Re: [Qemu-devel] [PATCH v2 3/9] virtio-9p: handle handle_9p_output() error

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Wed, Sep 21, 2016 at 06:57:12PM +0200, Greg Kurz wrote:
> A broken guest may send a request with only non-empty out buffers
> or only non-empty in buffers, virtqueue_pop() will then return a
> VirtQueueElement with out_num == 0 or in_num == 0 respectively.
> 
> All 9P requests are expected to start with the following 7-byte header:
> 
>             uint32_t size_le;
>             uint8_t id;
>             uint16_t tag_le;
> 
> If iov_to_buf() fails to return these 7 bytes, then something is wrong in
> the guest.
> 
> In both cases, it is wrong to crash QEMU, since the root cause lies in the
> guest. Let's switch the device to the broken state instead.
> 
> Signed-off-by: Greg Kurz <groug@kaod.org>
> ---
> v2: - added out_free_pdu: label for errors or when virtqueue is empty
> ---
>  hw/9pfs/virtio-9p-device.c |   20 ++++++++++++++++----
>  1 file changed, 16 insertions(+), 4 deletions(-)
> 
> diff --git a/hw/9pfs/virtio-9p-device.c b/hw/9pfs/virtio-9p-device.c
> index e7ea0e45f3dd..5f3a67cfc717 100644
> --- a/hw/9pfs/virtio-9p-device.c
> +++ b/hw/9pfs/virtio-9p-device.c
> @@ -52,17 +52,24 @@ static void handle_9p_output(VirtIODevice *vdev, VirtQueue *vq)
>  
>          elem = virtqueue_pop(vq, sizeof(VirtQueueElement));
>          if (!elem) {
> -            pdu_free(pdu);
> -            break;
> +            goto out_free_pdu;
>          }
>  
> -        BUG_ON(elem->out_num == 0 || elem->in_num == 0);
> +        if (elem->out_num == 0 || elem->in_num == 0) {
> +            virtio_error(vdev,
> +                         "The guest sent a VirtFS request without headers");
> +            goto out_free_pdu;
> +        }

I'm not sure this check is even necessary since we check
iov_to_buf(elem->out_sg[]) and pdu_marshall() avoids overflowing
elem->in_sg[].

Either way, your change is correct:

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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