bnxt: prevent skb UAF after handing over to PTP worker

When reading the timestamp is required bnxt_tx_int() hands
over the ownership of the completed skb to the PTP worker.
The skb should not be used afterwards, as the worker may
run before the rest of our code and free the skb, leading
to a use-after-free.

Since dev_kfree_skb_any() accepts NULL make the loss of
ownership more obvious and set skb to NULL.

Fixes: 83bb623c ("bnxt_en: Transmit and retrieve packet timestamps")
Reviewed-by: Andy Gospodarek <gospo@broadcom.com>
Reviewed-by: Michael Chan <michael.chan@broadcom.com>
Link: https://lore.kernel.org/r/20220921201005.335390-1-kuba@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/broadcom/bnxt/bnxt.c

@@ -659,7 +659,6 @@ static void bnxt_tx_int(struct bnxt *bp, struct bnxt_napi *bnapi, int nr_pkts)
 
 	for (i = 0; i < nr_pkts; i++) {
 		struct bnxt_sw_tx_bd *tx_buf;
-		bool compl_deferred = false;
 		struct sk_buff *skb;
 		int j, last;
 
@@ -668,6 +667,8 @@ static void bnxt_tx_int(struct bnxt *bp, struct bnxt_napi *bnapi, int nr_pkts)
 		skb = tx_buf->skb;
 		tx_buf->skb = NULL;
 
+		tx_bytes += skb->len;
+
 		if (tx_buf->is_push) {
 			tx_buf->is_push = 0;
 			goto next_tx_int;
@@ -688,8 +689,9 @@ static void bnxt_tx_int(struct bnxt *bp, struct bnxt_napi *bnapi, int nr_pkts)
 		}
 		if (unlikely(skb_shinfo(skb)->tx_flags & SKBTX_IN_PROGRESS)) {
 			if (bp->flags & BNXT_FLAG_CHIP_P5) {
+				/* PTP worker takes ownership of the skb */
 				if (!bnxt_get_tx_ts_p5(bp, skb))
-					compl_deferred = true;
+					skb = NULL;
 				else
 					atomic_inc(&bp->ptp_cfg->tx_avail);
 			}
@@ -698,8 +700,6 @@ static void bnxt_tx_int(struct bnxt *bp, struct bnxt_napi *bnapi, int nr_pkts)
 next_tx_int:
 		cons = NEXT_TX(cons);
 
-		tx_bytes += skb->len;
-		if (!compl_deferred)
 		dev_kfree_skb_any(skb);
 	}