linux-kernel - Re: [PATCH 7/7] video: fbdev: s3fb: Error out if 'pixclock' equals zero

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Date:   Mon, 4 Apr 2022 17:52:29 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Zheyu Ma <zheyuma97@...il.com>
Cc:     Helge Deller <deller@....de>,
        Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 7/7] video: fbdev: s3fb: Error out if 'pixclock' equals zero

On Mon, Apr 4, 2022 at 3:50 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 in s3fb_check_var().
>
> The following log reveals it:
>
> [  511.141561] divide error: 0000 [#1] PREEMPT SMP KASAN PTI
> [  511.141607] RIP: 0010:s3fb_check_var+0x3f3/0x530
> [  511.141693] Call Trace:
> [  511.141695]  <TASK>
> [  511.141716]  fb_set_var+0x367/0xeb0
> [  511.141815]  do_fb_ioctl+0x234/0x670
> [  511.141876]  fb_ioctl+0xdd/0x130
> [  511.141888]  do_syscall_64+0x3b/0x90
>
> Signed-off-by: Zheyu Ma <zheyuma97@...il.com>
> ---
>  drivers/video/fbdev/s3fb.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/drivers/video/fbdev/s3fb.c b/drivers/video/fbdev/s3fb.c
> index 5c74253e7b2c..b93c8eb02336 100644
> --- a/drivers/video/fbdev/s3fb.c
> +++ b/drivers/video/fbdev/s3fb.c
> @@ -549,6 +549,9 @@ static int s3fb_check_var(struct fb_var_screeninfo *var, struct fb_info *info)
>         int rv, mem, step;
>         u16 m, n, r;
>
> +       if (!var->pixclock)
> +               return -EINVAL;
> +

When passed an invalid value, .check_var() is supposed to
round up the invalid value to a valid value, if possible.

>         /* Find appropriate format */
>         rv = svga_match_format (s3fb_formats, var, NULL);

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