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

[Olivier Sobrie]

Hello Ahmed,

On Tue, Dec 23, 2014 at 05:46:54PM +0200, Ahmed S. Darwish wrote:
> 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>

Thank you for the fix.

> ---
>  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;

Instead of using an extra variable, you can also set skb to NULL here.
Or maybe better, you can move the dev_kfree_skb() in the two previous
tests (in the check of variables urb and buf).

Thank you,

Olivier

>   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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