[PATCH net v4 1/2] amt: send the relay's General Query directly from the receive path

An skb queued in a qdisc can outlive the tunnel it references
through a raw pointer in skb->cb. For example, with igmp_qrv set
to 1 on the relay a tunnel lives for 135s, so a netem delay of
180s on the amt device outlives it; 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

amt_send_igmp_gq() and amt_send_mld_gq() are only called from
amt_request_handler(), inside the rcu_read_lock_bh() section of
amt_rcv(). amt_request_handler() already has the tunnel the query is
for: it found or created it inside that section. Queuing the query with
dev_queue_xmit() only leads back into amt_dev_xmit(), which strips the
Ethernet header and calls amt_send_membership_query() for that tunnel.

Make that call directly from the two senders instead, the same way
amt_send_advertisement() transmits from the receive path. The query
never waits in a qdisc, the tunnel is only dereferenced inside the
RCU section that found or created it, and nothing is stored in
skb->cb, so no lookup or refcount is needed. Remove the query branch
of amt_dev_xmit(), amt_skb_cb() and struct amt_skb_cb, which have no
users left.

Behaviour changes:
 - The relay's own General Queries no longer pass through the amt
   device's egress path: its qdisc, tc egress (clsact/tcx), the
   netfilter egress hook and packet taps. They are still visible as
   UDP on the underlay.
 - A query that is sent successfully is no longer counted as
   tx_dropped. The old query branch left through the unlock label,
   which counted every sent query as dropped.
 - A query that reaches amt_dev_xmit() on a relay from elsewhere,
   such as a userspace querier, is now dropped at the IGMP/MLD type
   switch. Before, it trusted whatever skb->cb held, and a NULL
   tunnel hit the WARN_ON(1).

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: Omar Ramadan <redacted>
---
v4: no code change. Add a selftest (patch 2), as Taehee asked in the v2
  thread:
  https://lore.kernel.org/netdev/CAMArcTXsU+YbUzjF8BOLsVjL2L_Quasv54mdm8iiJN5QdoAPZA@mail.gmail.com/ (local)
v3: https://lore.kernel.org/netdev/20260928181601.85857-1-omar@blockcast.net/ (local)
v3 (Omar): send the GQ directly from amt_request_handler()'s RCU
  section, instead of storing (ip4, source_port) in skb->cb and looking
  the tunnel up again at dequeue. This also removes the per-query
  tunnel_list walk that Taehee raised on v1, and the v2 window Sashiko
  found in which a re-created tunnel could be matched before its nonce
  and mac were written. Cen agreed in the v2 thread to go this way if
  Taehee is fine with it.
v2: https://lore.kernel.org/netdev/20260922214150.13970-1-cenzhang@linux.microsoft.com/ (local)
v1: https://lore.kernel.org/netdev/20260818164825.63967-1-blbllhy@gmail.com/ (local)

Testing: Cen's KASAN reproducer, in which netem holds the General Query
for 160s past a 135s tunnel lifetime, reports the slab-use-after-free
in amt_dev_xmit() on net, and nothing with this patch or with v2
applied. tools/testing/selftests/net/amt.sh passes 5/5 with and without
this patch on a KASAN + lockdep kernel.

 drivers/net/amt.c | 48 +++++++++++++++--------------------------------
 include/net/amt.h |  4 ----
 2 files changed, 15 insertions(+), 37 deletions(-)

diff --git a/drivers/net/amt.c b/drivers/net/amt.c
index bddc24e18..b53f8ec55 100644
--- a/drivers/net/amt.c
+++ b/drivers/net/amt.c

@@ -80,15 +80,6 @@ static struct in6_addr mld2_all_node = MLD2_ALL_NODE_INIT;
 static struct mld2_grec mldv2_zero_grec;
 #endif
 
-static struct amt_skb_cb *amt_skb_cb(struct sk_buff *skb)
-{
-	BUILD_BUG_ON(sizeof(struct amt_skb_cb) + sizeof(struct tc_skb_cb) >
-		     sizeof_field(struct sk_buff, cb));
-
-	return (struct amt_skb_cb *)((void *)skb->cb +
-		sizeof(struct tc_skb_cb));
-}
-
 static void __amt_source_gc_work(void)
 {
 	struct amt_source_node *snode;

