Re: [Qemu-devel] [PATCH 1/3] virtio-pci: properly validate address before accessing config

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Apr 26, 2013 at 04:27:55PM +0200, Petr Matousek wrote:
> On Fri, Apr 26, 2013 at 04:34:02PM +0800, Jason Wang wrote:
> > There are several several issues in the current checking:
> > 
> > - The check was based on the minus of unsigned values which can overflow
> > - It was done after .{set|get}_config() which can lead crash when config_len is
> >   zero since vdev->config is NULL
> > 
> > Fix this by:
> > 
> > - Validate the address in virtio_pci_config_{read|write}() before
> >   .{set|get}_config
> > - Use addition instead minus to do the validation
> > 
> > Cc: Michael S. Tsirkin <mst@redhat.com>
> > Cc: Petr Matousek <pmatouse@redhat.com>
> > Signed-off-by: Jason Wang <jasowang@redhat.com>
> > ---
> >  hw/virtio/virtio-pci.c |    9 +++++++++
> >  hw/virtio/virtio.c     |   18 ------------------
> >  2 files changed, 9 insertions(+), 18 deletions(-)
> > 
> > diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
> > index a1f15a8..7f6c7d1 100644
> > --- a/hw/virtio/virtio-pci.c
> > +++ b/hw/virtio/virtio-pci.c
> > @@ -400,6 +400,10 @@ static uint64_t virtio_pci_config_read(void *opaque, hwaddr addr,
> >      }
> >      addr -= config;
> >  
> > +    if (addr + size > proxy->vdev->config_len) {
> > +        return (uint32_t)-1;
> > +    }
> > +
> 
> What is the range of values addr can be? I guess it's not arbitrary and
> not fully in guests hands. Can it be higher than corresponding pci
> config space size?
> 
> IOW, can guest touch anything interesting or will all accesses end in
> the first page in the qemu address space, considering vdev->config being
> NULL?

Good point.
I think it's only the first page of the qemu address space.




> -- 
> Petr Matousek / Red Hat Security Response Team