mac80211: initialize fast-xmit 'info' later

In ieee80211_xmit_fast(), 'info' is initialized to point to the skb
that's passed in, but that skb may later be replaced by a clone (if
it was shared), leading to an invalid pointer.

This can lead to use-after-free and also later crashes since the
real SKB's info->hw_queue doesn't get initialized properly.

Fix this by assigning info only later, when it's needed, after the
skb replacement (may have) happened.

Cc: stable@vger.kernel.org
Reported-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/tx.c

@@ -3287,7 +3287,7 @@ static bool ieee80211_xmit_fast(struct ieee80211_sub_if_data *sdata,
 	int extra_head = fast_tx->hdr_len - (ETH_HLEN - 2);
 	int hw_headroom = sdata->local->hw.extra_tx_headroom;
 	struct ethhdr eth;
-	struct ieee80211_tx_info *info = IEEE80211_SKB_CB(skb);
+	struct ieee80211_tx_info *info;
 	struct ieee80211_hdr *hdr = (void *)fast_tx->hdr;
 	struct ieee80211_tx_data tx;
 	ieee80211_tx_result r;
@@ -3351,6 +3351,7 @@ static bool ieee80211_xmit_fast(struct ieee80211_sub_if_data *sdata,
 	memcpy(skb->data + fast_tx->da_offs, eth.h_dest, ETH_ALEN);
 	memcpy(skb->data + fast_tx->sa_offs, eth.h_source, ETH_ALEN);
 
+	info = IEEE80211_SKB_CB(skb);
 	memset(info, 0, sizeof(*info));
 	info->band = fast_tx->band;
 	info->control.vif = &sdata->vif;