If verdict is NF_DROP in the target, the skb is freed.
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.
Signed-off-by: Taehee Yoo <ap420073@xxxxxxxxx>
---
net/ipv4/netfilter/ip_tables.c | 9 +++++----
1 file changed, 5 insertions(+), 4 deletions(-)
diff --git a/net/ipv4/netfilter/ip_tables.c b/net/ipv4/netfilter/ip_tables.c
index 2a55a40..622ed28 100644
--- a/net/ipv4/netfilter/ip_tables.c
+++ b/net/ipv4/netfilter/ip_tables.c
@@ -352,13 +352,14 @@ ipt_do_table(struct sk_buff *skb,
acpar.targinfo = t->data;
verdict = t->u.kernel.target->target(skb, &acpar);
- /* Target might have changed stuff. */
- ip = ip_hdr(skb);
- if (verdict == XT_CONTINUE)
+ if (verdict == XT_CONTINUE) {
+ /* Target might have changed stuff. */
+ ip = ip_hdr(skb);
e = ipt_next_entry(e);
- else
+ } else {
/* Verdict */
break;
+ }
} while (!acpar.hotdrop);
xt_write_recseq_end(addend);
--
2.9.3
--
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