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Re: [PATCH-for-6.2 v3 1/2] hw/block/fdc: Kludge missing floppy drive to
From: |
Philippe Mathieu-Daudé |
Subject: |
Re: [PATCH-for-6.2 v3 1/2] hw/block/fdc: Kludge missing floppy drive to fix CVE-2021-20196 |
Date: |
Tue, 23 Nov 2021 14:57:39 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.3.0 |
On 11/23/21 14:33, Hanna Reitz wrote:
> On 18.11.21 13:06, 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.
>>
>> Cc: qemu-stable@nongnu.org
>> 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
>> Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
>> 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 d21b717b7d6..6f94b6a6daa 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 device.
>> + */
>> + cur_drv->blk = blk_new(qemu_get_aio_context(), 0, BLK_PERM_ALL);
>
> So to me this looks basically like a mini version of
> floppy_drive_realize(), and I was wondering what else we might want to
> use from that function. fd_init() and fd_revalidate() look interesting,
> but it appears that fdctrl_realize_common() already did that for all
> drives so we should be good.
How the controller / bus / floppy / drive are connected is a bit
confusing. Did you ever try to hot-remove the magnetic medium from
the floppy plastic enclosure while the controller rotates it?
> Then again, fd_revalidate() behaves differently for the initial drv->blk
> == NULL (drv->drive is set to TYPE_NONE, and last_sect and max_track are
> set to 0) and for then later !blk_is_inserted() (drv->drive not changed
> (so I guess it stays TYPE_NONE?), but last_sect and max_track are set to
> 0xff). Not sure if that’s a problem. Probably not, given that I think
> drv->disk and drv->drive both stay TYPE_NONE.
I'm not sure about the future plans for this device model...
> Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Thanks!
- [PATCH-for-6.2 v3 0/2] hw/block/fdc: Fix CVE-2021-20196, Philippe Mathieu-Daudé, 2021/11/18
- [PATCH-for-6.2 v3 1/2] hw/block/fdc: Kludge missing floppy drive to fix CVE-2021-20196, Philippe Mathieu-Daudé, 2021/11/18
- [PATCH-for-6.2 v3 2/2] tests/qtest/fdc-test: Add a regression test for CVE-2021-20196, Philippe Mathieu-Daudé, 2021/11/18
- Re: [PATCH-for-6.2 v3 2/2] tests/qtest/fdc-test: Add a regression test for CVE-2021-20196, Hanna Reitz, 2021/11/23
- Re: [PATCH-for-6.2 v3 2/2] tests/qtest/fdc-test: Add a regression test for CVE-2021-20196, Philippe Mathieu-Daudé, 2021/11/23
- Re: [PATCH-for-6.2 v3 2/2] tests/qtest/fdc-test: Add a regression test for CVE-2021-20196, Hanna Reitz, 2021/11/23
- Re: [PATCH-for-6.2 v3 2/2] tests/qtest/fdc-test: Add a regression test for CVE-2021-20196, Philippe Mathieu-Daudé, 2021/11/24
- Re: [PATCH-for-6.2 v3 2/2] tests/qtest/fdc-test: Add a regression test for CVE-2021-20196, Hanna Reitz, 2021/11/24
- Re: [PATCH-for-6.2 v3 2/2] tests/qtest/fdc-test: Add a regression test for CVE-2021-20196, Philippe Mathieu-Daudé, 2021/11/24
- Re: [PATCH-for-6.2 v3 2/2] tests/qtest/fdc-test: Add a regression test for CVE-2021-20196, Alexander Bulekov, 2021/11/23
Re: [PATCH-for-6.2 v3 0/2] hw/block/fdc: Fix CVE-2021-20196, Philippe Mathieu-Daudé, 2021/11/22