bonding: use local function pointer of bond->recv_probe in bond_handle_frame

The bond->recv_probe is called in bond_handle_frame() when
a packet is received, but bond_close() sets it to NULL. So,
a panic occurs when both functions work in parallel.

Why this happen:
After null pointer check of bond->recv_probe, an sk_buff is
duplicated and bond->recv_probe is called in bond_handle_frame.
So, a panic occurs when bond_close() is called between the
check and call of bond->recv_probe.

Patch:
This patch uses a local function pointer of bond->recv_probe
in bond_handle_frame(). So, it can avoid the null pointer
dereference.
Signed-off-by: Mitsuo Hayasaka <mitsuo.hayasaka.hu@hitachi.com>
Cc: Jay Vosburgh <fubar@us.ibm.com>
Cc: Andy Gospodarek <andy@greyhouse.net>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: WANG Cong <xiyou.wangcong@gmail.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/bonding/bond_main.c

@@ -1435,6 +1435,8 @@ static rx_handler_result_t bond_handle_frame(struct sk_buff **pskb)
 	struct sk_buff *skb = *pskb;
 	struct slave *slave;
 	struct bonding *bond;
+	void (*recv_probe)(struct sk_buff *, struct bonding *,
+				struct slave *);
 
 	skb = skb_share_check(skb, GFP_ATOMIC);
 	if (unlikely(!skb))
@@ -1448,11 +1450,12 @@ static rx_handler_result_t bond_handle_frame(struct sk_buff **pskb)
 	if (bond->params.arp_interval)
 		slave->dev->last_rx = jiffies;
 
-	if (bond->recv_probe) {
+	recv_probe = ACCESS_ONCE(bond->recv_probe);
+	if (recv_probe) {
 		struct sk_buff *nskb = skb_clone(skb, GFP_ATOMIC);
 
 		if (likely(nskb)) {
-			bond->recv_probe(nskb, bond, slave);
+			recv_probe(nskb, bond, slave);
 			dev_kfree_skb(nskb);
 		}
 	}