Re: [Qemu-devel] [PATCH] Allow mismatched virtio config-len

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

On Fri, Jun 27, 2014 at 03:42:10PM +0100, Dr. David Alan Gilbert wrote:
> * Michael S. Tsirkin (mst@redhat.com) wrote:
> > On Fri, Jun 27, 2014 at 09:34:38AM +0100, Dr. David Alan Gilbert (git) wrote:
> > > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > > 
> > > Commit 'virtio: validate config_len on load' restricted config_len
> > > loaded from the wire to match the config_len that the device had.
> > > 
> > > Unfortunately, there are cases where this isn't true, the one
> > > we found it on was the wqe addition in virtio-blk.
> > 
> > I think you mean wce.
> 
> Oops - yes.
> 
> > > Allow mismatched config-lengths:
> > >    *) If the version on the wire is shorter then ensure that the
> > >       remainder is 0xff filled (as virtio_config_read does on
> > >       out of range reads)
> > >    *) If the version on the wire is longer, load what we have space
> > >       for and skip the rest.
> > > 
> > > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > 
> > Looks good overall, but I am having thoughts about the
> > padding with 0xff.
> > We previously didn't do this (before virtio: validate config_len on
> > load) so it seems safest (at least for 2.1) not to do it now either.
> 
> Who allocates that memory? If it's known to be a value then I agree; however
> if it's uninitialised then I think it's best to pick a value rather than
> have behaviour that depends on random junk in the memory.

It's initialized: e.g. for net it includes the mac,
for block the wce value.

> > > ---
> > >  hw/virtio/virtio.c | 30 ++++++++++++++++++++++++++----
> > >  1 file changed, 26 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> > > index a3082d5..2b11142 100644
> > > --- a/hw/virtio/virtio.c
> > > +++ b/hw/virtio/virtio.c
> > > @@ -927,11 +927,33 @@ int virtio_load(VirtIODevice *vdev, QEMUFile *f)
> > >      }
> > >      config_len = qemu_get_be32(f);
> > >      if (config_len != vdev->config_len) {
> > > -        error_report("Unexpected config length 0x%x. Expected 0x%zx",
> > > -                     config_len, vdev->config_len);
> > > -        return -1;
> > > +        /*
> > > +         * Unfortunately the reality is that there are cases where we
> > > +         * see mismatched config lengths, so we have to deal with them
> > > +         * rather than rejecting them.
> > > +         */
> > > +
> > 
> > Drop extra line please.
> > 
> > > +        if (config_len < vdev->config_len) {
> > > +            /* This is normal in some devices when they add a new option */
> > > +            memset(vdev->config, 0xff, vdev->config_len);
> > > +            qemu_get_buffer(f, vdev->config, config_len);
> > > +        } else {
> > > +            int32_t diff;
> > > +            /* config_len > vdev->config_len
> > > +             * This is rarer, but is here to allow us to fix the case above
> > > +             */
> > > +            qemu_get_buffer(f, vdev->config, vdev->config_len);
> > > +            /*
> > > +             * Even though we expect the diff to be small, we can't use
> > > +             * qemu_file_skip because it's not safe for a large skip.
> > > +             */
> > > +            for (diff = config_len - vdev->config_len; diff > 0; diff--) {
> > > +                qemu_get_byte(f);
> > > +            }
> > > +        }
> > > +    } else {
> > > +        qemu_get_buffer(f, vdev->config, vdev->config_len);
> > >      }
> > > -    qemu_get_buffer(f, vdev->config, vdev->config_len);
> > >  
> > >      num = qemu_get_be32(f);
> > 
> > 
> > So I would say handle config_len < vdev->config_len and config_len ==
> > vdev->config_len the same:
> > 
> >         qemu_get_buffer(f, vdev->config, MIN(config_len, vdev->config_len));
> > 
> > and then skip the remainder if any
> >         while (config_len > vdev->config_len) {
> >                 qemu_get_byte(f);
> > 		config_len--;
> > 	}
> 
> That probably still needs that MIN splitting out (int32_t vs size_t); but
> other than that I guess I can redo that.
> 
> Dave
> 
> --
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK