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

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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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[RFC/PATCHSET 0/3] virtio-pstore: Implement virtio pstore device Namhyung Kim <namhyung@kernel.org> - 2016-07-18 06:40 +0200
  [PATCH 1/3] virtio: Basic implementation of virtio pstore driver Namhyung Kim <namhyung@kernel.org> - 2016-07-18 06:40 +0200
    Re: [PATCH 1/3] virtio: Basic implementation of virtio pstore driver Kees Cook <keescook@chromium.org> - 2016-07-18 07:20 +0200
      Re: [PATCH 1/3] virtio: Basic implementation of virtio pstore driver Namhyung Kim <namhyung@kernel.org> - 2016-07-18 08:00 +0200
        Re: [PATCH 1/3] virtio: Basic implementation of virtio pstore driver Kees Cook <keescook@chromium.org> - 2016-07-18 20:00 +0200
          Re: [PATCH 1/3] virtio: Basic implementation of virtio pstore driver Namhyung Kim <namhyung@kernel.org> - 2016-07-19 15:50 +0200
            Re: [PATCH 1/3] virtio: Basic implementation of virtio pstore driver Namhyung Kim <namhyung@kernel.org> - 2016-07-19 17:40 +0200
            Re: [PATCH 1/3] virtio: Basic implementation of virtio pstore driver Namhyung Kim <namhyung@kernel.org> - 2016-07-20 15:00 +0200
    Re: [PATCH 1/3] virtio: Basic implementation of virtio pstore  driver Cornelia Huck <cornelia.huck@de.ibm.com> - 2016-07-18 10:00 +0200
      Re: [PATCH 1/3] virtio: Basic implementation of virtio pstore driver Namhyung Kim <namhyung@kernel.org> - 2016-07-18 10:40 +0200
        Re: [PATCH 1/3] virtio: Basic implementation of virtio pstore  driver Cornelia Huck <cornelia.huck@de.ibm.com> - 2016-07-18 11:10 +0200
  [PATCH 2/3] qemu: Implement virtio-pstore device Namhyung Kim <namhyung@kernel.org> - 2016-07-18 06:40 +0200
    Re: [PATCH 2/3] qemu: Implement virtio-pstore device Christian Borntraeger <borntraeger@de.ibm.com> - 2016-07-18 09:30 +0200
      Re: [PATCH 2/3] qemu: Implement virtio-pstore device Namhyung Kim <namhyung@kernel.org> - 2016-07-18 10:40 +0200
    Re: [PATCH 2/3] qemu: Implement virtio-pstore device Stefan Hajnoczi <stefanha@gmail.com> - 2016-07-18 12:10 +0200
      Re: [PATCH 2/3] qemu: Implement virtio-pstore device Namhyung Kim <namhyung@kernel.org> - 2016-07-18 16:30 +0200
        Re: [PATCH 2/3] qemu: Implement virtio-pstore device Stefan Hajnoczi <stefanha@gmail.com> - 2016-07-20 13:40 +0200
          Re: [PATCH 2/3] qemu: Implement virtio-pstore device Namhyung Kim <namhyung@kernel.org> - 2016-07-20 14:50 +0200
      Re: [PATCH 2/3] qemu: Implement virtio-pstore device Namhyung Kim <namhyung@kernel.org> - 2016-07-19 17:50 +0200
        Re: [PATCH 2/3] qemu: Implement virtio-pstore device Stefan Hajnoczi <stefanha@gmail.com> - 2016-07-20 13:40 +0200
          Re: [PATCH 2/3] qemu: Implement virtio-pstore device Namhyung Kim <namhyung@kernel.org> - 2016-07-20 14:40 +0200
  [PATCH 3/3] kvmtool: Implement virtio-pstore device Namhyung Kim <namhyung@kernel.org> - 2016-07-18 06:40 +0200

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