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 first.

The following log reveals it:

[   43.861711] divide error: 0000 [#1] PREEMPT SMP KASAN PTI
[   43.861737] CPU: 2 PID: 11764 Comm: i740 Not tainted 5.14.0-rc2-00513-gac532c9bbcfb-dirty #224
[   43.861756] RIP: 0010:asiliantfb_check_var+0x4e/0x730
[   43.861843] Call Trace:
[   43.861848]  ? asiliantfb_remove+0x190/0x190
[   43.861858]  fb_set_var+0x2e4/0xeb0
[   43.861866]  ? fb_blank+0x1a0/0x1a0
[   43.861873]  ? lock_acquire+0x1ef/0x530
[   43.861884]  ? lock_release+0x810/0x810
[   43.861892]  ? lock_is_held_type+0x100/0x140
[   43.861903]  ? ___might_sleep+0x1ee/0x2d0
[   43.861914]  ? __mutex_lock+0x620/0x1190
[   43.861921]  ? do_fb_ioctl+0x313/0x700
[   43.861929]  ? mutex_lock_io_nested+0xfa0/0xfa0
[   43.861936]  ? __this_cpu_preempt_check+0x1d/0x30
[   43.861944]  ? _raw_spin_unlock_irqrestore+0x46/0x60
[   43.861952]  ? lockdep_hardirqs_on+0x59/0x100
[   43.861959]  ? _raw_spin_unlock_irqrestore+0x46/0x60
[   43.861967]  ? trace_hardirqs_on+0x6a/0x1c0
[   43.861978]  do_fb_ioctl+0x31e/0x700

Signed-off-by: Zheyu Ma <redacted>
---
Changes in v2:
    - Make commit log more descriptive
---
 drivers/video/fbdev/asiliantfb.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/video/fbdev/asiliantfb.c b/drivers/video/fbdev/asiliantfb.c
index 3e006da47752..84c56f525889 100644
--- a/drivers/video/fbdev/asiliantfb.c
+++ b/drivers/video/fbdev/asiliantfb.c

@@ -227,6 +227,9 @@ static int asiliantfb_check_var(struct fb_var_screeninfo *var,
 {
 	unsigned long Ftarget, ratio, remainder;
 
+	if (!var->pixclock)
+		return -EINVAL;
+
 	ratio = 1000000 / var->pixclock;
 	remainder = 1000000 % var->pixclock;
 	Ftarget = 1000000 * ratio + (1000000 * remainder) / var->pixclock;

-- 
2.17.6