netfilter: nf_conntrack: destroy stale expectfn expectations on unregister

NAT helpers such as nf_nat_h323 store a raw pointer to module text in
exp->expectfn (e.g. ip_nat_q931_expect). nf_ct_helper_expectfn_unregister()
only unlinks the callback descriptor and never walks the expectation table,
so an expectation pending at module removal survives with a dangling
exp->expectfn into freed module text.

When the expected connection arrives, init_conntrack() invokes
exp->expectfn(), now a stale pointer into the unloaded module. Reproduced
on a KASAN build by loading the H.323 helpers, creating a Q.931
expectation, unloading nf_nat_h323, then connecting to the expected port:

 Oops: int3: 0000 [#1] SMP KASAN NOPTI
 RIP: 0010:0xffffffffa06102d1
  init_conntrack.isra.0 (net/netfilter/nf_conntrack_core.c:1862)
  nf_conntrack_in (net/netfilter/nf_conntrack_core.c:2049)
  ipv4_conntrack_local (net/netfilter/nf_conntrack_proto.c:223)
  nf_hook_slow (net/netfilter/core.c:619)
  __ip_local_out (net/ipv4/ip_output.c:120)
  __tcp_transmit_skb (net/ipv4/tcp_output.c:1715)
  tcp_connect (net/ipv4/tcp_output.c:4374)
  tcp_v4_connect (net/ipv4/tcp_ipv4.c:345)
  __sys_connect (net/socket.c:2167)
 Modules linked in: nf_conntrack_h323 [last unloaded: nf_nat_h323]

Reaching the dangling state requires CAP_SYS_MODULE in the initial user
namespace to remove a NAT helper that still has live expectations, so this
is a robustness fix; leaving an expectation pointing at freed text is wrong
regardless.

Add nf_ct_helper_expectfn_destroy(), which walks the expectation table and
drops every expectation whose ->expectfn matches the descriptor being torn
down. Call it from each NAT helper's exit path after the existing RCU grace
period, so no expectation outlives the code it points at and no extra
synchronize_rcu() is introduced. With the fix, the same reproducer runs to
completion without the Oops.

Fixes: f587de0e2f ("[NETFILTER]: nf_conntrack/nf_nat: add H.323 helper port")
Reported-by: Xiang Mei <xmei5@asu.edu>
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/net/netfilter/nf_conntrack_helper.h

@@ -155,6 +155,7 @@ void nf_ct_helper_log(struct sk_buff *skb, const struct nf_conn *ct,
 
 void nf_ct_helper_expectfn_register(struct nf_ct_helper_expectfn *n);
 void nf_ct_helper_expectfn_unregister(struct nf_ct_helper_expectfn *n);
+void nf_ct_helper_expectfn_destroy(const struct nf_ct_helper_expectfn *n);
 struct nf_ct_helper_expectfn *
 nf_ct_helper_expectfn_find_by_name(const char *name);
 struct nf_ct_helper_expectfn *

net/ipv4/netfilter/nf_nat_h323.c

@@ -555,6 +555,8 @@ static void __exit nf_nat_h323_fini(void)
 	nf_ct_helper_expectfn_unregister(&q931_nat);
 	nf_ct_helper_expectfn_unregister(&callforwarding_nat);
 	synchronize_rcu();
+	nf_ct_helper_expectfn_destroy(&q931_nat);
+	nf_ct_helper_expectfn_destroy(&callforwarding_nat);
 }
 
 /****************************************************************************/

net/netfilter/nf_conntrack_helper.c

@@ -283,6 +283,25 @@ void nf_ct_helper_expectfn_unregister(struct nf_ct_helper_expectfn *n)
 }
 EXPORT_SYMBOL_GPL(nf_ct_helper_expectfn_unregister);
 
+static bool expect_iter_expectfn(struct nf_conntrack_expect *exp, void *data)
+{
+	const struct nf_ct_helper_expectfn *n = data;
+
+	/* Relies on registered expectfn descriptors having unique ->expectfn
+	 * pointers, which holds for the in-tree NAT helpers.
+	 */
+	return exp->expectfn == n->expectfn;
+}
+
+/* Destroy expectations still pointing at @n->expectfn; call after the
+ * caller's RCU grace period so none outlives the (often modular) callback.
+ */
+void nf_ct_helper_expectfn_destroy(const struct nf_ct_helper_expectfn *n)
+{
+	nf_ct_expect_iterate_destroy(expect_iter_expectfn, (void *)n);
+}
+EXPORT_SYMBOL_GPL(nf_ct_helper_expectfn_destroy);
+
 /* Caller should hold the rcu lock */
 struct nf_ct_helper_expectfn *
 nf_ct_helper_expectfn_find_by_name(const char *name)

net/netfilter/nf_nat_core.c

@@ -1341,6 +1341,7 @@ static int __init nf_nat_init(void)
 		RCU_INIT_POINTER(nf_nat_hook, NULL);
 		nf_ct_helper_expectfn_unregister(&follow_master_nat);
 		synchronize_net();
+		nf_ct_helper_expectfn_destroy(&follow_master_nat);
 		unregister_pernet_subsys(&nat_net_ops);
 		kvfree(nf_nat_bysource);
 	}
@@ -1358,6 +1359,7 @@ static void __exit nf_nat_cleanup(void)
 	RCU_INIT_POINTER(nf_nat_hook, NULL);
 
 	synchronize_net();
+	nf_ct_helper_expectfn_destroy(&follow_master_nat);
 	kvfree(nf_nat_bysource);
 	unregister_pernet_subsys(&nat_net_ops);
 }

net/netfilter/nf_nat_sip.c

@@ -655,6 +655,7 @@ static void __exit nf_nat_sip_fini(void)
 	RCU_INIT_POINTER(nf_nat_sip_hooks, NULL);
 	nf_ct_helper_expectfn_unregister(&sip_nat);
 	synchronize_rcu();
+	nf_ct_helper_expectfn_destroy(&sip_nat);
 }
 
 static const struct nf_nat_sip_hooks sip_hooks = {