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

In the Linux kernel, the following vulnerability has been resolved:

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

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.