Re: [PATCH-for-6.2 v2 1/2] hw/block/fdc: Kludge missing floppy drive to fix CVE-2021-20196

[Darren Kenny]

Hi Philippe,

A small nit below, but otherwise looks good.

On Thursday, 2021-11-18 at 00:24:21 +01, Philippe Mathieu-Daudé wrote:
> Guest might select another drive on the bus by setting the
> DRIVE_SEL bit of the DIGITAL OUTPUT REGISTER (DOR).
> The current controller model doesn't expect a BlockBackend
> to be NULL. A simple way to fix CVE-2021-20196 is to create
> an empty BlockBackend when it is missing. All further
> accesses will be safely handled, and the controller state
> machines keep behaving correctly.
>
> Fixes: CVE-2021-20196
> Reported-by: Gaoning Pan (Ant Security Light-Year Lab) <pgn@zju.edu.cn>
> BugLink: https://bugs.launchpad.net/qemu/+bug/1912780
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/338
> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> ---
>  hw/block/fdc.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/hw/block/fdc.c b/hw/block/fdc.c
> index fa933cd3263..eab17e946d6 100644
> --- a/hw/block/fdc.c
> +++ b/hw/block/fdc.c
> @@ -1161,7 +1161,19 @@ static FDrive *get_drv(FDCtrl *fdctrl, int unit)
>  
>  static FDrive *get_cur_drv(FDCtrl *fdctrl)
>  {
> -    return get_drv(fdctrl, fdctrl->cur_drv);
> +    FDrive *cur_drv = get_drv(fdctrl, fdctrl->cur_drv);
> +
> +    if (!cur_drv->blk) {
> +        /*
> +         * Kludge: empty drive line selected. Create an anonymous
> +         * BlockBackend to avoid NULL deref with various BlockBackend
> +         * API calls within this model (CVE-2021-20196).
> +         * Due to the controller QOM model limitations, we don't
> +         * attach the created to the controller.
>
The created .... to the controller

Something is missing here, maybe 'device'?

Thanks,

Darren.

> +         */
> +        cur_drv->blk = blk_new(qemu_get_aio_context(), 0, BLK_PERM_ALL);
> +    }
> +    return cur_drv;
>  }
>  
>  /* Status A register : 0x00 (read-only) */
> -- 
> 2.31.1