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

Ahmed S. Darwish
2014-12-24T15:52:57Z

Hi Olivier,

On Wed, Dec 24, 2014 at 01:31:20PM +0100, Olivier Sobrie wrote:
> Hello Ahmed,
>
> On Tue, Dec 23, 2014 at 05:46:54PM +0200, Ahmed S. Darwish wrote:
> > From: Ahmed S. Darwish <ahmed.darwish@valeo.com>
> >
> > 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@valeo.com>
>
> 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).
>

Nice, I'll move dev_kfree_skb() to the two earlier tests then.

Thanks,

P.S. mailer and patch identation; had to manually fix them
before replying (but thanks for the quick review, ofc ;-))

> 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;
> > }
> >