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

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

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.

> 
> 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.

> ---
>  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--;
	}


>  
> -- 
> 1.9.3