ravb: Fix use-after-free ravb_tstamp_skb

When a Tx timestamp is requested, a pointer to the skb is stored in the
ravb_tstamp_skb struct. This was done without an skb_get. There exists
the possibility that the skb could be freed by ravb_tx_free (when
ravb_tx_free is called from ravb_start_xmit) before the timestamp was
processed, leading to a use-after-free bug.

Use skb_get when filling a ravb_tstamp_skb struct, and add appropriate
frees/consumes when a ravb_tstamp_skb struct is freed.

Fixes: c156633f1353 ("Renesas Ethernet AVB driver proper")
Signed-off-by: Tho Vu <[e-mail]>
Signed-off-by: Kazuya Mizuguchi <[e-mail]>
Signed-off-by: Simon Horman <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/ethernet/renesas/ravb_main.c

···
 // SPDX-License-Identifier: GPL-2.0
 /* Renesas Ethernet AVB device driver
  *
- * Copyright (C) 2014-2015 Renesas Electronics Corporation
+ * Copyright (C) 2014-2019 Renesas Electronics Corporation
  * Copyright (C) 2015 Renesas Solutions Corp.
  * Copyright (C) 2015-2016 Cogent Embedded, Inc. <[e-mail]>
  *
···
 			kfree(ts_skb);
 			if (tag == tfa_tag) {
 				skb_tstamp_tx(skb, &shhwtstamps);
+				dev_consume_skb_any(skb);
 				break;
+			} else {
+				dev_kfree_skb_any(skb);
 			}
 		}
 		ravb_modify(ndev, TCCR, TCCR_TFR, TCCR_TFR);
···
 			}
 			goto unmap;
 		}
-		ts_skb->skb = skb;
+		ts_skb->skb = skb_get(skb);
 		ts_skb->tag = priv->ts_skb_tag++;
 		priv->ts_skb_tag &= 0x3ff;
 		list_add_tail(&ts_skb->list, &priv->ts_skb_list);
···
 	/* Clear the timestamp list */
 	list_for_each_entry_safe(ts_skb, ts_skb2, &priv->ts_skb_list, list) {
 		list_del(&ts_skb->list);
+		kfree_skb(ts_skb->skb);
 		kfree(ts_skb);
 	}