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

commit b442723fcec445fb0ae1104888dd22cd285e0a91 upstream.

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.

Note:

Finding no more URBs/transmit-contexts and returning NETDEV_TX_BUSY
is a driver bug in and out of itself: it means that our start/stop
queue flow control is broken.

This patch only fixes the (buggy) error handling code; the root
cause shall be fixed in a later commit.

Acked-by: Olivier Sobrie <olivier@sobrie.be>
Signed-off-by: Ahmed S. Darwish <ahmed.darwish@valeo.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>

diff --git a/drivers/net/can/usb/kvaser_usb.c b/drivers/net/can/usb/kvaser_usb.c
index cc3df8a..f6e30be 100644 (file)
--- a/drivers/net/can/usb/kvaser_usb.c
+++ b/drivers/net/can/usb/kvaser_usb.c
@@ -1286,12 +1286,14 @@ static netdev_tx_t kvaser_usb_start_xmit(struct sk_buff *skb,
        if (!urb) {
                netdev_err(netdev, "No memory left for URBs\n");
                stats->tx_dropped++;
-               goto nourbmem;
+               dev_kfree_skb(skb);
+               return NETDEV_TX_OK;
        }
 
        buf = kmalloc(sizeof(struct kvaser_msg), GFP_ATOMIC);
        if (!buf) {
                stats->tx_dropped++;
+               dev_kfree_skb(skb);
                goto nobufmem;
        }
 
@@ -1326,6 +1328,7 @@ static netdev_tx_t kvaser_usb_start_xmit(struct sk_buff *skb,
                }
        }
 
+       /* This should never happen; it implies a flow control bug */
        if (!context) {
                netdev_warn(netdev, "cannot find free context\n");
                ret =  NETDEV_TX_BUSY;
@@ -1356,9 +1359,6 @@ 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) */
-
                atomic_dec(&priv->active_tx_urbs);
                usb_unanchor_urb(urb);
 
@@ -1380,8 +1380,6 @@ releasebuf:
        kfree(buf);
 nobufmem:
        usb_free_urb(urb);
-nourbmem:
-       dev_kfree_skb(skb);
        return ret;
 }