linux-kernel - Re: [PATCH v2 1/3] video: fbdev: asiliantfb: Error out if 'pixclock' equals zero

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Date:   Fri, 10 Sep 2021 17:16:44 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Zheyu Ma <zheyuma97@...il.com>
Cc:     "Antonino A. Daplas" <adaplas@...il.com>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH v2 1/3] video: fbdev: asiliantfb: Error out if 'pixclock'
 equals zero

Hi Zheyu,

On Mon, Jul 26, 2021 at 12:04 PM Zheyu Ma <zheyuma97@...il.com> 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 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 <zheyuma97@...il.com>

Thanks for your patch!

> ---
> 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;

While this fixes the crash, it is not correct: according to the
fbdev API, invalid values must be rounded up to a supported value,
if possible.  -EINVAL should only be returned if rounding up values
in fb_var_screeninfo cannot give a valid mode.

The same comment applies to the other patches in this series:
[PATCH v2 2/3] video: fbdev: kyro: Error out if 'pixclock' equals zero
[PATCH v2 3/3] video: fbdev: riva: Error out if 'pixclock' equals zero

> +
>         ratio = 1000000 / var->pixclock;
>         remainder = 1000000 % var->pixclock;
>         Ftarget = 1000000 * ratio + (1000000 * remainder) / var->pixclock;

Gr{oetje,eeting}s,

                        Geert


--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds