[PATCH AUTOSEL 4.14 070/123] netfilter: conntrack: fix cloned unconfirmed skb->_nfct race in __nf_conntrack_confirm

Sasha Levin
2019-03-27T18:49:43Z

From: Chieh-Min Wang <chiehminw@synology.com>

[ Upstream commit 13f5251fd17088170c18844534682d9cab5ff5aa ]

For bridge(br_flood) or broadcast/multicast packets, they could clone
skb with unconfirmed conntrack which break the rule that unconfirmed
skb->_nfct is never shared. With nfqueue running on my system, the race
can be easily reproduced with following warning calltrace:

[13257.707525] CPU: 0 PID: 12132 Comm: main Tainted: P W 4.4.60 #7744
[13257.707568] Hardware name: Qualcomm (Flattened Device Tree)
[13257.714700] [<c021f6dc>] (unwind_backtrace) from [<c021bce8>] (show_stack+0x10/0x14)
[13257.720253] [<c021bce8>] (show_stack) from [<c0449e10>] (dump_stack+0x94/0xa8)
[13257.728240] [<c0449e10>] (dump_stack) from [<c022a7e0>] (warn_slowpath_common+0x94/0xb0)
[13257.735268] [<c022a7e0>] (warn_slowpath_common) from [<c022a898>] (warn_slowpath_null+0x1c/0x24)
[13257.743519] [<c022a898>] (warn_slowpath_null) from [<c06ee450>] (__nf_conntrack_confirm+0xa8/0x618)
[13257.752284] [<c06ee450>] (__nf_conntrack_confirm) from [<c0772670>] (ipv4_confirm+0xb8/0xfc)
[13257.761049] [<c0772670>] (ipv4_confirm) from [<c06e7a60>] (nf_iterate+0x48/0xa8)
[13257.769725] [<c06e7a60>] (nf_iterate) from [<c06e7af0>] (nf_hook_slow+0x30/0xb0)
[13257.777108] [<c06e7af0>] (nf_hook_slow) from [<c07f20b4>] (br_nf_post_routing+0x274/0x31c)
[13257.784486] [<c07f20b4>] (br_nf_post_routing) from [<c06e7a60>] (nf_iterate+0x48/0xa8)
[13257.792556] [<c06e7a60>] (nf_iterate) from [<c06e7af0>] (nf_hook_slow+0x30/0xb0)
[13257.800458] [<c06e7af0>] (nf_hook_slow) from [<c07e5580>] (br_forward_finish+0x94/0xa4)
[13257.808010] [<c07e5580>] (br_forward_finish) from [<c07f22ac>] (br_nf_forward_finish+0x150/0x1ac)
[13257.815736] [<c07f22ac>] (br_nf_forward_finish) from [<c06e8df0>] (nf_reinject+0x108/0x170)
[13257.824762] [<c06e8df0>] (nf_reinject) from [<c06ea854>] (nfqnl_recv_verdict+0x3d8/0x420)
[13257.832924] [<c06ea854>] (nfqnl_recv_verdict) from [<c06e940c>] (nfnetlink_rcv_msg+0x158/0x248)
[13257.841256] [<c06e940c>] (nfnetlink_rcv_msg) from [<c06e5564>] (netlink_rcv_skb+0x54/0xb0)
[13257.849762] [<c06e5564>] (netlink_rcv_skb) from [<c06e4ec8>] (netlink_unicast+0x148/0x23c)
[13257.858093] [<c06e4ec8>] (netlink_unicast) from [<c06e5364>] (netlink_sendmsg+0x2ec/0x368)
[13257.866348] [<c06e5364>] (netlink_sendmsg) from [<c069fb8c>] (sock_sendmsg+0x34/0x44)
[13257.874590] [<c069fb8c>] (sock_sendmsg) from [<c06a03dc>] (___sys_sendmsg+0x1ec/0x200)
[13257.882489] [<c06a03dc>] (___sys_sendmsg) from [<c06a11c8>] (__sys_sendmsg+0x3c/0x64)
[13257.890300] [<c06a11c8>] (__sys_sendmsg) from [<c0209b40>] (ret_fast_syscall+0x0/0x34)

The original code just triggered the warning but do nothing. It will
caused the shared conntrack moves to the dying list and the packet be
droppped (nf_ct_resolve_clash returns NF_DROP for dying conntrack).

- Reproduce steps:

+----------------------------+
| br0(bridge) |
| |
+-+---------+---------+------+
| eth0| | eth1| | eth2|
| | | | | |
+--+--+ +--+--+ +---+-+
| | |
| | |
+--+-+ +-+--+ +--+-+
| PC1| | PC2| | PC3|
+----+ +----+ +----+

iptables -A FORWARD -m mark --mark 0x1000000/0x1000000 -j NFQUEUE --queue-num 100 --queue-bypass

ps: Our nfq userspace program will set mark on packets whose connection
has already been processed.

PC1 sends broadcast packets simulated by hping3:

hping3 --rand-source --udp 192.168.1.255 -i u100

- Broadcast racing flow chart is as follow:

br_handle_frame
BR_HOOK(NFPROTO_BRIDGE, NF_BR_PRE_ROUTING, br_handle_frame_finish)
// skb->_nfct (unconfirmed conntrack) is constructed at PRE_ROUTING stage
br_handle_frame_finish
// check if this packet is broadcast
br_flood_forward
br_flood
list_for_each_entry_rcu(p, &br->port_list, list) // iterate through each port
maybe_deliver
deliver_clone
skb = skb_clone(skb)
__br_forward
BR_HOOK(NFPROTO_BRIDGE, NF_BR_FORWARD,...)
// queue in our nfq and received by our userspace program
// goto __nf_conntrack_confirm with process context on CPU 1
br_pass_frame_up
BR_HOOK(NFPROTO_BRIDGE, NF_BR_LOCAL_IN,...)
// goto __nf_conntrack_confirm with softirq context on CPU 0

Because conntrack confirm can happen at both INPUT and POSTROUTING
stage. So with NFQUEUE running, skb->_nfct with the same unconfirmed
conntrack could race on different core.

This patch fixes a repeating kernel splat, now it is only displayed
once.

Signed-off-by: Chieh-Min Wang <chiehminw@synology.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
net/netfilter/nf_conntrack_core.c | 14 +++++++++++---
1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
index f07357ba9629..06520bf30f29 100644
--- a/net/netfilter/nf_conntrack_core.c
+++ b/net/netfilter/nf_conntrack_core.c
@@ -763,10 +763,18 @@ __nf_conntrack_confirm(struct sk_buff *skb)
* REJECT will give spurious warnings here.
*/

- /* No external references means no one else could have
- * confirmed us.
+ /* Another skb with the same unconfirmed conntrack may
+ * win the race. This may happen for bridge(br_flood)
+ * or broadcast/multicast packets do skb_clone with
+ * unconfirmed conntrack.
*/
- WARN_ON(nf_ct_is_confirmed(ct));
+ if (unlikely(nf_ct_is_confirmed(ct))) {
+ WARN_ON_ONCE(1);
+ nf_conntrack_double_unlock(hash, reply_hash);
+ local_bh_enable();
+ return NF_DROP;
+ }
+
pr_debug("Confirming conntrack %p\n", ct);
/* We have to check the DYING flag after unlink to prevent
* a race against nf_ct_get_next_corpse() possibly called from
--
2.19.1