@@ -791,6 +782,11 @@ static void amt_send_request(struct amt_dev *amt, bool v6)
 	rcu_read_unlock();
 }
 
+static bool amt_send_membership_query(struct amt_dev *amt,
+				      struct sk_buff *skb,
+				      struct amt_tunnel_list *tunnel,
+				      bool v6);
+
 static void amt_send_igmp_gq(struct amt_dev *amt,
 			     struct amt_tunnel_list *tunnel)
 {

@@ -800,8 +796,11 @@ static void amt_send_igmp_gq(struct amt_dev *amt,
 	if (!skb)
 		return;
 
-	amt_skb_cb(skb)->tunnel = tunnel;
-	dev_queue_xmit(skb);
+	skb_pull(skb, sizeof(struct ethhdr));
+	if (amt_send_membership_query(amt, skb, tunnel, false)) {
+		amt->dev->stats.tx_dropped++;
+		kfree_skb(skb);
+	}
 }
 
 #if IS_ENABLED(CONFIG_IPV6)

@@ -885,8 +884,11 @@ static void amt_send_mld_gq(struct amt_dev *amt, struct amt_tunnel_list *tunnel)
 	if (!skb)
 		return;
 
-	amt_skb_cb(skb)->tunnel = tunnel;
-	dev_queue_xmit(skb);
+	skb_pull(skb, sizeof(struct ethhdr));
+	if (amt_send_membership_query(amt, skb, tunnel, true)) {
+		amt->dev->stats.tx_dropped++;
+		kfree_skb(skb);
+	}
 }
 #else
 static void amt_send_mld_gq(struct amt_dev *amt, struct amt_tunnel_list *tunnel)

@@ -1186,7 +1188,6 @@ static netdev_tx_t amt_dev_xmit(struct sk_buff *skb, struct net_device *dev)
 #endif
 	bool report = false;
 	struct igmphdr *ih;
-	bool query = false;
 	struct iphdr *iph;
 	bool data = false;
 	bool v6 = false;

@@ -1204,9 +1205,6 @@ static netdev_tx_t amt_dev_xmit(struct sk_buff *skb, struct net_device *dev)
 			case IGMP_HOST_MEMBERSHIP_REPORT:
 				report = true;
 				break;
-			case IGMP_HOST_MEMBERSHIP_QUERY:
-				query = true;
-				break;
 			default:
 				goto free;
 			}

@@ -1228,9 +1226,6 @@ static netdev_tx_t amt_dev_xmit(struct sk_buff *skb, struct net_device *dev)
 			case ICMPV6_MLD2_REPORT:
 				report = true;
 				break;
-			case ICMPV6_MGM_QUERY:
-				query = true;
-				break;
 			default:
 				goto free;
 			}

@@ -1261,19 +1256,6 @@ static netdev_tx_t amt_dev_xmit(struct sk_buff *skb, struct net_device *dev)
 			goto free;
 		goto unlock;
 	} else if (amt->mode == AMT_MODE_RELAY) {
-		if (query) {
-			tunnel = amt_skb_cb(skb)->tunnel;
-			if (!tunnel) {
-				WARN_ON(1);
-				goto free;
-			}
-
-			/* Do not forward unexpected query */
-			if (amt_send_membership_query(amt, skb, tunnel, v6))
-				goto free;
-			goto unlock;
-		}
-
 		if (!data)
 			goto free;
 		list_for_each_entry_rcu(tunnel, &amt->tunnel_list, list) {

diff --git a/include/net/amt.h b/include/net/amt.h
index a0255491f..2846dde0c 100644
--- a/include/net/amt.h
+++ b/include/net/amt.h

@@ -231,10 +231,6 @@ struct amt_relay_headers {
 	};
 } __packed;
 
-struct amt_skb_cb {
-	struct amt_tunnel_list *tunnel;
-};
-
 struct amt_tunnel_list {
 	struct list_head	list;
 	/* Protect All resources under an amt_tunne_list */

-- 
2.47.3