[PATCH] video: fbdev: arkfb: Fix a divide-by-zero bug in ark_set_pixclock()

[Helge Deller]

On 8/3/22 11:23, Zheyu Ma wrote:
> Since the user can control the arguments of the ioctl() from the user
> space, under special arguments that may result in a divide-by-zero bug
> in:
>   drivers/video/fbdev/arkfb.c:784: ark_set_pixclock(info, (hdiv * info->var.pixclock) / hmul);
> with hdiv=1, pixclock=1 and hmul=2 you end up with (1*1)/2 = (int) 0.
> and then in:
>   drivers/video/fbdev/arkfb.c:504: rv = dac_set_freq(par->dac, 0, 1000000000 / pixclock);
> we'll get a division-by-zero.
>
> The following log can reveal it:
>
> divide error: 0000 [#1] PREEMPT SMP KASAN PTI
> RIP: 0010:ark_set_pixclock drivers/video/fbdev/arkfb.c:504 [inline]
> RIP: 0010:arkfb_set_par+0x10fc/0x24c0 drivers/video/fbdev/arkfb.c:784
> Call Trace:
>  fb_set_var+0x604/0xeb0 drivers/video/fbdev/core/fbmem.c:1034
>  do_fb_ioctl+0x234/0x670 drivers/video/fbdev/core/fbmem.c:1110
>  fb_ioctl+0xdd/0x130 drivers/video/fbdev/core/fbmem.c:1189
>
> Fix this by checking the argument of ark_set_pixclock() first.
>
> Fixes: 681e14730c73 ("arkfb: new framebuffer driver for ARK Logic cards")
> Signed-off-by: Zheyu Ma <zheyuma97 at gmail.com>

applied to fbdev git tree.

Thanks!
Helge

> ---
>  drivers/video/fbdev/arkfb.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/video/fbdev/arkfb.c b/drivers/video/fbdev/arkfb.c
> index eb3e47c58c5f..ed76ddc7df3d 100644
> --- a/drivers/video/fbdev/arkfb.c
> +++ b/drivers/video/fbdev/arkfb.c
> @@ -781,7 +781,12 @@ static int arkfb_set_par(struct fb_info *info)
>  		return -EINVAL;
>  	}
>
> -	ark_set_pixclock(info, (hdiv * info->var.pixclock) / hmul);
> +	value = (hdiv * info->var.pixclock) / hmul;
> +	if (!value) {
> +		fb_dbg(info, "invalid pixclock\n");
> +		value = 1;
> +	}
> +	ark_set_pixclock(info, value);
>  	svga_set_timings(par->state.vgabase, &ark_timing_regs, &(info->var), hmul, hdiv,
>  			 (info->var.vmode & FB_VMODE_DOUBLE)     ? 2 : 1,
>  			 (info->var.vmode & FB_VMODE_INTERLACED) ? 2 : 1,