[PATCH 4.19 29/58] can: vxcan: vxcan_xmit: fix use after free bug

[Greg Kroah-Hartman]

From: Vincent Mailhol <mailhol.vincent@xxxxxxxxxx>

[ Upstream commit 75854cad5d80976f6ea0f0431f8cedd3bcc475cb ]

After calling netif_rx_ni(skb), dereferencing skb is unsafe.
Especially, the canfd_frame cfd which aliases skb memory is accessed
after the netif_rx_ni().

Fixes: a8f820a380a2 ("can: add Virtual CAN Tunnel driver (vxcan)")
Link: https://lore.kernel.org/r/20210120114137.200019-3-mailhol.vincent@xxxxxxxxxx
Signed-off-by: Vincent Mailhol <mailhol.vincent@xxxxxxxxxx>
Signed-off-by: Marc Kleine-Budde <mkl@xxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 drivers/net/can/vxcan.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/net/can/vxcan.c b/drivers/net/can/vxcan.c
index ed6828821fbd3..ccd758ba3fb09 100644
--- a/drivers/net/can/vxcan.c
+++ b/drivers/net/can/vxcan.c
@@ -49,6 +49,7 @@ static netdev_tx_t vxcan_xmit(struct sk_buff *skb, struct net_device *dev)
 	struct net_device *peer;
 	struct canfd_frame *cfd = (struct canfd_frame *)skb->data;
 	struct net_device_stats *peerstats, *srcstats = &dev->stats;
+	u8 len;
 
 	if (can_dropped_invalid_skb(dev, skb))
 		return NETDEV_TX_OK;
@@ -71,12 +72,13 @@ static netdev_tx_t vxcan_xmit(struct sk_buff *skb, struct net_device *dev)
 	skb->dev        = peer;
 	skb->ip_summed  = CHECKSUM_UNNECESSARY;
 
+	len = cfd->len;
 	if (netif_rx_ni(skb) == NET_RX_SUCCESS) {
 		srcstats->tx_packets++;
-		srcstats->tx_bytes += cfd->len;
+		srcstats->tx_bytes += len;
 		peerstats = &peer->stats;
 		peerstats->rx_packets++;
-		peerstats->rx_bytes += cfd->len;
+		peerstats->rx_bytes += len;
 	}
 
 out_unlock:
-- 
2.27.0