linux-kernel - Re: [PATCH v6] fbdev: fbmem: Fix the implicit type casting

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Date:   Wed, 2 Feb 2022 17:07:37 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Yizhuo Zhai <yzhai003@....edu>
Cc:     Helge Deller <deller@....de>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Matthew Wilcox <willy@...radead.org>,
        Sam Ravnborg <sam@...nborg.org>,
        Xiyu Yang <xiyuyang19@...an.edu.cn>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Alex Deucher <alexander.deucher@....com>,
        Zhen Lei <thunder.leizhen@...wei.com>,
        Zheyu Ma <zheyuma97@...il.com>, linux-fbdev@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6] fbdev: fbmem: Fix the implicit type casting

On Wed, Feb 02, 2022 at 03:58:09PM -0800, Yizhuo Zhai wrote:
> In function do_fb_ioctl(), the "arg" is the type of unsigned long,
> and in "case FBIOBLANK:" this argument is casted into an int before
> passig to fb_blank(). In fb_blank(), the comparision
> if (blank > FB_BLANK_POWERDOWN) would be bypass if the original
> "arg" is a large number, which is possible because it comes from
> the user input. Fix this by adding the check before the function
> call.
> 
> Signed-off-by: Yizhuo Zhai <yzhai003@....edu>

Reviewed-by: Guenter Roeck <linux@...ck-us.net>

On a side note, change logs would be useful.

Guenter

> ---
>  drivers/video/fbdev/core/fbmem.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
> index 0fa7ede94fa6..13083ad8d751 100644
> --- a/drivers/video/fbdev/core/fbmem.c
> +++ b/drivers/video/fbdev/core/fbmem.c
> @@ -1160,6 +1160,8 @@ static long do_fb_ioctl(struct fb_info *info, unsigned int cmd,
>  		ret = fbcon_set_con2fb_map_ioctl(argp);
>  		break;
>  	case FBIOBLANK:
> +		if (arg > FB_BLANK_POWERDOWN)
> +			return -EINVAL;
>  		console_lock();
>  		lock_fb_info(info);
>  		ret = fb_blank(info, arg);
> -- 
> 2.25.1
>