linux-kernel - Re: [PATCH 1/7] video: fbdev: i740fb: Error out if 'pixclock' equals zero

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Message-ID: <f68dde85-5ead-3c5c-3bf3-3e6bd7458f2e@gmx.de>
Date:   Mon, 11 Apr 2022 08:18:09 +0200
From:   Helge Deller <deller@....de>
To:     Ondrej Zary <linux@...y.sk>, Zheyu Ma <zheyuma97@...il.com>
Cc:     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: [PATCH 1/7] video: fbdev: i740fb: Error out if 'pixclock' equals
 zero

On 4/10/22 11:02, Ondrej Zary wrote:
> On Friday 08 April 2022 03:58:10 Zheyu Ma wrote:
>> On Fri, Apr 8, 2022 at 3:50 AM Helge Deller <deller@....de> wrote:
>>>
>>> On 4/4/22 10:47, Zheyu Ma wrote:
>>>> The userspace program could pass any values to the driver through
>>>> ioctl() interface. If the driver doesn't check the value of 'pixclock',
>>>> it may cause divide error.
>>>>
>>>> Fix this by checking whether 'pixclock' is zero in the function
>>>> i740fb_check_var().
>>>>
>>>> The following log reveals it:
>>>>
>>>> divide error: 0000 [#1] PREEMPT SMP KASAN PTI
>>>> RIP: 0010:i740fb_decode_var drivers/video/fbdev/i740fb.c:444 [inline]
>>>> RIP: 0010:i740fb_set_par+0x272f/0x3bb0 drivers/video/fbdev/i740fb.c:739
>>>> Call Trace:
>>>>     fb_set_var+0x604/0xeb0 drivers/video/fbdev/core/fbmem.c:1036
>>>>     do_fb_ioctl+0x234/0x670 drivers/video/fbdev/core/fbmem.c:1112
>>>>     fb_ioctl+0xdd/0x130 drivers/video/fbdev/core/fbmem.c:1191
>>>>     vfs_ioctl fs/ioctl.c:51 [inline]
>>>>     __do_sys_ioctl fs/ioctl.c:874 [inline]
>>>>
>>>> Signed-off-by: Zheyu Ma <zheyuma97@...il.com>
>>>
>>> Hello Zheyu,
>>>
>>> I've applied the patches #2-#7 of this series, but left
>>> out this specific patch (for now).
>>> As discussed on the mailing list we can try to come up with a
>>> better fix (to round up the pixclock when it's invalid).
>>> If not, I will apply this one later.
>>
>> I'm also looking forward to a more appropriate patch for this driver!
>
> I was not able to reproduce it at first but finally found it: the
> monitor must be unplugged. If a valid EDID is present,
> fb_validate_mode() call in i740fb_check_var() will refuse zero
> pixclock.
>
> Haven't found any obvious way to correct zero pixclock value. Most other drivers simply return -EINVAL.

Thanks for checking, Ondrej!

So, I'll apply the EINVAL-patch from Zheyu for now.

Helge