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SubjectRe: [PATCH] video: fbdev: cirrusfb: check pixclock to avoid divide by zero
On Mon, Oct 25, 2021 at 02:01:30PM -0500, George Kennedy wrote:
> Do a sanity check on pixclock value before using it as a divisor.
>
> Syzkaller reported a divide error in cirrusfb_check_pixclock.
>
> divide error: 0000 [#1] SMP KASAN PTI
> CPU: 0 PID: 14938 Comm: cirrusfb_test Not tainted 5.15.0-rc6 #1
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.11.0-2
> RIP: 0010:cirrusfb_check_var+0x6f1/0x1260
>
> Call Trace:
> fb_set_var+0x398/0xf90
> do_fb_ioctl+0x4b8/0x6f0
> fb_ioctl+0xeb/0x130
> __x64_sys_ioctl+0x19d/0x220
> do_syscall_64+0x3a/0x80
> entry_SYSCALL_64_after_hwframe+0x44/0xae
>
> Signed-off-by: George Kennedy <george.kennedy@oracle.com>
> ---
> drivers/video/fbdev/cirrusfb.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/drivers/video/fbdev/cirrusfb.c b/drivers/video/fbdev/cirrusfb.c
> index 93802ab..099ddcb 100644
> --- a/drivers/video/fbdev/cirrusfb.c
> +++ b/drivers/video/fbdev/cirrusfb.c
> @@ -477,6 +477,9 @@ static int cirrusfb_check_pixclock(const struct fb_var_screeninfo *var,
> struct cirrusfb_info *cinfo = info->par;
> unsigned maxclockidx = var->bits_per_pixel >> 3;
>
> + if (!var->pixclock)
> + return -EINVAL;

Shouldn't you be checking further up the call chain where this got set
to 0?

What logic allows this to be a valid value? What about all other fb
drivers?

thanks,

greg k-h

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