can: xilinx_can: xcan_write_frame(): fix use-after-free of transmitted SKB

can_put_echo_skb() takes ownership of the SKB and it may be freed
during or after the call.

However, xilinx_can xcan_write_frame() keeps using SKB after the call.

Fix that by only calling can_put_echo_skb() after the code is done
touching the SKB.

The tx_lock is held for the entire xcan_write_frame() execution and
also on the can_get_echo_skb() side so the order of operations does not
matter.

An earlier fix commit 3d3c817c3a40 ("can: xilinx_can: Fix usage of skb
memory") did not move the can_put_echo_skb() call far enough.

Signed-off-by: Anssi Hannula <[e-mail]>
Fixes: 1598efe57b3e ("can: xilinx_can: refactor code in preparation for CAN FD support")
Link: https://patch.msgid.link/[e-mail]
[mkl: add "commit" in front of sha1 in patch description]
[mkl: fix indention]
Signed-off-by: Marc Kleine-Budde <[e-mail]>

drivers/net/can/xilinx_can.c

···
 		dlc |= XCAN_DLCR_EDL_MASK;
 	}
 
-	if (!(priv->devtype.flags & XCAN_FLAG_TX_MAILBOXES) &&
-	    (priv->devtype.flags & XCAN_FLAG_TXFEMP))
-		can_put_echo_skb(skb, ndev, priv->tx_head % priv->tx_max, 0);
-	else
-		can_put_echo_skb(skb, ndev, 0, 0);
-
-	priv->tx_head++;
-
 	priv->write_reg(priv, XCAN_FRAME_ID_OFFSET(frame_offset), id);
 	/* If the CAN frame is RTR frame this write triggers transmission
 	 * (not on CAN FD)
···
 					data[1]);
 		}
 	}
+
+	if (!(priv->devtype.flags & XCAN_FLAG_TX_MAILBOXES) &&
+	    (priv->devtype.flags & XCAN_FLAG_TXFEMP))
+		can_put_echo_skb(skb, ndev, priv->tx_head % priv->tx_max, 0);
+	else
+		can_put_echo_skb(skb, ndev, 0, 0);
+
+	priv->tx_head++;
 }
 
 /**