Subject: Re: [PATCH net v2] net: add xmit recursion limit to tunnel drivers

From: Weiming Shi <[email-masked]>
Date: Fri, 6 Mar 2026 01:08:16 +0800

On 26-03-05 15:34, Paolo Abeni wrote:
> On 3/3/26 5:43 PM, [email-masked] wrote:
> > 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>
> > 
> > Rather than adding an expensive recursion check to the common qdisc
> > fast path, add recursion detection directly in tunnel drivers where
> > route lookups can create these loops, as suggested by Eric Dumazet.
> > 
> > Use a lower limit (IP_TUNNEL_RECURSION_LIMIT=4) than XMIT_RECURSION_LIMIT
> > (8) because each tunnel recursion level involves route lookups and full
> > IP output processing, consuming significantly more stack per level.
> > 
> > Move dev_xmit_recursion helpers from the private net/core/dev.h header
> > to the public include/linux/netdevice.h so tunnel drivers can use them.
> > 
> > Fixes: 745e20f1b626 ("net: add a recursion limit in xmit path")
> > Reported-by: Xiang Mei <[email-masked]>
> > Signed-off-by: Weiming Shi <[email-masked]>
> > ---
> > v2:
> >   - Move recursion check from qdisc path to tunnel drivers (Eric Dumazet)
> >   - Add IPv6 tunnel (ip6_tunnel) coverage
> >   - Use lower recursion limit (4) for tunnel-specific stack consumption
> > 
> >  include/linux/netdevice.h | 32 ++++++++++++++++++++++++++++++++
> >  include/net/ip_tunnels.h  |  7 +++++++
> >  net/core/dev.h            | 34 ----------------------------------
> >  net/ipv4/ip_tunnel.c      | 10 ++++++++++
> >  net/ipv6/ip6_tunnel.c     | 10 ++++++++++
> >  5 files changed, 59 insertions(+), 34 deletions(-)
> > 
> > diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> > index d4e6e00bb90a..1a4d2542dbab 100644
> > --- a/include/linux/netdevice.h
> > +++ b/include/linux/netdevice.h
> > @@ -3576,17 +3576,49 @@ struct page_pool_bh {
> >  };
> >  DECLARE_PER_CPU(struct page_pool_bh, system_page_pool);
> >  
> > +#define XMIT_RECURSION_LIMIT	8
> > +
> >  #ifndef CONFIG_PREEMPT_RT
> >  static inline int dev_recursion_level(void)
> >  {
> >  	return this_cpu_read(softnet_data.xmit.recursion);
> >  }
> > +
> > +static inline bool dev_xmit_recursion(void)
> > +{
> > +	return unlikely(__this_cpu_read(softnet_data.xmit.recursion) >
> > +			XMIT_RECURSION_LIMIT);
> > +}
> > +
> > +static inline void dev_xmit_recursion_inc(void)
> > +{
> > +	__this_cpu_inc(softnet_data.xmit.recursion);
> > +}
> > +
> > +static inline void dev_xmit_recursion_dec(void)
> > +{
> > +	__this_cpu_dec(softnet_data.xmit.recursion);
> > +}
> >  #else
> >  static inline int dev_recursion_level(void)
> >  {
> >  	return current->net_xmit.recursion;
> >  }
> >  
> > +static inline bool dev_xmit_recursion(void)
> > +{
> > +	return unlikely(current->net_xmit.recursion > XMIT_RECURSION_LIMIT);
> > +}
> > +
> > +static inline void dev_xmit_recursion_inc(void)
> > +{
> > +	current->net_xmit.recursion++;
> > +}
> > +
> > +static inline void dev_xmit_recursion_dec(void)
> > +{
> > +	current->net_xmit.recursion--;
> > +}
> >  #endif
> >  
> >  void __netif_schedule(struct Qdisc *q);
