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

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Message-ID: <eb2edc5a-afad-f0c9-012f-9b9f226d2e5a@gmx.de>
Date:   Thu, 7 Apr 2022 21:49:03 +0200
From:   Helge Deller <deller@....de>
To:     Zheyu Ma <zheyuma97@...il.com>
Cc:     linux-fbdev@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/7] video: fbdev: i740fb: Error out if 'pixclock' equals
 zero

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.

Thanks!
Helge


> ---
>  drivers/video/fbdev/i740fb.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/drivers/video/fbdev/i740fb.c b/drivers/video/fbdev/i740fb.c
> index 52cce0db8bd3..b595437a5752 100644
> --- a/drivers/video/fbdev/i740fb.c
> +++ b/drivers/video/fbdev/i740fb.c
> @@ -657,6 +657,9 @@ static int i740fb_decode_var(const struct fb_var_screeninfo *var,
>
>  static int i740fb_check_var(struct fb_var_screeninfo *var, struct fb_info *info)
>  {
> +	if (!var->pixclock)
> +		return -EINVAL;
> +
>  	switch (var->bits_per_pixel) {
>  	case 8:
>  		var->red.offset	= var->green.offset = var->blue.offset = 0;