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Re: [PATCH-for-6.2 v2 1/2] hw/block/fdc: Kludge missing floppy drive to
From: |
Philippe Mathieu-Daudé |
Subject: |
Re: [PATCH-for-6.2 v2 1/2] hw/block/fdc: Kludge missing floppy drive to fix CVE-2021-20196 |
Date: |
Thu, 18 Nov 2021 12:54:24 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.2.0 |
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.