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Date: Sat, 13 Dec 2025 14:27:37 +0100
From: Florian Westphal <fw@strlen.de>
To: Jakub Kicinski <kuba@kernel.org>
Cc: syzbot <syzbot+4393c47753b7808dac7d@syzkaller.appspotmail.com>,
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Subject: Re: [syzbot] [netfilter?] WARNING in nf_conntrack_cleanup_net_list
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 <20251213080716.27a25928@kernel.org>
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Jakub Kicinski <kuba@kernel.org> 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.