Re: [Qemu-devel] [PATCH] virtio: abort on zero config length

[Michael S. Tsirkin]

On Thu, Apr 25, 2013 at 03:20:20PM -0500, Anthony Liguori wrote:
> Jason Wang <address@hidden> writes:
> 
> > In fact we don't support zero length config length for virtio device.
> 
> virtio-rng?

It has config_len == 0?  In that case guest using virtio-rng can crash
qemu or read qemu memory:

uint32_t virtio_config_readb(VirtIODevice *vdev, uint32_t addr)
{
    uint8_t val;

    vdev->get_config(vdev, vdev->config);

    if (addr > (vdev->config_len - sizeof(val)))

^^^^^^^^^ quiz: spot a bug above if config_len is 0    :)


        return (uint32_t)-1;

    val = ldub_p(vdev->config + addr);
    return val;
}

how about corrupting qemu memory? That too:

void virtio_config_writeb(VirtIODevice *vdev, uint32_t addr, uint32_t data)
{
    uint8_t val = data;

    if (addr > (vdev->config_len - sizeof(val)))
        return;

    stb_p(vdev->config + addr, val);

    if (vdev->set_config)
        vdev->set_config(vdev, vdev->config);
}



> > And it can lead outbound memory access. So abort on zero config length
> > to catch the bug earlier.
> 
> Not sure what you mean, but virtio-rng has a zero length config space.
> 
> Regards,
> 
> Anthony Liguori
> 
> >
> > Signed-off-by: Jason Wang <address@hidden>
> > ---
> >  hw/virtio/virtio.c |    7 ++-----
> >  1 files changed, 2 insertions(+), 5 deletions(-)
> >
> > diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> > index 1c2282c..a6fa667 100644
> > --- a/hw/virtio/virtio.c
> > +++ b/hw/virtio/virtio.c
> > @@ -923,6 +923,7 @@ void virtio_init(VirtIODevice *vdev, const char *name,
> >                   uint16_t device_id, size_t config_size)
> >  {
> >      int i;
> > +    assert(config_size);
> >      vdev->device_id = device_id;
> >      vdev->status = 0;
> >      vdev->isr = 0;
> > @@ -938,11 +939,7 @@ void virtio_init(VirtIODevice *vdev, const char *name,
> >  
> >      vdev->name = name;
> >      vdev->config_len = config_size;
> > -    if (vdev->config_len) {
> > -        vdev->config = g_malloc0(config_size);
> > -    } else {
> > -        vdev->config = NULL;
> > -    }
> > +    vdev->config = g_malloc0(config_size);
> >      vdev->vmstate = qemu_add_vm_change_state_handler(virtio_vmstate_change,
> >                                                       vdev);
> >  }
> > -- 
> > 1.7.1