On Tue, 13 Mar 2018 20:06:50 +0100
Mohammed Gamal <mga...@redhat.com> wrote:

> Dring high network traffic changes to network interface parameters
> such as number of channels or MTU can cause a kernel panic with a NULL
> pointer dereference. This is due to netvsc_device_remove() being
> called and deallocating the channel ring buffers, which can then be
> accessed by netvsc_send_pkt() before they're allocated on calling
> netvsc_device_add()
> 
> The patch fixes this problem by checking the channel state and returning
> ENODEV if not yet opened. We also move the call to hv_ringbuf_avail_percent()
> which may access the uninitialized ring buffer.
> 
> Signed-off-by: Mohammed Gamal <mga...@redhat.com>
> ---
>  drivers/net/hyperv/netvsc.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/hyperv/netvsc.c b/drivers/net/hyperv/netvsc.c
> index 0265d70..44a8358 100644
> --- a/drivers/net/hyperv/netvsc.c
> +++ b/drivers/net/hyperv/netvsc.c
> @@ -757,7 +757,7 @@ static inline int netvsc_send_pkt(
>       struct netdev_queue *txq = netdev_get_tx_queue(ndev, packet->q_idx);
>       u64 req_id;
>       int ret;
> -     u32 ring_avail = hv_ringbuf_avail_percent(&out_channel->outbound);
> +     u32 ring_avail;
>  
>       nvmsg.hdr.msg_type = NVSP_MSG1_TYPE_SEND_RNDIS_PKT;
>       if (skb)
> @@ -773,7 +773,7 @@ static inline int netvsc_send_pkt(
>  
>       req_id = (ulong)skb;
>  
> -     if (out_channel->rescind)
> +     if (out_channel->rescind || out_channel->state != CHANNEL_OPENED_STATE)
>               return -ENODEV;
>  
>       if (packet->page_buf_cnt) {
> @@ -791,6 +791,7 @@ static inline int netvsc_send_pkt(
>                                      
> VMBUS_DATA_PACKET_FLAG_COMPLETION_REQUESTED);
>       }
>  
> +     ring_avail = hv_ringbuf_avail_percent(&out_channel->outbound);
>       if (ret == 0) {
>               atomic_inc_return(&nvchan->queue_sends);
>  

Thanks for your patch. Yes there are races with the current update
logic. The root cause goes higher up in the flow; the send queues should
be stopped before netvsc_device_remove is called. Solving it where you tried
to is racy and not going to work reliably.

Network patches should go to net...@vger.kernel.org

You can't move the ring_avail check until after the vmbus_sendpacket because
that will break the flow control logic.

Instead, you should just move the avail_read check until just after the 
existing rescind
check.

Also, you shouldn't need to check for OPENED_STATE, just rescind is enough.




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