net: hdlc_x25: Prevent racing between "x25_close" and "x25_xmit"/"x25_rx"

"x25_close" is called by "hdlc_close" in "hdlc.c", which is called by
hardware drivers' "ndo_stop" function.
"x25_xmit" is called by "hdlc_start_xmit" in "hdlc.c", which is hardware
drivers' "ndo_start_xmit" function.
"x25_rx" is called by "hdlc_rcv" in "hdlc.c", which receives HDLC frames
from "net/core/dev.c".

"x25_close" races with "x25_xmit" and "x25_rx" because their callers race.

However, we need to ensure that the LAPB APIs called in "x25_xmit" and
"x25_rx" are called before "lapb_unregister" is called in "x25_close".

This patch adds locking to ensure when "x25_xmit" and "x25_rx" are doing
their work, "lapb_unregister" is not yet called in "x25_close".

Reasons for not solving the racing between "x25_close" and "x25_xmit" by
calling "netif_tx_disable" in "x25_close":
1. We still need to solve the racing between "x25_close" and "x25_rx";
2. The design of the HDLC subsystem assumes the HDLC hardware drivers
have full control over the TX queue, and the HDLC protocol drivers (like
this driver) have no control. Controlling the queue here in the protocol
driver may interfere with hardware drivers' control of the queue.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Xie He <xie.he.0141@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/wan/hdlc_x25.c

@@ -23,6 +23,8 @@
 
 struct x25_state {
 	x25_hdlc_proto settings;
+	bool up;
+	spinlock_t up_lock; /* Protects "up" */
 };
 
 static int x25_ioctl(struct net_device *dev, struct ifreq *ifr);
@@ -104,6 +106,8 @@ static void x25_data_transmit(struct net_device *dev, struct sk_buff *skb)
 
 static netdev_tx_t x25_xmit(struct sk_buff *skb, struct net_device *dev)
 {
+	hdlc_device *hdlc = dev_to_hdlc(dev);
+	struct x25_state *x25st = state(hdlc);
 	int result;
 
 	/* There should be a pseudo header of 1 byte added by upper layers.
@@ -114,11 +118,19 @@ static netdev_tx_t x25_xmit(struct sk_buff *skb, struct net_device *dev)
 		return NETDEV_TX_OK;
 	}
 
+	spin_lock_bh(&x25st->up_lock);
+	if (!x25st->up) {
+		spin_unlock_bh(&x25st->up_lock);
+		kfree_skb(skb);
+		return NETDEV_TX_OK;
+	}
+
 	switch (skb->data[0]) {
 	case X25_IFACE_DATA:	/* Data to be transmitted */
 		skb_pull(skb, 1);
 		if ((result = lapb_data_request(dev, skb)) != LAPB_OK)
 			dev_kfree_skb(skb);
+		spin_unlock_bh(&x25st->up_lock);
 		return NETDEV_TX_OK;
 
 	case X25_IFACE_CONNECT:
@@ -147,6 +159,7 @@ static netdev_tx_t x25_xmit(struct sk_buff *skb, struct net_device *dev)
 		break;
 	}
 
+	spin_unlock_bh(&x25st->up_lock);
 	dev_kfree_skb(skb);
 	return NETDEV_TX_OK;
 }
@@ -164,6 +177,7 @@ static int x25_open(struct net_device *dev)
 		.data_transmit = x25_data_transmit,
 	};
 	hdlc_device *hdlc = dev_to_hdlc(dev);
+	struct x25_state *x25st = state(hdlc);
 	struct lapb_parms_struct params;
 	int result;
 
@@ -190,6 +204,10 @@ static int x25_open(struct net_device *dev)
 	if (result != LAPB_OK)
 		return -EINVAL;
 
+	spin_lock_bh(&x25st->up_lock);
+	x25st->up = true;
+	spin_unlock_bh(&x25st->up_lock);
+
 	return 0;
 }
 
@@ -197,6 +215,13 @@ static int x25_open(struct net_device *dev)
 
 static void x25_close(struct net_device *dev)
 {
+	hdlc_device *hdlc = dev_to_hdlc(dev);
+	struct x25_state *x25st = state(hdlc);
+
+	spin_lock_bh(&x25st->up_lock);
+	x25st->up = false;
+	spin_unlock_bh(&x25st->up_lock);
+
 	lapb_unregister(dev);
 }
 
@@ -205,15 +230,28 @@ static void x25_close(struct net_device *dev)
 static int x25_rx(struct sk_buff *skb)
 {
 	struct net_device *dev = skb->dev;
+	hdlc_device *hdlc = dev_to_hdlc(dev);
+	struct x25_state *x25st = state(hdlc);
 
 	if ((skb = skb_share_check(skb, GFP_ATOMIC)) == NULL) {
 		dev->stats.rx_dropped++;
 		return NET_RX_DROP;
 	}
 
-	if (lapb_data_received(dev, skb) == LAPB_OK)
+	spin_lock_bh(&x25st->up_lock);
+	if (!x25st->up) {
+		spin_unlock_bh(&x25st->up_lock);
+		kfree_skb(skb);
+		dev->stats.rx_dropped++;
+		return NET_RX_DROP;
+	}
+
+	if (lapb_data_received(dev, skb) == LAPB_OK) {
+		spin_unlock_bh(&x25st->up_lock);
 		return NET_RX_SUCCESS;
+	}
 
+	spin_unlock_bh(&x25st->up_lock);
 	dev->stats.rx_errors++;
 	dev_kfree_skb_any(skb);
 	return NET_RX_DROP;
@@ -298,6 +336,8 @@ static int x25_ioctl(struct net_device *dev, struct ifreq *ifr)
 			return result;
 
 		memcpy(&state(hdlc)->settings, &new_settings, size);
+		state(hdlc)->up = false;
+		spin_lock_init(&state(hdlc)->up_lock);
 
 		/* There's no header_ops so hard_header_len should be 0. */
 		dev->hard_header_len = 0;