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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* 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.

> > ---
> >  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