Re: [PATCH] netfilter: conntrack: drop expectations before freeing templates

Florian Westphal
2025-08-20T00:24:13Z

Florian Westphal <fw@strlen.de> wrote:
> Qingjie Xing <xqjcool@gmail.com> wrote:
> > With an iptables-configured TFTP helper in place, a UDP packet
> > (10.65.41.36:1069 → 10.65.36.2:69, TFTP RRQ) triggered creation of an expectation.
> > Later, iptables changes removed the rule’s per-rule template nf_conn.
> > When the expectation’s timer expired, nf_ct_unlink_expect_report()
> > ran and dereferenced the freed master, causing a crash.
>
> Sorry, I do not see the problem.
> A template should never be listed as exp->master.
>
> Can you make a reproducer/selftest for this bug?
>
> I worry we paper over a different bug.

Or maybe this will provide a clue (not even compile tested).

@@ -299,6 +302,9 @@ struct nf_conntrack_expect *nf_ct_expect_alloc(struct nf_conn *me)
{
struct nf_conntrack_expect *new;

+ if (WARN_ON_ONCE(nf_ct_is_template(me)))
+ return NULL;
+
new = kmem_cache_alloc(nf_ct_expect_cachep, GFP_ATOMIC);
if (!new)