wifi: mac80211: check skb_shared in ieee80211_8023_xmit()

Add a missing skb_shared check into 802.3 path to prevent potential
use-after-free from happening. This also uses skb_share_check()
instead of open-coding in tx path.
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Link: https://lore.kernel.org/r/e7a73aaf7742b17e43421c56625646dfc5c4d2cb.1653571902.git.ryder.lee@mediatek.comSigned-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/tx.c

@@ -2818,19 +2818,10 @@ static struct sk_buff *ieee80211_build_hdr(struct ieee80211_sub_if_data *sdata,
 	/*
 	 * If the skb is shared we need to obtain our own copy.
 	 */
-	if (skb_shared(skb)) {
-		struct sk_buff *tmp_skb = skb;
-
-		/* can't happen -- skb is a clone if info_id != 0 */
-		WARN_ON(info_id);
-
-		skb = skb_clone(skb, GFP_ATOMIC);
-		kfree_skb(tmp_skb);
-
-		if (!skb) {
-			ret = -ENOMEM;
-			goto free;
-		}
+	skb = skb_share_check(skb, GFP_ATOMIC);
+	if (unlikely(!skb)) {
+		ret = -ENOMEM;
+		goto free;
 	}
 
 	hdr.frame_control = fc;
@@ -3539,15 +3530,9 @@ static bool ieee80211_xmit_fast(struct ieee80211_sub_if_data *sdata,
 
 	/* after this point (skb is modified) we cannot return false */
 
-	if (skb_shared(skb)) {
-		struct sk_buff *tmp_skb = skb;
-
-		skb = skb_clone(skb, GFP_ATOMIC);
-		kfree_skb(tmp_skb);
-
-		if (!skb)
-			return true;
-	}
+	skb = skb_share_check(skb, GFP_ATOMIC);
+	if (unlikely(!skb))
+		return true;
 
 	if ((hdr->frame_control & cpu_to_le16(IEEE80211_STYPE_QOS_DATA)) &&
 	    ieee80211_amsdu_aggregate(sdata, sta, fast_tx, skb))
@@ -4437,7 +4422,7 @@ static void ieee80211_8023_xmit(struct ieee80211_sub_if_data *sdata,
 				struct net_device *dev, struct sta_info *sta,
 				struct ieee80211_key *key, struct sk_buff *skb)
 {
-	struct ieee80211_tx_info *info = IEEE80211_SKB_CB(skb);
+	struct ieee80211_tx_info *info;
 	struct ieee80211_local *local = sdata->local;
 	struct tid_ampdu_tx *tid_tx;
 	u8 tid;
@@ -4452,6 +4437,11 @@ static void ieee80211_8023_xmit(struct ieee80211_sub_if_data *sdata,
 	    test_bit(SDATA_STATE_OFFCHANNEL, &sdata->state))
 		goto out_free;
 
+	skb = skb_share_check(skb, GFP_ATOMIC);
+	if (unlikely(!skb))
+		return;
+
+	info = IEEE80211_SKB_CB(skb);
 	memset(info, 0, sizeof(*info));
 
 	ieee80211_aggr_check(sdata, sta, skb);