linux-kernel - [PATCH] netfilter: fix dangling pointer access of fake

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Date:   Tue,  9 Apr 2019 06:56:12 +0000
From:   Rundong Ge <rdong.ge@...il.com>
To:     pablo@...filter.org
Cc:     kadlec@...ckhole.kfki.hu, fw@...len.de, roopa@...ulusnetworks.com,
        davem@...emloft.net, netfilter-devel@...r.kernel.org,
        coreteam@...filter.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, rdong.ge@...il.com
Subject: [PATCH] netfilter: fix dangling pointer access of fake_rtable

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@...il.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