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) <xmei5@asu.edu>
Reported-by: Cen Zhang (Microsoft Security FORGE Labs) <cenzhang@linux.microsoft.com>
Signed-off-by: Omar Ramadan <omar@blockcast.net>
Link: https://patch.msgid.link/20260928202312.74574-2-omar@blockcast.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 0 insertions, 4 deletions

diff --git a/include/net/amt.h b/include/net/amt.h
index a0255491f5b0..2846dde0cadc 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 */