insertions(+), 10 deletions(-)

diff --git a/include/net/ip6_tunnel.h b/include/net/ip6_tunnel.h
index b99805ee2fd1..6e76e50a4406 100644
--- a/include/net/ip6_tunnel.h
+++ b/include/net/ip6_tunnel.h
@@ -106,22 +106,22 @@ static inline int ip6_encap_hlen(struct ip_tunnel_encap *e)
 	return hlen;
 }
 
-static inline int ip6_tnl_encap(struct sk_buff *skb, struct ip6_tnl *t,
+static inline int ip6_tnl_encap(struct sk_buff *skb, struct ip_tunnel_encap *e,
 				u8 *protocol, struct flowi6 *fl6)
 {
 	const struct ip6_tnl_encap_ops *ops;
 	int ret = -EINVAL;
 
-	if (t->encap.type == TUNNEL_ENCAP_NONE)
+	if (e->type == TUNNEL_ENCAP_NONE)
 		return 0;
 
-	if (t->encap.type >= MAX_IPTUN_ENCAP_OPS)
+	if (e->type >= MAX_IPTUN_ENCAP_OPS)
 		return -EINVAL;
 
 	rcu_read_lock();
-	ops = rcu_dereference(ip6tun_encaps[t->encap.type]);
+	ops = rcu_dereference(ip6tun_encaps[e->type]);
 	if (likely(ops && ops->build_header))
-		ret = ops->build_header(skb, &t->encap, protocol, fl6);
+		ret = ops->build_header(skb, e, protocol, fl6);
 	rcu_read_unlock();
 
 	return ret;
diff --git a/net/ipv6/ip6_tunnel.c b/net/ipv6/ip6_tunnel.c
index ebf83f090376..d47757e8a388 100644
--- a/net/ipv6/ip6_tunnel.c
+++ b/net/ipv6/ip6_tunnel.c
@@ -1102,6 +1102,7 @@ int ip6_tnl_xmit(struct sk_buff *skb, struct net_device *dev, __u8 dsfield,
 		 __u8 proto)
 {
 	struct ip6_tnl *t = netdev_priv(dev);
+	struct ip_tunnel_encap ipencap;
 	struct net *net = t->net;
 	struct ipv6hdr *ipv6h;
 	struct ipv6_tel_txoption opt;
@@ -1109,10 +1110,11 @@ int ip6_tnl_xmit(struct sk_buff *skb, struct net_device *dev, __u8 dsfield,
 	struct net_device *tdev;
 	int err_count, mtu;
 	unsigned int eth_hlen = t->dev->type == ARPHRD_ETHER ? ETH_HLEN : 0;
-	unsigned int psh_hlen = sizeof(struct ipv6hdr) + t->encap_hlen;
-	unsigned int max_headroom = psh_hlen;
+	unsigned int max_headroom;
 	__be16 payload_protocol;
 	bool use_cache = false;
+	unsigned int psh_hlen;
+	int encap_hlen;
 	u8 hop_limit;
 	int err = -1;
 
@@ -1202,6 +1204,15 @@ int ip6_tnl_xmit(struct sk_buff *skb, struct net_device *dev, __u8 dsfield,
 				     t->parms.name);
 		goto tx_err_dst_release;
 	}
+
+	/* Can tear, but hlen and build_header() use the same snapshot. */
+	ipencap = data_race(t->encap);
+	encap_hlen = ip6_encap_hlen(&ipencap);
+	if (unlikely(encap_hlen < 0))
+		goto tx_err_dst_release;
+	psh_hlen = sizeof(struct ipv6hdr) + encap_hlen;
+	max_headroom = psh_hlen;
+
 	mtu = dst6_mtu(dst) - eth_hlen - psh_hlen - t->tun_hlen;
 	if (encap_limit >= 0) {
 		max_headroom += 8;
@@ -1251,7 +1262,7 @@ int ip6_tnl_xmit(struct sk_buff *skb, struct net_device *dev, __u8 dsfield,
 	}
 
 	if (t->parms.collect_md) {
-		if (t->encap.type != TUNNEL_ENCAP_NONE)
+		if (ipencap.type != TUNNEL_ENCAP_NONE)
 			goto tx_err_dst_release;
 	} else {
 		if (use_cache && ndst)
@@ -1272,10 +1283,10 @@ int ip6_tnl_xmit(struct sk_buff *skb, struct net_device *dev, __u8 dsfield,
 	 * needed_headroom if necessary.
 	 */
 	max_headroom = LL_RESERVED_SPACE(tdev) + sizeof(struct ipv6hdr)
-			+ dst->header_len + t->hlen;
+			+ dst->header_len + t->tun_hlen + encap_hlen;
 	ip_tunnel_adj_headroom(dev, max_headroom);
 
-	err = ip6_tnl_encap(skb, t, &proto, fl6);
+	err = ip6_tnl_encap(skb, &ipencap, &proto, fl6);
 	if (err)
 		return err;
 
-- 
2.34.1