netfilter: ctnetlink: suspicious RCU usage in ctnetlink_dump_helpinfo

Two patches listed below removed ctnetlink_dump_helpinfo call from under
rcu_read_lock. Now its rcu_dereference generates following warning:
=============================
WARNING: suspicious RCU usage
5.13.0+ #5 Not tainted
-----------------------------
net/netfilter/nf_conntrack_netlink.c:221 suspicious rcu_dereference_check() usage!

other info that might help us debug this:
rcu_scheduler_active = 2, debug_locks = 1
stack backtrace:
CPU: 1 PID: 2251 Comm: conntrack Not tainted 5.13.0+ #5
Call Trace:
 dump_stack+0x7f/0xa1
 ctnetlink_dump_helpinfo+0x134/0x150 [nf_conntrack_netlink]
 ctnetlink_fill_info+0x2c2/0x390 [nf_conntrack_netlink]
 ctnetlink_dump_table+0x13f/0x370 [nf_conntrack_netlink]
 netlink_dump+0x10c/0x370
 __netlink_dump_start+0x1a7/0x260
 ctnetlink_get_conntrack+0x1e5/0x250 [nf_conntrack_netlink]
 nfnetlink_rcv_msg+0x613/0x993 [nfnetlink]
 netlink_rcv_skb+0x50/0x100
 nfnetlink_rcv+0x55/0x120 [nfnetlink]
 netlink_unicast+0x181/0x260
 netlink_sendmsg+0x23f/0x460
 sock_sendmsg+0x5b/0x60
 __sys_sendto+0xf1/0x160
 __x64_sys_sendto+0x24/0x30
 do_syscall_64+0x36/0x70
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Fixes: 49ca022b ("netfilter: ctnetlink: don't dump ct extensions of unconfirmed conntracks")
Fixes: 0b35f603 ("netfilter: Remove duplicated rcu_read_lock.")
Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
Reviewed-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_conntrack_netlink.c

@@ -218,6 +218,7 @@ static int ctnetlink_dump_helpinfo(struct sk_buff *skb,
 	if (!help)
 		return 0;
 
+	rcu_read_lock();
 	helper = rcu_dereference(help->helper);
 	if (!helper)
 		goto out;
@@ -233,9 +234,11 @@ static int ctnetlink_dump_helpinfo(struct sk_buff *skb,
 
 	nla_nest_end(skb, nest_helper);
 out:
+	rcu_read_unlock();
 	return 0;
 
 nla_put_failure:
+	rcu_read_unlock();
 	return -1;
 }