netfilter: conntrack: refetch conntrack after nf_conntrack_update()

__nf_conntrack_update() might refresh the conntrack object that is
attached to the skbuff. Otherwise, this triggers UAF.

[  633.200434] ==================================================================
[  633.200472] BUG: KASAN: use-after-free in nf_conntrack_update+0x34e/0x770 [nf_conntrack]
[  633.200478] Read of size 1 at addr ffff888370804c00 by task nfqnl_test/6769

[  633.200487] CPU: 1 PID: 6769 Comm: nfqnl_test Not tainted 5.8.0-rc2+ #388
[  633.200490] Hardware name: LENOVO 23259H1/23259H1, BIOS G2ET32WW (1.12 ) 05/30/2012
[  633.200491] Call Trace:
[  633.200499]  dump_stack+0x7c/0xb0
[  633.200526]  ? nf_conntrack_update+0x34e/0x770 [nf_conntrack]
[  633.200532]  print_address_description.constprop.6+0x1a/0x200
[  633.200539]  ? _raw_write_lock_irqsave+0xc0/0xc0
[  633.200568]  ? nf_conntrack_update+0x34e/0x770 [nf_conntrack]
[  633.200594]  ? nf_conntrack_update+0x34e/0x770 [nf_conntrack]
[  633.200598]  kasan_report.cold.9+0x1f/0x42
[  633.200604]  ? call_rcu+0x2c0/0x390
[  633.200633]  ? nf_conntrack_update+0x34e/0x770 [nf_conntrack]
[  633.200659]  nf_conntrack_update+0x34e/0x770 [nf_conntrack]
[  633.200687]  ? nf_conntrack_find_get+0x30/0x30 [nf_conntrack]

Closes: https://bugzilla.netfilter.org/show_bug.cgi?id=1436
Fixes: ee04805f ("netfilter: conntrack: make conntrack userspace helpers work again")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_conntrack_core.c

@@ -2158,6 +2158,8 @@ static int nf_conntrack_update(struct net *net, struct sk_buff *skb)
 		err = __nf_conntrack_update(net, skb, ct, ctinfo);
 		if (err < 0)
 			return err;
+
+		ct = nf_ct_get(skb, &ctinfo);
 	}
 
 	return nf_confirm_cthelper(skb, ct, ctinfo);