netdev - [PATCH v4 2/3] can: vxcan: vxcan_xmit: fix use after free bug

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Message-Id: <20210120114137.200019-3-mailhol.vincent@wanadoo.fr>
Date:   Wed, 20 Jan 2021 20:41:36 +0900
From:   Vincent Mailhol <mailhol.vincent@...adoo.fr>
To:     Marc Kleine-Budde <mkl@...gutronix.de>,
        Oliver Hartkopp <socketcan@...tkopp.net>,
        linux-can@...r.kernel.org
Cc:     netdev@...r.kernel.org, Wolfgang Grandegger <wg@...ndegger.com>,
        Stephane Grosjean <s.grosjean@...k-system.com>,
        Loris Fauster <loris.fauster@...ontrol.com>,
        Alejandro Concepcion Rodriguez <alejandro@...ro.eu>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Vincent Mailhol <mailhol.vincent@...adoo.fr>
Subject: [PATCH v4 2/3] can: vxcan: vxcan_xmit: fix use after free bug

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)")
Signed-off-by: Vincent Mailhol <mailhol.vincent@...adoo.fr>
---
 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 fa47bab510bb..f9a524c5f6d6 100644
--- a/drivers/net/can/vxcan.c
+++ b/drivers/net/can/vxcan.c
@@ -39,6 +39,7 @@ 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;
@@ -61,12 +62,13 @@ 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:
-- 
2.26.2