mac80211: Properly handle SKB with radiotap only

The monitor interface Rx handling of SKBs that contain only
radiotap information was buggy as it tried to access the
SKB assuming it contains a frame.

To fix this, check the RX_FLAG_NO_PSDU flag in the Rx status
(indicting that the SKB contains only radiotap information),
and do not perform data path specific processing when the flag
is set.
Signed-off-by: Ilan Peer <ilan.peer@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/rx.c

@@ -753,6 +753,7 @@ ieee80211_rx_monitor(struct ieee80211_local *local, struct sk_buff *origskb,
 	struct ieee80211_sub_if_data *monitor_sdata =
 		rcu_dereference(local->monitor_sdata);
 	bool only_monitor = false;
+	unsigned int min_head_len;
 
 	if (status->flag & RX_FLAG_RADIOTAP_HE)
 		rtap_space += sizeof(struct ieee80211_radiotap_he);
@@ -766,6 +767,8 @@ ieee80211_rx_monitor(struct ieee80211_local *local, struct sk_buff *origskb,
 		rtap_space += sizeof(*rtap) + rtap->len + rtap->pad;
 	}
 
+	min_head_len = rtap_space;
+
 	/*
 	 * First, we may need to make a copy of the skb because
 	 *  (1) we need to modify it for radiotap (if not present), and
@@ -775,18 +778,23 @@ ieee80211_rx_monitor(struct ieee80211_local *local, struct sk_buff *origskb,
 	 * the SKB because it has a bad FCS/PLCP checksum.
 	 */
 
-	if (ieee80211_hw_check(&local->hw, RX_INCLUDES_FCS)) {
-		if (unlikely(origskb->len <= FCS_LEN)) {
-			/* driver bug */
-			WARN_ON(1);
-			dev_kfree_skb(origskb);
-			return NULL;
+	if (!(status->flag & RX_FLAG_NO_PSDU)) {
+		if (ieee80211_hw_check(&local->hw, RX_INCLUDES_FCS)) {
+			if (unlikely(origskb->len <= FCS_LEN + rtap_space)) {
+				/* driver bug */
+				WARN_ON(1);
+				dev_kfree_skb(origskb);
+				return NULL;
+			}
+			present_fcs_len = FCS_LEN;
 		}
-		present_fcs_len = FCS_LEN;
+
+		/* also consider the hdr->frame_control */
+		min_head_len += 2;
 	}
 
-	/* ensure hdr->frame_control and vendor radiotap data are in skb head */
-	if (!pskb_may_pull(origskb, 2 + rtap_space)) {
+	/* ensure that the expected data elements are in skb head */
+	if (!pskb_may_pull(origskb, min_head_len)) {
 		dev_kfree_skb(origskb);
 		return NULL;
 	}