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

[George Kennedy <george.kennedy@oracle.com>]

Hi Geert,

On 10/26/2021 4:30 AM, Geert Uytterhoeven wrote:
> Hi George,
>
> On Mon, Oct 25, 2021 at 9:37 PM George Kennedy
> <george.kennedy@oracle.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@oracle.com>
>>>> ---
>>>>    drivers/video/fbdev/cirrusfb.c | 3 +++
>>>>    1 file changed, 3 insertions(+)
>>>>
>>>> diff --git a/drivers/video/fbdev/cirrusfb.c b/drivers/video/fbdev/cirrusfb.c
>>>> index 93802ab..099ddcb 100644
>>>> --- 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

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.

I would think we would want to keep this driver consistent with the 
other fb drivers that return failure with the incorrect value.

Thank you,
George
>
>>> 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@linux-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