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

[Eric Dumazet]

On Sat, Dec 13, 2025 at 2:40 PM Florian Westphal <fw@xxxxxxxxx> wrote:
>
> Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
> > > > 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.
> >
> > What kind of packets ? TCP ones ?
>
> UDP, but I can't say yet if thats an udp specific issue or not.
> (the packets are generated via ip_defrag.c).

skb_release_head_state() does not follow the fraglist. Oh well.

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index a00808f7be6a1b86c595183f8b131996e3d0afcc..f597769d8c206dc063b53938a18edbe9620101d9
100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -1497,7 +1497,9 @@ void napi_consume_skb(struct sk_buff *skb, int budget)

        DEBUG_NET_WARN_ON_ONCE(!in_softirq());

-       if (skb->alloc_cpu != smp_processor_id() && !skb_shared(skb)) {
+       if (skb->alloc_cpu != smp_processor_id() &&
+           !skb_shared(skb) &&
+           !skb_has_frag_list(skb)) {
                skb_release_head_state(skb);
                return skb_attempt_defer_free(skb);
        }