2024-03-05 08:21:40

by Roman Smirnov

Subject: [PATCH] fbmon: prevent division by zero in fb_videomode_from_videomode()

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 <[email protected]>
Reviewed-by: Sergey Shtylyov <[email protected]>
---
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;
/* a mode must have htotal and vtotal != 0 or it is invalid */
} else {
fbmode->refresh = 0;
--
2.34.1

2024-03-05 10:32:51

by Thomas Zimmermann

Subject: Re: [PATCH] fbmon: prevent division by zero in fb_videomode_from_videomode()

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 <[email protected]>
> Reviewed-by: Sergey Shtylyov <[email protected]>
> ---
> 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)