linux-kernel - Re: [PATCH] video: fbdev: cirrusfb: check pixclock to avoid divide by zero

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Date:   Tue, 26 Oct 2021 11:47:57 -0400
From:   George Kennedy <george.kennedy@...cle.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Sam Ravnborg <sam@...nborg.org>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] video: fbdev: cirrusfb: check pixclock to avoid divide by
 zero

Hi Geert,

On 10/26/2021 10:11 AM, Geert Uytterhoeven wrote:
> Hi George,
>
> On Tue, Oct 26, 2021 at 3:38 PM George Kennedy
> <george.kennedy@...cle.com> wrote:
>> On 10/26/2021 4:30 AM, Geert Uytterhoeven wrote:
>>> On Mon, Oct 25, 2021 at 9:37 PM George Kennedy
>>> <george.kennedy@...cle.com> wrote:
>>>> On 10/25/2021 3:07 PM, Greg KH wrote:
>>>>> On Mon, Oct 25, 2021 at 02:01:30PM -0500, George Kennedy wrote:
>>>>>> Do a sanity check on pixclock value before using it as a divisor.
>>>>>>
>>>>>> Syzkaller reported a divide error in cirrusfb_check_pixclock.
>>>>>>
>>>>>> divide error: 0000 [#1] SMP KASAN PTI
>>>>>> CPU: 0 PID: 14938 Comm: cirrusfb_test Not tainted 5.15.0-rc6 #1
>>>>>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.11.0-2
>>>>>> RIP: 0010:cirrusfb_check_var+0x6f1/0x1260
>>>>>>
>>>>>> Call Trace:
>>>>>>     fb_set_var+0x398/0xf90
>>>>>>     do_fb_ioctl+0x4b8/0x6f0
>>>>>>     fb_ioctl+0xeb/0x130
>>>>>>     __x64_sys_ioctl+0x19d/0x220
>>>>>>     do_syscall_64+0x3a/0x80
>>>>>>     entry_SYSCALL_64_after_hwframe+0x44/0xae
>>>>>>
>>>>>> Signed-off-by: George Kennedy <george.kennedy@...cle.com>
>>>>>> --- a/drivers/video/fbdev/cirrusfb.c
>>>>>> +++ b/drivers/video/fbdev/cirrusfb.c
>>>>>> @@ -477,6 +477,9 @@ static int cirrusfb_check_pixclock(const struct fb_var_screeninfo *var,
>>>>>>        struct cirrusfb_info *cinfo = info->par;
>>>>>>        unsigned maxclockidx = var->bits_per_pixel >> 3;
>>>>>>
>>>>>> +    if (!var->pixclock)
>>>>>> +            return -EINVAL;
>>> This is not correct: fbdev drivers should round up invalid values,
>>> and only return an error if rounding up cannot yield a valid value.
>> What default value would you recommend? Here are examples of some of the
>> possible cirrusfb pixclock values:
>> 40000: 25MHz
>> 20000: 50Mhz
>> 12500: 80Mhz
> You should pick the lowest supported value.

In bestclock() the frequency value ("freq") is not allowed to go below 8000.

         if (freq < 8000)
                 freq = 8000;

If pixclock is passed in as zero to cirrusfb_check_pixclock(), is it ok 
to then set the value of pixclock to 125000, which will result in "freq" 
being set to 8000 (or adjust the passed in pixclock value to make sure 
"freq" does not get below 8000)?

Thank you,
George
>
>> We can plug in a default value, but I believe it is just covering up the
>> fact that an incorrect value has been copied in.
> Passing zero is not incorrect.  The driver is supposed to round it
> up to a valid value.
>
>> I would think we would want to keep this driver consistent with the
>> other fb drivers that return failure with the incorrect value.
> I disagree: non-conformant behavior should be fixed, not copied.
>
>>>>> Shouldn't you be checking further up the call chain where this got set
>>>>> to 0?
>>>> The same pixclock check is done in these fb drivers:
>>>>
>>>> arch/arm/mach-rpc/include/mach/acornfb.h:    if (!var->pixclock)
>>>> drivers/video/fbdev/asiliantfb.c:    if (!var->pixclock)
>>>> drivers/video/fbdev/clps711x-fb.c:    if (!var->pixclock)
>>>> drivers/video/fbdev/core/fbmon.c:    if (!var->pixclock)
>>>> drivers/video/fbdev/core/modedb.c:    if (!var->pixclock)
>>>> drivers/video/fbdev/cirrusfb.c:    if (!var->pixclock)
>>>> drivers/video/fbdev/kyro/fbdev.c:    if (!var->pixclock)
>>>> drivers/video/fbdev/riva/fbdev.c:    if (!var->pixclock)
>>>> drivers/video/fbdev/uvesafb.c:    if (!var->pixclock)
>>>>
>>>>> What logic allows this to be a valid value?  What about all other fb
>>>>> drivers?
>>>> The "check_var" function, which is set into the ".fb_check_var" element
>>>> of the fb_ops struct, should do the check, but in the case of cirrusfb,
>>>> that is not being done.
>>>>
>>>> All this patch does is add the same pixclock check that the other above
>>>> fb drivers do.
>>> Indeed, several drivers are not following the rounding rules.
> Gr{oetje,eeting}s,
>
>                          Geert
>
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org
>
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
>                                  -- Linus Torvalds