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Subject[PATCH] netfilter: fix dangling pointer access of fake_rtable
Date
With  bridge-nf-call-iptables enabeled, Skbs go through the bridge
and enqueued between <NF_BR_PRE_ROUTING,NF_BR_PRI_BRNF> and
<NF_BR_FORWARD,NF_BR_PRI_BRNF - 1> won't be flushed when bridge is
down. Then _skb_refdst of skbs in the nfqueue become dangling pointer.

Reproduce steps:
1.Create a bridge on the box.
2.echo 1 >/proc/sys/net/bridge/bridge-nf-call-iptables
3.Add a netfilter hook function to queue the packets to nfqueuenum 0.
The hook point must between <NF_BR_PRE_ROUTING,NF_BR_PRI_BRNF> and
<NF_BR_FORWARD,NF_BR_PRI_BRNF - 1>.
4.Add a userspace process "nfqueue_rcv" to continuously read and
set_verdict "NF_ACCEPT" to packets from queue 0.
5.Continuosly ping client1 from client0
6.Send "Ctrl + Z" to pause the "nfqueue_rcv" to simulate the queue
congestion.
7.Using "ifconfig br0 down&&brctl delbr br0" to delete the bridge.
8.At this time the _skb_refdst of skbs in the nfqueue become dangling
pointer. If we send "fg" to resume the "nfqueue_rcv", the kernel
may try to access the freed memory.

Debug log:
Here I add debug logs in "netdev_freemem" and "dst_release" to prove
the freed memory access. As the log shows, the "dst_release" accessed
bridge's fake_rtable after the bridge was freed.

Apr 8 22:25:14 raydon kernel: [62139.005062] netdev_freemem name:br0,
fake_rtable:000000009d76cef0

Apr 8 22:25:21 raydon kernel: [62145.967133] dst_release
dst:000000009d76cef0 dst->dev->name: řKU¡TH

Apr 8 22:25:21 raydon kernel: [62145.967154] dst_release
dst:000000009d76cef0 dst->dev->name: řKU¡TH

Apr 8 22:25:21 raydon kernel: [62145.967180] dst_release
dst:000000009d76cef0 dst->dev->name: řKU¡TH

Apr 8 22:25:21 raydon kernel: [62145.967197] dst_release
dst:000000009d76cef0 dst->dev->name: řKU¡TH

The reason why the hook point should be after <NF_BR_PRE_ROUTING,
NF_BR_PRI_BRNF> is skbs reference bridge's fake_rtable in
"br_nf_pre_routing_finish" hooked at <NF_BR_PRE_ROUTING,NF_BR_PRI_BRNF>.

And the reason why the hook point should be before <NF_BR_FORWARD,
NF_BR_PRI_BRNF - 1> is "br_nf_forward_ip" will set the state.out to
bridge dev. After this hook point, the "nfqnl_dev_drop" triggered by
the bridge's NETDEV_DOWN event can flush the queued skbs before
bridge's memory is freed, because the state.out now matches the
bridge's dev.

Signed-off-by: Rundong Ge <rdong.ge@gmail.com>
---
net/netfilter/nfnetlink_queue.c | 24 ++++++++++++++++++------
1 file changed, 18 insertions(+), 6 deletions(-)

diff --git a/net/netfilter/nfnetlink_queue.c b/net/netfilter/nfnetlink_queue.c
index 0dcc359..57eb02d 100644
--- a/net/netfilter/nfnetlink_queue.c
+++ b/net/netfilter/nfnetlink_queue.c
@@ -905,13 +905,25 @@ static void free_entry(struct nf_queue_entry *entry)
dev_cmp(struct nf_queue_entry *entry, unsigned long ifindex)
{
#if IS_ENABLED(CONFIG_BRIDGE_NETFILTER)
- int physinif, physoutif;
+ struct net_device *physindev, *physoutdev;
+ struct net_bridge_port *port;

- physinif = nf_bridge_get_physinif(entry->skb);
- physoutif = nf_bridge_get_physoutif(entry->skb);
-
- if (physinif == ifindex || physoutif == ifindex)
- return 1;
+ physindev = nf_bridge_get_physindev(entry->skb);
+ physoutdev = nf_bridge_get_physoutdev(entry->skb);
+ if (physindev) {
+ if (physindev->ifindex == ifindex)
+ return 1;
+ port = br_port_get_rcu(physindev);
+ if (port && port->br->dev->ifindex == ifindex)
+ return 1;
+ }
+ if (physoutdev) {
+ if (physoutdev->ifindex == ifindex)
+ return 1;
+ port = br_port_get_rcu(physoutdev);
+ if (port && port->br->dev->ifindex == ifindex)
+ return 1;
+ }
#endif
if (entry->state.in)
if (entry->state.in->ifindex == ifindex)
--
1.8.3.1
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