If verdict is NF_STOLEN in the SYNPROXY target,
the skb is consumed.
However, ipt_do_table() always tries to get ip header from the skb.
So that, KASAN triggers the use-after-free message.

We can reproduce this message using below command.
  # iptables -I INPUT -p tcp -j SYNPROXY --mss 1460

[ 193.542265] BUG: KASAN: use-after-free in ipt_do_table+0x1405/0x1c10
[ ... ]
[ 193.578603] Call Trace:
[ 193.581590] <IRQ>
[ 193.584107] dump_stack+0x68/0xa0
[ 193.588168] print_address_description+0x78/0x290
[ 193.593828] ? ipt_do_table+0x1405/0x1c10
[ 193.598690] kasan_report+0x230/0x340
[ 193.603194] __asan_report_load2_noabort+0x19/0x20
[ 193.608950] ipt_do_table+0x1405/0x1c10
[ 193.613591] ? rcu_read_lock_held+0xae/0xd0
[ 193.618631] ? ip_route_input_rcu+0x27d7/0x4270
[ 193.624348] ? ipt_do_table+0xb68/0x1c10
[ 193.629124] ? do_add_counters+0x620/0x620
[ 193.634234] ? iptable_filter_net_init+0x60/0x60
[ ... ]

After this patch, only when verdict is XT_CONTINUE,
ipt_do_table() tries to get ip header.
Also arpt_do_table() is modified because it has same bug.
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>