[PATCH] can: kvaser_usb: Don't free packets when tight on URBs

[Ahmed S. Darwish]

From: Ahmed S. Darwish <ahmed.darwish@xxxxxxxxx>

Flooding the Kvaser CAN to USB dongle with multiple reads and
writes in high frequency caused seemingly-random panics in the
kernel.

On further inspection, it seems the driver erroneously freed the
to-be-transmitted packet upon getting tight on URBs and returning
NETDEV_TX_BUSY, leading to invalid memory writes and double frees
at a later point in time.

Signed-off-by: Ahmed S. Darwish <ahmed.darwish@xxxxxxxxx>
---
 drivers/net/can/usb/kvaser_usb.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

 (Generated over 3.19.0-rc1)

diff --git a/drivers/net/can/usb/kvaser_usb.c b/drivers/net/can/usb/kvaser_usb.c
index 541fb7a..34c35d8 100644
--- a/drivers/net/can/usb/kvaser_usb.c
+++ b/drivers/net/can/usb/kvaser_usb.c
@@ -1286,6 +1286,7 @@ static netdev_tx_t kvaser_usb_start_xmit(struct sk_buff *skb,
 	struct kvaser_msg *msg;
 	int i, err;
 	int ret = NETDEV_TX_OK;
+	bool kfree_skb_on_error = true;
 
 	if (can_dropped_invalid_skb(netdev, skb))
 		return NETDEV_TX_OK;
@@ -1336,6 +1337,7 @@ static netdev_tx_t kvaser_usb_start_xmit(struct sk_buff *skb,
 
 	if (!context) {
 		netdev_warn(netdev, "cannot find free context\n");
+		kfree_skb_on_error = false;
 		ret =  NETDEV_TX_BUSY;
 		goto releasebuf;
 	}
@@ -1364,8 +1366,7 @@ static netdev_tx_t kvaser_usb_start_xmit(struct sk_buff *skb,
 	if (unlikely(err)) {
 		can_free_echo_skb(netdev, context->echo_index);
 
-		skb = NULL; /* set to NULL to avoid double free in
-			     * dev_kfree_skb(skb) */
+		kfree_skb_on_error = false;
 
 		atomic_dec(&priv->active_tx_urbs);
 		usb_unanchor_urb(urb);
@@ -1389,7 +1390,8 @@ releasebuf:
 nobufmem:
 	usb_free_urb(urb);
 nourbmem:
-	dev_kfree_skb(skb);
+	if (kfree_skb_on_error)
+		dev_kfree_skb(skb);
 	return ret;
 }

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