> > 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)
> >  
> > diff --git a/net/core/dev.h b/net/core/dev.h
> > index 98793a738f43..ec974b3c42d9 100644
> > --- a/net/core/dev.h
> > +++ b/net/core/dev.h
> > @@ -366,40 +366,6 @@ static inline void napi_assert_will_not_race(const struct napi_struct *napi)
> >  
> >  void kick_defer_list_purge(unsigned int cpu);
> >  
> > -#define XMIT_RECURSION_LIMIT	8
> > -
> > -#ifndef CONFIG_PREEMPT_RT
> > -static inline bool dev_xmit_recursion(void)
> > -{
> > -	return unlikely(__this_cpu_read(softnet_data.xmit.recursion) >
> > -			XMIT_RECURSION_LIMIT);
> > -}
> > -
> > -static inline void dev_xmit_recursion_inc(void)
> > -{
> > -	__this_cpu_inc(softnet_data.xmit.recursion);
> > -}
> > -
> > -static inline void dev_xmit_recursion_dec(void)
> > -{
> > -	__this_cpu_dec(softnet_data.xmit.recursion);
> > -}
> > -#else
> > -static inline bool dev_xmit_recursion(void)
> > -{
> > -	return unlikely(current->net_xmit.recursion > XMIT_RECURSION_LIMIT);
> > -}
> > -
> > -static inline void dev_xmit_recursion_inc(void)
> > -{
> > -	current->net_xmit.recursion++;
> > -}
> > -
> > -static inline void dev_xmit_recursion_dec(void)
> > -{
> > -	current->net_xmit.recursion--;
> > -}
> > -#endif
> >  
> >  int dev_set_hwtstamp_phylib(struct net_device *dev,
> >  			    struct kernel_hwtstamp_config *cfg,
> > diff --git a/net/ipv4/ip_tunnel.c b/net/ipv4/ip_tunnel.c
> > index 50d0f5fe4e4c..39822e845a06 100644
> > --- a/net/ipv4/ip_tunnel.c
> > +++ b/net/ipv4/ip_tunnel.c
> > @@ -683,6 +683,14 @@ void ip_tunnel_xmit(struct sk_buff *skb, struct net_device *dev,
> >  	__be32 dst;
> >  	__be16 df;
> >  
> > +	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;
> > +	}
> > +
> >  	inner_iph = (const struct iphdr *)skb_inner_network_header(skb);
> >  	connected = (tunnel->parms.iph.daddr != 0);
> >  	payload_protocol = skb_protocol(skb, true);
> > @@ -842,8 +850,10 @@ void ip_tunnel_xmit(struct sk_buff *skb, struct net_device *dev,
> >  
> >  	ip_tunnel_adj_headroom(dev, max_headroom);
> >  
> > +	dev_xmit_recursion_inc();
> >  	iptunnel_xmit(NULL, rt, skb, fl4.saddr, fl4.daddr, protocol, tos, ttl,
> >  		      df, !net_eq(tunnel->net, dev_net(dev)), 0);
> 
> I think this check should go in iptunnel_xmit(), or UDP tunnels will
> remain unprotected.
> 
> Same thing for the IPv6 case.
> 
> > +	dev_xmit_recursion_dec();
> >  	return;
> >  
> >  #if IS_ENABLED(CONFIG_IPV6)
> > diff --git a/net/ipv6/ip6_tunnel.c b/net/ipv6/ip6_tunnel.c
> > index 4c29aa94e86e..55bedd5cd656 100644
> > --- a/net/ipv6/ip6_tunnel.c
> > +++ b/net/ipv6/ip6_tunnel.c
> > @@ -1101,6 +1101,14 @@ int ip6_tnl_xmit(struct sk_buff *skb, struct net_device *dev, __u8 dsfield,
> >  	u8 hop_limit;
> >  	int err = -1;
> >  
> > +	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 -1;
> 
> I think an standard error code is expected here. Possibly -ELOOP?
> 
> /P
> 
Hi Paolo,

Thanks for the review.
I agree that moving the check into iptunnel_xmit() / ip6tunnel_xmit()
is better, as it also covers UDP tunnels. Will do in v3.

Thanks,
Weiming Shi