Re: [PATCH 2/3] qemu: Implement virtio-pstore device

[Namhyung Kim <namhyung@kernel.org>]

Hello,

On Mon, Jul 18, 2016 at 11:03:53AM +0100, Stefan Hajnoczi wrote:
> On Mon, Jul 18, 2016 at 01:37:40PM +0900, Namhyung Kim wrote:
> > +static void virtio_pstore_handle_io(VirtIODevice *vdev, VirtQueue *vq)
> > +{
> > +    VirtIOPstore *s = VIRTIO_PSTORE(vdev);
> > +    VirtQueueElement *elem;
> > +    struct virtio_pstore_hdr *hdr;
> > +    ssize_t len;
> > +
> > +    for (;;) {
> > +        elem = virtqueue_pop(vq, sizeof(VirtQueueElement));
> > +        if (!elem) {
> > +            return;
> > +        }
> > +
> > +        hdr = elem->out_sg[0].iov_base;
> > +        if (elem->out_sg[0].iov_len != sizeof(*hdr)) {
> > +            error_report("invalid header size: %u",
> > +                         (unsigned)elem->out_sg[0].iov_len);
> > +            exit(1);
> > +        }
> 
> Please use iov_to_buf() instead of directly accessing out_sg[].  Virtio
> devices are not supposed to assume a particular iovec layout.  In other
> words, virtio_pstore_hdr could be split across multiple out_sg[] iovecs.
> 
> You must also copy in data (similar to Linux syscall implementations) to
> prevent the guest from modifying data while the command is processed.
> Such race conditions could lead to security bugs.

By accessing elem->out_sg[0].iov_base directly, I abused it as an
in-and-out buffer.  But it seems not allowed by the virtio spec, do I
have to use separate buffers for request and response?

Thanks,
Namhyung