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

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 11/18/21 11:44, Darren Kenny wrote:
> 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'?

OK. I forgot to add Cc: qemu-stable@nongnu.org so will respin.

Thanks,

Phil.