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Message-ID: <a4a77ad1-1f3c-45ca-9385-436a8bfe5bc3@suse.de>
Date: Tue, 5 Mar 2024 11:18:05 +0100
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Subject: Re: [PATCH] fbmon: prevent division by zero in
 fb_videomode_from_videomode()
Content-Language: en-US
To: Roman Smirnov <r.smirnov@omp.ru>, Daniel Vetter <daniel@ffwll.ch>,
 Helge Deller <deller@gmx.de>
Cc: Sergey Shtylyov <s.shtylyov@omp.ru>, Karina Yankevich
 <k.yankevich@omp.ru>, linux-fbdev@vger.kernel.org,
 dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
 lvc-project@linuxtesting.org
References: <20240305082040.7445-1-r.smirnov@omp.ru>
From: Thomas Zimmermann <tzimmermann@suse.de>
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In-Reply-To: <20240305082040.7445-1-r.smirnov@omp.ru>
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Hi

Am 05.03.24 um 09:20 schrieb Roman Smirnov:
> The expression htotal * vtotal can have a zero value
> on overflow. It is necessary to prevent division by
> zero like in fb_var_to_videomode().
>
> Found by Linux Verification Center (linuxtesting.org) with Svace.
>
> Signed-off-by: Roman Smirnov <r.smirnov@omp.ru>
> Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
> ---
>   drivers/video/fbdev/core/fbmon.c | 5 +++--
>   1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/video/fbdev/core/fbmon.c b/drivers/video/fbdev/core/fbmon.c
> index 79e5bfbdd34c..bd98b138da6a 100644
> --- a/drivers/video/fbdev/core/fbmon.c
> +++ b/drivers/video/fbdev/core/fbmon.c
> @@ -1311,7 +1311,7 @@ int fb_get_mode(int flags, u32 val, struct fb_var_screeninfo *var, struct fb_inf
>   int fb_videomode_from_videomode(const struct videomode *vm,
>   				struct fb_videomode *fbmode)
>   {
> -	unsigned int htotal, vtotal;
> +	unsigned int htotal, vtotal, hfreq;
>   
>   	fbmode->xres = vm->hactive;
>   	fbmode->left_margin = vm->hback_porch;
> @@ -1345,7 +1345,8 @@ int fb_videomode_from_videomode(const struct videomode *vm,
>   		 vm->vsync_len;
>   	/* prevent division by zero */
>   	if (htotal && vtotal) {
> -		fbmode->refresh = vm->pixelclock / (htotal * vtotal);
> +		hfreq = vm->pixelclock / htotal;
> +		fbmode->refresh = hfreq / vtotal;

I think this can change the end result because of integer rounding on 
the intermediate result. Maybe use

   if (htotal && vtotal && (vm->pixelclock / htotal >= vtotal))

for the test. That rules out overflowing multiplication and sets refresh 
to 0 in such cases.

Best regards
Thomas

>   	/* a mode must have htotal and vtotal != 0 or it is invalid */
>   	} else {
>   		fbmode->refresh = 0;

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)