[PATCH net v2] amt: do not store tunnel pointer in skb control block
From: Cen Zhang (Microsoft Security FORGE Labs) <hidden>
Date: 2026-09-22 21:42:16
Also in:
lkml
Subsystem:
amt (automatic multicast tunneling), networking drivers, networking [general], the rest · Maintainers:
Taehee Yoo, Andrew Lunn, "David S. Miller", Eric Dumazet, Jakub Kicinski, Paolo Abeni, Linus Torvalds
An skb queued in a qdisc can outlive the tunnel it references
through a raw pointer in skb->cb. For example, a netem delay of
180s exceeds the default tunnel lifetime of 135s (igmp_qrv=1);
when the tunnel expires and is freed, the subsequent dequeue
triggers a use-after-free in amt_dev_xmit().
BUG: KASAN: slab-use-after-free in amt_dev_xmit+0x2763/0x2e20
Call Trace:
amt_dev_xmit+0x2763/0x2e20 [drivers/net/amt.c:1262]
dev_hard_start_xmit+0x22f/0x620
sch_direct_xmit+0x12e/0xac0
netem_dequeue+0x333/0xc50
net_tx_action+0x35c/0xa60
Store the tunnel identity (ip4 + source_port) in skb->cb instead
of a pointer, and re-lookup the tunnel under RCU in amt_dev_xmit().
If the tunnel is gone, the query is simply dropped.
A refcount fix would be hard to keep balanced here, as the skb may
be dropped or cloned by the qdisc layer before reaching
amt_dev_xmit().
Fixes: cbc21dc1cfe9 ("amt: add data plane of amt interface")
Reported-by: AutonomousCodeSecurity@microsoft.com
Reported-by: Xiang Mei (Microsoft) <redacted>
Reported-by: Cen Zhang (Microsoft Security FORGE Labs) <redacted>
Signed-off-by: Cen Zhang (Microsoft Security FORGE Labs) <redacted>
---
v2:
- Drop the comment above struct amt_skb_cb (Taehee Yoo)
- Mention in the commit message why a refcount is not used
(Taehee Yoo)
- Rebase on net/main
v1: https://lore.kernel.org/netdev/20260818164825.63967-1-blbllhy@gmail.com/ (local)
drivers/net/amt.c | 34 +++++++++++++++++++++++++---------
include/net/amt.h | 3 ++-
2 files changed, 27 insertions(+), 10 deletions(-)
diff --git a/drivers/net/amt.c b/drivers/net/amt.c
index bddc24e1856..b660cebf248 100644
--- a/drivers/net/amt.c
+++ b/drivers/net/amt.c@@ -791,6 +791,18 @@ static void amt_send_request(struct amt_dev *amt, bool v6) rcu_read_unlock(); } +static struct amt_tunnel_list *amt_lookup_tunnel(struct amt_dev *amt, + __be32 ip4, __be16 source_port) +{ + struct amt_tunnel_list *tunnel; + + list_for_each_entry_rcu(tunnel, &amt->tunnel_list, list) + if (tunnel->ip4 == ip4 && tunnel->source_port == source_port) + return tunnel; + + return NULL; +} + static void amt_send_igmp_gq(struct amt_dev *amt, struct amt_tunnel_list *tunnel) {
@@ -800,7 +812,8 @@ static void amt_send_igmp_gq(struct amt_dev *amt, if (!skb) return; - amt_skb_cb(skb)->tunnel = tunnel; + amt_skb_cb(skb)->tunnel_ip4 = tunnel->ip4; + amt_skb_cb(skb)->tunnel_port = tunnel->source_port; dev_queue_xmit(skb); }
@@ -885,7 +898,8 @@ static void amt_send_mld_gq(struct amt_dev *amt, struct amt_tunnel_list *tunnel) if (!skb) return; - amt_skb_cb(skb)->tunnel = tunnel; + amt_skb_cb(skb)->tunnel_ip4 = tunnel->ip4; + amt_skb_cb(skb)->tunnel_port = tunnel->source_port; dev_queue_xmit(skb); } #else
@@ -1262,15 +1276,17 @@ static netdev_tx_t amt_dev_xmit(struct sk_buff *skb, struct net_device *dev) goto unlock; } else if (amt->mode == AMT_MODE_RELAY) { if (query) { - tunnel = amt_skb_cb(skb)->tunnel; - if (!tunnel) { - WARN_ON(1); - goto free; - } - + rcu_read_lock(); + tunnel = amt_lookup_tunnel(amt, + amt_skb_cb(skb)->tunnel_ip4, + amt_skb_cb(skb)->tunnel_port); /* Do not forward unexpected query */ - if (amt_send_membership_query(amt, skb, tunnel, v6)) + if (!tunnel || + amt_send_membership_query(amt, skb, tunnel, v6)) { + rcu_read_unlock(); goto free; + } + rcu_read_unlock(); goto unlock; }
diff --git a/include/net/amt.h b/include/net/amt.h
index a0255491f5b..8727cf007bc 100644
--- a/include/net/amt.h
+++ b/include/net/amt.h@@ -232,7 +232,8 @@ struct amt_relay_headers { } __packed; struct amt_skb_cb { - struct amt_tunnel_list *tunnel; + __be32 tunnel_ip4; + __be16 tunnel_port; }; struct amt_tunnel_list {
--
2.53.0