linux-kernel - Re: [BUG] fbdev: i740fb: Divide error when ‘var->pixclock’ is zero

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Message-ID: <a564f6af-31fa-79a2-72c3-578f2c095b23@gmx.de>
Date:   Tue, 5 Apr 2022 08:33:57 +0200
From:   Helge Deller <deller@....de>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Zheyu Ma <zheyuma97@...il.com>,
        Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [BUG] fbdev: i740fb: Divide error when ‘var->pixclock’ is zero

Hello Geert,

On 4/4/22 13:46, Geert Uytterhoeven wrote:
> Hi Helge,
>
> On Sun, Apr 3, 2022 at 5:41 PM Helge Deller <deller@....de> wrote:
>> On 4/3/22 13:26, Zheyu Ma wrote:
>>> I found a bug in the function i740fb_set_par().
>>
>> Nice catch!
>>
>>> When the user calls the ioctl system call without setting the value to
>>> 'var->pixclock', the driver will throw a divide error.
>>>
>>> This bug occurs because the driver uses the value of 'var->pixclock'
>>> without checking it, as the following code snippet show:
>>>
>>> if ((1000000 / var->pixclock) > DACSPEED8) {
>>>      dev_err(info->device, "requested pixclock %i MHz out of range
>>> (max. %i MHz at 8bpp)\n",
>>>          1000000 / var->pixclock, DACSPEED8);
>>>     return -EINVAL;x
>>> }
>>>
>>> We can fix this by checking the value of 'var->pixclock' in the
>>> function i740fb_check_var() similar to commit
>>> b36b242d4b8ea178f7fd038965e3cac7f30c3f09, or we should set the lowest
>>> supported value when this field is zero.
>>> I have no idea about which solution is better.
>>
>> Me neither.
>> I think a solution like commit b36b242d4b8ea178f7fd038965e3cac7f30c3f09
>> is sufficient.
>>
>> Note that i740fb_set_par() is called in i740fb_resume() as well.
>> Since this doesn't comes form userspace I think adding a check for
>> the return value there isn't necessary.
>>
>> Would you mind sending a patch like b36b242d4b8ea178f7fd038965e3cac7f30c3f09 ?
>
> When passed an invalid value, .check_var() is supposed to
> round up the invalid to a valid value, if possible.

I don't disagree.
The main problem probably is: what is the next valid value?
This needs to be analyzed on a per-driver base and ideally tested.
Right now a division-by-zero is tiggered which is probably more worse.

That said, currently I'd prefer to apply the zero-checks patches over
any untested patches. It's easy to revert such checks if a better solution
becomes available.

Thoughts?

> Commit b36b242d4b8ea178 ("video: fbdev: asiliantfb: Error out if
> 'pixclock' equals zero") does not do that.

Helge