linux-kernel - Re: [PATCH v5] fbdev: fbmem: Fix the implicit type casting

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Date:   Wed, 2 Feb 2022 15:45:27 -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>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Zhen Lei <thunder.leizhen@...wei.com>,
        Zheyu Ma <zheyuma97@...il.com>,
        Alex Deucher <alexander.deucher@....com>,
        Xiyu Yang <xiyuyang19@...an.edu.cn>,
        linux-fbdev@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5] fbdev: fbmem: Fix the implicit type casting

On 2/2/22 15:33, 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>
> ---
>   drivers/video/fbdev/core/fbmem.c | 5 +++++
>   1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
> index 0fa7ede94fa6..d5dec24c4d16 100644
> --- a/drivers/video/fbdev/core/fbmem.c
> +++ b/drivers/video/fbdev/core/fbmem.c
> @@ -1162,6 +1162,11 @@ static long do_fb_ioctl(struct fb_info *info, unsigned int cmd,
>   	case FBIOBLANK:
>   		console_lock();
>   		lock_fb_info(info);
> +		if (arg > FB_BLANK_POWERDOWN) {
> +			unlock_fb_info(info);
> +			console_unlock();
> +			return -EINVAL;
> +		}

Is it really necessary to perform this check within the lock ?
arg is a passed in value, and FB_BLANK_POWERDOWN.
It seems to me that

	case FBIOBLANK:
		if (arg > FB_BLANK_POWERDOWN)
			return -EINVAL;
		console_lock();
		...

should be sufficient.

Sorry if I missed some previous discussion; this is the first time
I looked at this patch.

Guenter