On Wed, Jul 20, 2016 at 12:48:39AM +0900, Namhyung Kim wrote:
> 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?
Yes, a virtqueue element has (host read-only) out buffers followed by
(host write-only) in buffers.
Stefan
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