rt2x00: Fix double usage of skb->cb in USB RX path.

It is not safe to use the skb->cb area for both the rxd and
skb_frame_desc data at the same time, while they occupy an overlapping
piece of memory. This can lead to hard to trace crashes as pointers
within skb_frame_desc are pointing into nowhere, or the rxd data is
overwritten with non-sense.

Fix it by copying the rxd to a small buffer on the stack.
Signed-off-by: Gertjan van Wingerde <gwingerde@kpnplanet.nl>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/rt2x00/rt2500usb.c

@@ -1156,14 +1156,10 @@ static void rt2500usb_fill_rxdone(struct queue_entry *entry,
 	u32 word1;
 
 	/*
-	 * Copy descriptor to the skb->cb array, this has 2 benefits:
-	 * 1) Each descriptor word is 4 byte aligned.
-	 * 2) Descriptor is safe  from moving of frame data in rt2x00usb.
+	 * Copy descriptor to the skbdesc->desc buffer, making it safe from moving of
+	 * frame data in rt2x00usb.
 	 */
-	skbdesc->desc_len =
-	    min_t(u16, entry->queue->desc_size, sizeof(entry->skb->cb));
-	memcpy(entry->skb->cb, rxd, skbdesc->desc_len);
-	skbdesc->desc = entry->skb->cb;
+	memcpy(skbdesc->desc, rxd, skbdesc->desc_len);
 	rxd = (__le32 *)skbdesc->desc;
 
 	/*

drivers/net/wireless/rt2x00/rt2x00usb.c

@@ -267,6 +267,7 @@ static void rt2x00usb_interrupt_rxdone(struct urb *urb)
 	struct sk_buff *skb;
 	struct skb_frame_desc *skbdesc;
 	struct rxdone_entry_desc rxdesc;
+	u8 rxd[32];
 
 	if (!test_bit(DEVICE_ENABLED_RADIO, &rt2x00dev->flags) ||
 	    !test_and_clear_bit(ENTRY_OWNER_DEVICE_DATA, &entry->flags))
@@ -286,15 +287,12 @@ static void rt2x00usb_interrupt_rxdone(struct urb *urb)
 	skbdesc = get_skb_frame_desc(entry->skb);
 	memset(skbdesc, 0, sizeof(*skbdesc));
 	skbdesc->entry = entry;
+	skbdesc->desc = rxd;
+	skbdesc->desc_len = entry->queue->desc_size;
 
 	memset(&rxdesc, 0, sizeof(rxdesc));
 	rt2x00dev->ops->lib->fill_rxdone(entry, &rxdesc);
 
-	/*
-	 * Trim the skb to the correct size.
-	 */
-	skb_trim(entry->skb, rxdesc.size);
-
 	/*
 	 * Allocate a new sk buffer to replace the current one.
 	 * If allocation fails, we should drop the current frame

drivers/net/wireless/rt2x00/rt73usb.c

@@ -1428,14 +1428,10 @@ static void rt73usb_fill_rxdone(struct queue_entry *entry,
 	u32 word1;
 
 	/*
-	 * Copy descriptor to the skb->cb array, this has 2 benefits:
-	 * 1) Each descriptor word is 4 byte aligned.
-	 * 2) Descriptor is safe  from moving of frame data in rt2x00usb.
+	 * Copy descriptor to the skbdesc->desc buffer, making it safe from moving of
+	 * frame data in rt2x00usb.
 	 */
-	skbdesc->desc_len =
-	    min_t(u16, entry->queue->desc_size, sizeof(entry->skb->cb));
-	memcpy(entry->skb->cb, rxd, skbdesc->desc_len);
-	skbdesc->desc = entry->skb->cb;
+	memcpy(skbdesc->desc, rxd, skbdesc->desc_len);
 	rxd = (__le32 *)skbdesc->desc;
 
 	/*