linux-kernel - RE: [PATCH v2] fbdev: fbmem: Fix the implicit type casting

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Date:   Wed, 2 Feb 2022 09:02:52 +0000
From:   David Laight <David.Laight@...LAB.COM>
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>,
        "Zhen Lei" <thunder.leizhen@...wei.com>,
        Guenter Roeck <linux@...ck-us.net>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Zheyu Ma <zheyuma97@...il.com>,
        Alex Deucher <alexander.deucher@....com>,
        Xiyu Yang <xiyuyang19@...an.edu.cn>,
        "linux-fbdev@...r.kernel.org" <linux-fbdev@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v2] fbdev: fbmem: Fix the implicit type casting

From: Yizhuo Zhai
> Sent: 01 February 2022 02:36
> 
> 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.

Doesn't this convert invalid values (> FB_BLANK_POWERDOWN)
that should generate errors into valid requests?

	David

> 
> Signed-off-by: Yizhuo Zhai <yzhai003@....edu>
> ---
>  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..f08326efff54 100644
> --- a/drivers/video/fbdev/core/fbmem.c
> +++ b/drivers/video/fbdev/core/fbmem.c
> @@ -1162,6 +1162,8 @@ static long do_fb_ioctl(struct fb_info *info, unsigned int cmd,
>  	case FBIOBLANK:
>  		console_lock();
>  		lock_fb_info(info);
> +		if (blank > FB_BLANK_POWERDOWN)
> +			blank = FB_BLANK_POWERDOWN;
>  		ret = fb_blank(info, arg);
>  		/* might again call into fb_blank */
>  		fbcon_fb_blanked(info, arg);
> --
> 2.25.1

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