Subject: Re: [syzbot] [netfilter?] WARNING in nf_conntrack_cleanup_net_list

From: Florian Westphal <[email-masked]>
Date: Sat, 13 Dec 2025 14:27:37 +0100

Jakub Kicinski <[email-masked]> wrote:
> On Thu, 11 Dec 2025 10:38:31 -0800 syzbot wrote:
> > ------------[ cut here ]------------
> > conntrack cleanup blocked for 60s
> > WARNING: net/netfilter/nf_conntrack_core.c:2512 at
> 
> Yes, I was about to comment on the patch which added the warning..
> 
> There is still a leak somewhere. Running ip_defrag.sh and then load /
> unload ipvlan repros this (modprobe ipvlan is a quick check if the
> cleanup thread is wedged, if it is modprobe will hang, if it isn't
> run ip_defrag.sh, again etc).
> 
> I looked around last night but couldn't find an skb stuck anywhere.
> The nf_conntrack_net->count was == 1

Its caused skb skb fraglist skbs that still hold nf_conn references
on the softnet data defer lists.

setting net.core.skb_defer_max=0 makes the hang disappear for me.