[IEEE80211]: avoid integer underflow for runt rx frames

Reported by Chris Evans <scarybeasts@gmail.com>:

> The summary is that an evil 80211 frame can crash out a victim's
> machine. It only applies to drivers using the 80211 wireless code, and
> only then to certain drivers (and even then depends on a card's
> firmware not dropping a dubious packet). I must confess I'm not
> keeping track of Linux wireless support, and the different protocol
> stacks etc.
>
> Details are as follows:
>
> ieee80211_rx() does not explicitly check that "skb->len >= hdrlen".
> There are other skb->len checks, but not enough to prevent a subtle
> off-by-two error if the frame has the IEEE80211_STYPE_QOS_DATA flag
> set.
>
> This leads to integer underflow and crash here:
>
> if (frag != 0)
>    flen -= hdrlen;
>
> (flen is subsequently used as a memcpy length parameter).

How about this?
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ieee80211/ieee80211_rx.c

@@ -366,6 +366,12 @@ int ieee80211_rx(struct ieee80211_device *ieee, struct sk_buff *skb,
 	frag = WLAN_GET_SEQ_FRAG(sc);
 	hdrlen = ieee80211_get_hdrlen(fc);
 
+	if (skb->len < hdrlen) {
+		printk(KERN_INFO "%s: invalid SKB length %d\n",
+			dev->name, skb->len);
+		goto rx_dropped;
+	}
+
 	/* Put this code here so that we avoid duplicating it in all
 	 * Rx paths. - Jean II */
 #ifdef CONFIG_WIRELESS_EXT