Subject: [PATCH net] net/core: add xmit recursion limit to qdisc transmit path

From: [email-masked]
Date: Tue, 3 Mar 2026 10:29:48 +0800

From: Weiming Shi <[email-masked]>

__dev_queue_xmit() has two transmit code paths depending on whether the
device has a qdisc attached:

  1. Qdisc path (q->enqueue): calls __dev_xmit_skb()
  2. No-qdisc path: calls dev_hard_start_xmit() directly

Commit 745e20f1b626 ("net: add a recursion limit in xmit path") added
recursion protection to the no-qdisc path via dev_xmit_recursion()
check and dev_xmit_recursion_inc()/dec() tracking. However, the qdisc
path performs no recursion depth checking at all.

This allows unbounded recursion through qdisc-attached devices. For
example, a bond interface in broadcast mode with gretap slaves whose
remote endpoints route back through the bond creates an infinite
transmit loop that exhausts the kernel stack:

  BUG: KASAN: stack-out-of-bounds in blake2s.constprop.0+0xe7/0x160
  Write of size 32 at addr ffff88810033fed0 by task kworker/0:1/11
  Workqueue: mld mld_ifc_work
  Call Trace:
   <TASK>
   __build_flow_key.constprop.0 (net/ipv4/route.c:515)
   ip_rt_update_pmtu (net/ipv4/route.c:1073)
   iptunnel_xmit (net/ipv4/ip_tunnel_core.c:84)
   ip_tunnel_xmit (net/ipv4/ip_tunnel.c:847)
   gre_tap_xmit (net/ipv4/ip_gre.c:779)
   dev_hard_start_xmit (net/core/dev.c:3887)
   sch_direct_xmit (net/sched/sch_generic.c:347)
   __dev_queue_xmit (net/core/dev.c:4802)
   bond_dev_queue_xmit (drivers/net/bonding/bond_main.c:312)
   bond_xmit_broadcast (drivers/net/bonding/bond_main.c:5279)
   bond_start_xmit (drivers/net/bonding/bond_main.c:5530)
   dev_hard_start_xmit (net/core/dev.c:3887)
   __dev_queue_xmit (net/core/dev.c:4841)
   ip_finish_output2 (net/ipv4/ip_output.c:237)
   ip_output (net/ipv4/ip_output.c:438)
   iptunnel_xmit (net/ipv4/ip_tunnel_core.c:86)
   gre_tap_xmit (net/ipv4/ip_gre.c:779)
   dev_hard_start_xmit (net/core/dev.c:3887)
   sch_direct_xmit (net/sched/sch_generic.c:347)
   __dev_queue_xmit (net/core/dev.c:4802)
   bond_dev_queue_xmit (drivers/net/bonding/bond_main.c:312)
   bond_xmit_broadcast (drivers/net/bonding/bond_main.c:5279)
   bond_start_xmit (drivers/net/bonding/bond_main.c:5530)
   dev_hard_start_xmit (net/core/dev.c:3887)
   __dev_queue_xmit (net/core/dev.c:4841)
   ip_finish_output2 (net/ipv4/ip_output.c:237)
   ip_output (net/ipv4/ip_output.c:438)
   iptunnel_xmit (net/ipv4/ip_tunnel_core.c:86)
   ip_tunnel_xmit (net/ipv4/ip_tunnel.c:847)
   gre_tap_xmit (net/ipv4/ip_gre.c:779)
   dev_hard_start_xmit (net/core/dev.c:3887)
   sch_direct_xmit (net/sched/sch_generic.c:347)
   __dev_queue_xmit (net/core/dev.c:4802)
   bond_dev_queue_xmit (drivers/net/bonding/bond_main.c:312)
   bond_xmit_broadcast (drivers/net/bonding/bond_main.c:5279)
   bond_start_xmit (drivers/net/bonding/bond_main.c:5530)
   dev_hard_start_xmit (net/core/dev.c:3887)
   __dev_queue_xmit (net/core/dev.c:4841)
   mld_sendpack
   mld_ifc_work
   process_one_work
   worker_thread
   </TASK>

  poc (76) used greatest stack depth: 8 bytes left

The per-queue qdisc_run_begin() serialization does not prevent this
because each gretap slave can have multiple TX queues, so each
recursion level may select a different queue. The q->owner check also
fails because each level operates on a different qdisc instance.

Fix by adding the same recursion protection to the qdisc path that
the no-qdisc path already has: check dev_xmit_recursion() before
entering __dev_xmit_skb(), and bracket the call with
dev_xmit_recursion_inc()/dec() to properly track nesting depth
across both transmit paths.

Fixes: bbd8a0d3a3b6 ("net: Avoid enqueuing skb for default qdiscs")
Reported-by: Xiang Mei <[email-masked]>
Signed-off-by: Weiming Shi <[email-masked]>
---
 net/core/dev.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/net/core/dev.c b/net/core/dev.c
index c1a9f7fdcffa..d5d929df67be 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -4799,7 +4799,17 @@ int __dev_queue_xmit(struct sk_buff *skb, struct net_device *sb_dev)
 
 	trace_net_dev_queue(skb);
 	if (q->enqueue) {
+		if (unlikely(dev_xmit_recursion())) {
+			net_crit_ratelimited("Dead loop on virtual device %s, fix it urgently!\n",
+					     dev->name);
+			rc = -ENETDOWN;
+			dev_core_stats_tx_dropped_inc(dev);
+			kfree_skb_list(skb);
+			goto out;
+		}
+		dev_xmit_recursion_inc();
 		rc = __dev_xmit_skb(skb, q, dev, txq);
+		dev_xmit_recursion_dec();
 		goto out;
 	}
 
-- 
2.43.0