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Date:   Wed, 27 Oct 2021 11:22:04 -0400
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Subject: Re: [PATCH] video: fbdev: cirrusfb: check pixclock to avoid divide by
 zero
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To:     Geert Uytterhoeven <geert@linux-m68k.org>
Cc:     Greg KH <gregkh@linuxfoundation.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        Sam Ravnborg <sam@ravnborg.org>,
        DRI Development <dri-devel@lists.freedesktop.org>,
        Linux Fbdev development list <linux-fbdev@vger.kernel.org>,
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From:   George Kennedy <george.kennedy@oracle.com>
Organization: Oracle Corporation
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On 10/27/2021 2:53 AM, Geert Uytterhoeven wrote:
> Hi George,
>
> On Wed, Oct 27, 2021 at 3:13 AM George Kennedy
> <george.kennedy@oracle.com> wrote:
>> On 10/26/2021 1:12 PM, Geert Uytterhoeven wrote:
>>> On Tue, Oct 26, 2021 at 5:48 PM George Kennedy
>>> <george.kennedy@oracle.com> wrote:
>>>> On 10/26/2021 10:11 AM, Geert Uytterhoeven wrote:
>>>>> On Tue, Oct 26, 2021 at 3:38 PM George Kennedy
>>>>> <george.kennedy@oracle.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@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>
>>>>>>>>>> --- 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)?
>>> No, clock rate is the inverse of clock period.
>>> So the smallest clock period (fb_var_screeninfo.pixclock) corresponds
>>> to the largest clock rate (freq in bestclock()).
>> How about this?
>>
>> This gets the frequency derived from pixclock to maxclock or rounds up
>> pixclock to get the frequency as close to maxclock as possible.
>>
>> diff --git a/drivers/video/fbdev/cirrusfb.c b/drivers/video/fbdev/cirrusfb.c
>> index 93802ab..2e8e620 100644
>> --- a/drivers/video/fbdev/cirrusfb.c
>> +++ b/drivers/video/fbdev/cirrusfb.c
>> @@ -620,6 +620,18 @@ static int cirrusfb_check_var(struct
>> fb_var_screeninfo *var,
>>                   return -EINVAL;
>>           }
>>
>> +       if (!var->pixclock) {
>> +               long maxclock;
>> +               unsigned maxclockidx = var->bits_per_pixel >> 3;
>> +
>> +               maxclock =
>> cirrusfb_board_info[cinfo->btype].maxclock[maxclockidx];
>> +
>> +               var->pixclock = KHZ2PICOS(maxclock);
>> +               while (PICOS2KHZ(var->pixclock) > maxclock) {
>> +                       var->pixclock++;
>> +               }
>> +       }
>> +
>>           if (cirrusfb_check_pixclock(var, info))
>>                   return -EINVAL;
>>
>> The work can't be done in cirrusfb_check_pixclock() as var->pixclock is
>> read-only because "var" is "const struct fb_var_screeninfo *var".
> Perhaps the const should be dropped from the var parameter, so the
> rounding can be done in the function where it makes most sense,
> and where most of the above operations are already done?
>
> Then, you can simplify:
>
> -        freq = PICOS2KHZ(var->pixclock);
> +        freq = PICOS2KHZ(var->pixclock ? : 1);
>
> and change the "if (freq > maxclock) return -EINVAL" to use maxclock
> instead.

Geert,

Does this look ok?

diff --git a/drivers/video/fbdev/cirrusfb.c b/drivers/video/fbdev/cirrusfb.c
index 93802ab..3d47c34 100644
--- a/drivers/video/fbdev/cirrusfb.c
+++ b/drivers/video/fbdev/cirrusfb.c
@@ -469,7 +469,7 @@ static int cirrusfb_check_mclk(struct fb_info *info, 
long freq)
         return 0;
  }

-static int cirrusfb_check_pixclock(const struct fb_var_screeninfo *var,
+static int cirrusfb_check_pixclock(struct fb_var_screeninfo *var,
                                    struct fb_info *info)
  {
         long freq;
@@ -478,9 +478,7 @@ static int cirrusfb_check_pixclock(const struct 
fb_var_screeninfo *var,
         unsigned maxclockidx = var->bits_per_pixel >> 3;

         /* convert from ps to kHz */
-       freq = PICOS2KHZ(var->pixclock);
-
-       dev_dbg(info->device, "desired pixclock: %ld kHz\n", freq);
+       freq = PICOS2KHZ(var->pixclock ? : 1);

         maxclock = cirrusfb_board_info[cinfo->btype].maxclock[maxclockidx];
         cinfo->multiplexing = 0;
@@ -488,11 +486,13 @@ static int cirrusfb_check_pixclock(const struct 
fb_var_screeninfo *var,
         /* If the frequency is greater than we can support, we might be 
able
          * to use multiplexing for the video mode */
         if (freq > maxclock) {
-               dev_err(info->device,
-                       "Frequency greater than maxclock (%ld kHz)\n",
-                       maxclock);
-               return -EINVAL;
+               var->pixclock = KHZ2PICOS(maxclock);
+
+               while ((freq = PICOS2KHZ(var->pixclock)) > maxclock)
+                       var->pixclock++;
         }
+       dev_dbg(info->device, "desired pixclock: %ld kHz\n", freq);
+
         /*
          * Additional constraint: 8bpp uses DAC clock doubling to allow 
maximum
          * pixel clock

Is the pixclock round-up still needed? Without it the frequency may be 
slightly above maxclock in some cases.

Thank you,
George

>
> 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