net: add xmit recursion limit to tunnel xmit functions

Tunnel xmit functions (iptunnel_xmit, ip6tunnel_xmit) lack their own
recursion limit. When a bond device in broadcast mode has GRE tap
interfaces as slaves, and those GRE tunnels route back through the
bond, multicast/broadcast traffic triggers infinite recursion between
bond_xmit_broadcast() and ip_tunnel_xmit()/ip6_tnl_xmit(), causing
kernel stack overflow.

The existing XMIT_RECURSION_LIMIT (8) in the no-qdisc path is not
sufficient because tunnel recursion involves route lookups and full IP
output, consuming much more stack per level. Use a lower limit of 4
(IP_TUNNEL_RECURSION_LIMIT) to prevent overflow.

Add recursion detection using dev_xmit_recursion helpers directly in
iptunnel_xmit() and ip6tunnel_xmit() to cover all IPv4/IPv6 tunnel
paths including UDP encapsulated tunnels (VXLAN, Geneve, etc.).

Move dev_xmit_recursion helpers from net/core/dev.h to public header
include/linux/netdevice.h so they can be used by tunnel code.

 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>

Fixes: 745e20f1b626 ("net: add a recursion limit in xmit path")
Reported-by: Xiang Mei <xmei5@asu.edu>
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
Link: https://patch.msgid.link/20260306160133.3852900-2-bestswngs@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

2 files changed, 19 insertions, 0 deletions

diff --git a/include/net/ip6_tunnel.h b/include/net/ip6_tunnel.h
index 120db2865811..1253cbb4b0a4 100644
--- a/include/net/ip6_tunnel.h
+++ b/include/net/ip6_tunnel.h
@@ -156,6 +156,16 @@ static inline void ip6tunnel_xmit(struct sock *sk, struct sk_buff *skb,
 {
 	int pkt_len, err;
 
+	if (dev_recursion_level() > IP_TUNNEL_RECURSION_LIMIT) {
+		net_crit_ratelimited("Dead loop on virtual device %s, fix it urgently!\n",
+				     dev->name);
+		DEV_STATS_INC(dev, tx_errors);
+		kfree_skb(skb);
+		return;
+	}
+
+	dev_xmit_recursion_inc();
+
 	memset(skb->cb, 0, sizeof(struct inet6_skb_parm));
 	IP6CB(skb)->flags = ip6cb_flags;
 	pkt_len = skb->len - skb_inner_network_offset(skb);
@@ -166,6 +176,8 @@ static inline void ip6tunnel_xmit(struct sock *sk, struct sk_buff *skb,
 			pkt_len = -1;
 		iptunnel_xmit_stats(dev, pkt_len);
 	}
+
+	dev_xmit_recursion_dec();
 }
 #endif
 #endif
diff --git a/include/net/ip_tunnels.h b/include/net/ip_tunnels.h
index 4021e6a73e32..80662f812080 100644
--- a/include/net/ip_tunnels.h
+++ b/include/net/ip_tunnels.h
@@ -27,6 +27,13 @@
 #include <net/ip6_route.h>
 #endif
 
+/* Recursion limit for tunnel xmit to detect routing loops.
+ * Unlike XMIT_RECURSION_LIMIT (8) used in the no-qdisc path, tunnel
+ * recursion involves route lookups and full IP output, consuming much
+ * more stack per level, so a lower limit is needed.
+ */
+#define IP_TUNNEL_RECURSION_LIMIT	4
+
 /* Keep error state on tunnel for 30 sec */
 #define IPTUNNEL_ERR_TIMEO	(30*HZ)