Re: [PATCH v2] fbdev: Detect integer underflow at "struct fbcon_ops"->clear_margins.

[Bartlomiej Zolnierkiewicz]

On 7/23/20 4:21 PM, Greg Kroah-Hartman wrote:
> On Wed, Jul 22, 2020 at 10:07:06AM +0200, Daniel Vetter wrote:
>> On Tue, Jul 21, 2020 at 6:08 PM Greg Kroah-Hartman
>> <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>>>
>>> On Thu, Jul 16, 2020 at 08:27:21PM +0900, Tetsuo Handa wrote:
>>>> On 2020/07/16 19:00, Daniel Vetter wrote:
>>>>> On Thu, Jul 16, 2020 at 12:29:00AM +0900, Tetsuo Handa wrote:
>>>>>> On 2020/07/16 0:12, Dan Carpenter wrote:
>>>>>>> I've complained about integer overflows in fbdev for a long time...
>>>>>>>
>>>>>>> What I'd like to see is something like the following maybe.  I don't
>>>>>>> know how to get the vc_data in fbmem.c so it doesn't include your checks
>>>>>>> for negative.
>>>>>>
>>>>>> Yes. Like I said "Thus, I consider that we need more sanity/constraints checks." at
>>>>>> https://lore.kernel.org/lkml/b1e7dd6a-fc22-bba8-0abb-d3e779329bce@xxxxxxxxxxxxxxxxxxx/ ,
>>>>>> we want basic checks. That's a task for fbdev people who should be familiar with
>>>>>> necessary constraints.
>>>>>
>>>>> I think the worldwide supply of people who understand fbdev and willing to
>>>>> work on it is roughly 0. So if someone wants to fix this mess properly
>>>>> (which likely means adding tons of over/underflow checks at entry points,
>>>>> since you're never going to catch the driver bugs, there's too many and
>>>>> not enough people who care) they need to fix this themselves.
>>>>
>>>> But I think we can enforce reasonable constraint which is much stricter than Dan's basic_checks()
>>>> (which used INT_MAX). For example, do we need to accept var->{xres,yres} >= 1048576, for
>>>> "32768 rows or cols" * "32 pixels per character" = 1045876 and vc_do_resize() accepts only
>>>> rows and cols < 32768 ?
>>>>
>>>>>
>>>>> Just to avoid confusion here.
>>>>>
>>>>>> Anyway, my two patches are small and low cost; can we apply these patches regardless
>>>>>> of basic checks?
>>>>>
>>>>> Which two patches where?
>>>>
>>>> [PATCH v3] vt: Reject zero-sized screen buffer size.
>>>>  from https://lkml.kernel.org/r/20200712111013.11881-1-penguin-kernel@xxxxxxxxxxxxxxxxxxx
>>>
>>> This is now in my tree.
>>>
>>>> [PATCH v2] fbdev: Detect integer underflow at "struct fbcon_ops"->clear_margins.
>>>>  from https://lkml.kernel.org/r/20200715015102.3814-1-penguin-kernel@xxxxxxxxxxxxxxxxxxx
>>>
>>> That should be taken by the fbdev maintainer, but I can take it too if
>>> people want.
>>
>> Just missed this weeks pull request train and feeling like not worth
>> making this an exception (it's been broken forever after all), so
>> maybe best if you just add this to vt.
>>
>> Acked-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
>>
>> Also this avoids the impression I know what's going on in fbdev code,
>> maybe with sufficient abandon from my side someone will pop up who
>> cares an fixes the bazillion of syzkaller issues we seem to have
>> around console/vt and everything related.
> 
> Great, will go queue it up now, thanks!
Fine with me, thanks!

PS I'll later queue the patch from George to drm-misc-next (after
reading both fbdev patches in detail it seems that both are needed).

Best regards,
--
Bartlomiej Zolnierkiewicz
Samsung R&D Institute Poland
Samsung Electronics