From afae89de73dd693cfa329580ba0fba63bd3f0a11 Mon Sep 17 00:00:00 2001 From: Omar Ramadan Date: Mon, 28 Sep 2026 23:23:11 +0300 Subject: [PATCH] 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) Reported-by: Cen Zhang (Microsoft Security FORGE Labs) Signed-off-by: Omar Ramadan Link: https://patch.msgid.link/20260928202312.74574-2-omar@blockcast.net Signed-off-by: Jakub Kicinski --- 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 bddc24e1856d..b53f8ec55661 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 @@ out: 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 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 */