p54usb: fix nasty use after free

In theory, the firmware acks the received a data frame, before signaling the driver to free it again.
However Artur Skawina <art.08.09@gmail.com> has shown that it can happen in reverse order as well.
This is very bad and could lead to memory corruptions, oopses and panics.

Thanks to Artur Skawina <art.08.09@gmail.com> for reporting and debugging this issue.
Signed-off-by: Christian Lamparter <chunkeey@web.de>
Tested-by: Artur Skawina <art.08.09@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/p54/p54usb.c

@@ -144,10 +144,7 @@ static void p54u_tx_cb(struct urb *urb)
 	struct sk_buff *skb = urb->context;
 	struct ieee80211_hw *dev = (struct ieee80211_hw *)
 		usb_get_intfdata(usb_ifnum_to_if(urb->dev, 0));
-	struct p54u_priv *priv = dev->priv;
 
-	skb_pull(skb, priv->common.tx_hdr_len);
-	if (FREE_AFTER_TX(skb))
 	p54_free_skb(dev, skb);
 }
 
@@ -230,7 +227,8 @@ static void p54u_tx_3887(struct ieee80211_hw *dev, struct sk_buff *skb)
 			  p54u_tx_dummy_cb, dev);
 	usb_fill_bulk_urb(data_urb, priv->udev,
 			  usb_sndbulkpipe(priv->udev, P54U_PIPE_DATA),
-			  skb->data, skb->len, p54u_tx_cb, skb);
+			  skb->data, skb->len, FREE_AFTER_TX(skb) ?
+			  p54u_tx_cb : p54u_tx_dummy_cb, skb);
 
 	usb_anchor_urb(addr_urb, &priv->submitted);
 	err = usb_submit_urb(addr_urb, GFP_ATOMIC);
@@ -269,28 +267,24 @@ static void p54u_tx_lm87(struct ieee80211_hw *dev, struct sk_buff *skb)
 {
 	struct p54u_priv *priv = dev->priv;
 	struct urb *data_urb;
-	struct lm87_tx_hdr *hdr;
-	__le32 checksum;
-	__le32 addr = ((struct p54_hdr *)skb->data)->req_id;
+	struct lm87_tx_hdr *hdr = (void *)skb->data - sizeof(*hdr);
 
 	data_urb = usb_alloc_urb(0, GFP_ATOMIC);
 	if (!data_urb)
 		return;
 
-	checksum = p54u_lm87_chksum((__le32 *)skb->data, skb->len);
-	hdr = (struct lm87_tx_hdr *)skb_push(skb, sizeof(*hdr));
-	hdr->chksum = checksum;
-	hdr->device_addr = addr;
+	hdr->chksum = p54u_lm87_chksum((__le32 *)skb->data, skb->len);
+	hdr->device_addr = ((struct p54_hdr *)skb->data)->req_id;
 
 	usb_fill_bulk_urb(data_urb, priv->udev,
 			  usb_sndbulkpipe(priv->udev, P54U_PIPE_DATA),
-			  skb->data, skb->len, p54u_tx_cb, skb);
+			  hdr, skb->len + sizeof(*hdr),  FREE_AFTER_TX(skb) ?
+			  p54u_tx_cb : p54u_tx_dummy_cb, skb);
 	data_urb->transfer_flags |= URB_ZERO_PACKET;
 
 	usb_anchor_urb(data_urb, &priv->submitted);
 	if (usb_submit_urb(data_urb, GFP_ATOMIC)) {
 		usb_unanchor_urb(data_urb);
-		skb_pull(skb, sizeof(*hdr));
 		p54_free_skb(dev, skb);
 	}
 	usb_free_urb(data_urb);
@@ -300,11 +294,9 @@ static void p54u_tx_net2280(struct ieee80211_hw *dev, struct sk_buff *skb)
 {
 	struct p54u_priv *priv = dev->priv;
 	struct urb *int_urb, *data_urb;
-	struct net2280_tx_hdr *hdr;
+	struct net2280_tx_hdr *hdr = (void *)skb->data - sizeof(*hdr);
 	struct net2280_reg_write *reg;
 	int err = 0;
-	__le32 addr = ((struct p54_hdr *) skb->data)->req_id;
-	__le16 len = cpu_to_le16(skb->len);
 
 	reg = kmalloc(sizeof(*reg), GFP_ATOMIC);
 	if (!reg)
@@ -327,10 +319,9 @@ static void p54u_tx_net2280(struct ieee80211_hw *dev, struct sk_buff *skb)
 	reg->addr = cpu_to_le32(P54U_DEV_BASE);
 	reg->val = cpu_to_le32(ISL38XX_DEV_INT_DATA);
 
-	hdr = (void *)skb_push(skb, sizeof(*hdr));
 	memset(hdr, 0, sizeof(*hdr));
-	hdr->len = len;
-	hdr->device_addr = addr;
+	hdr->len = cpu_to_le16(skb->len);
+	hdr->device_addr = ((struct p54_hdr *) skb->data)->req_id;
 
 	usb_fill_bulk_urb(int_urb, priv->udev,
 		usb_sndbulkpipe(priv->udev, P54U_PIPE_DEV), reg, sizeof(*reg),
@@ -345,7 +336,8 @@ static void p54u_tx_net2280(struct ieee80211_hw *dev, struct sk_buff *skb)
 
 	usb_fill_bulk_urb(data_urb, priv->udev,
 			  usb_sndbulkpipe(priv->udev, P54U_PIPE_DATA),
-			  skb->data, skb->len, p54u_tx_cb, skb);
+			  hdr, skb->len + sizeof(*hdr), FREE_AFTER_TX(skb) ?
+			  p54u_tx_cb : p54u_tx_dummy_cb, skb);
 
 	usb_anchor_urb(int_urb, &priv->submitted);
 	err = usb_submit_urb(int_urb, GFP_ATOMIC);