mac80211: fix blockack-req processing

Daniel reported that the paged RX changes had
broken blockack request frame processing due
to using data that wasn't really part of the
skb data.

Fix this using skb_copy_bits() for the needed
data. As a side effect, this adds a check on
processing too short frames, which previously
this code could do.
Reported-by: NDaniel Halperin <dhalperi@cs.washington.edu>
Signed-off-by: NJohannes Berg <johannes@sipsolutions.net>
Acked-by: NDaniel Halperin <dhalperi@cs.washington.edu>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

net/mac80211/rx.c

@@ -1818,17 +1818,26 @@ ieee80211_rx_h_ctrl(struct ieee80211_rx_data *rx, struct sk_buff_head *frames)
 		return RX_CONTINUE;
 
 	if (ieee80211_is_back_req(bar->frame_control)) {
+		struct {
+			__le16 control, start_seq_num;
+		} __packed bar_data;
+
 		if (!rx->sta)
 			return RX_DROP_MONITOR;
+
+		if (skb_copy_bits(skb, offsetof(struct ieee80211_bar, control),
+				  &bar_data, sizeof(bar_data)))
+			return RX_DROP_MONITOR;
+
 		spin_lock(&rx->sta->lock);
-		tid = le16_to_cpu(bar->control) >> 12;
+		tid = le16_to_cpu(bar_data.control) >> 12;
 		if (!rx->sta->ampdu_mlme.tid_active_rx[tid]) {
 			spin_unlock(&rx->sta->lock);
 			return RX_DROP_MONITOR;
 		}
 		tid_agg_rx = rx->sta->ampdu_mlme.tid_rx[tid];
 
-		start_seq_num = le16_to_cpu(bar->start_seq_num) >> 4;
+		start_seq_num = le16_to_cpu(bar_data.start_seq_num) >> 4;
 
 		/* reset session timer */
 		if (tid_agg_rx->timeout)