Re: [PATCH] ui: fix DIV_BY_ZERO in tightvnc

[Marc-André Lureau]

Hi

On Wed, Dec 13, 2023 at 11:08 AM Dmitry Frolov <frolov@swemel.ru> wrote:
>
> Division by zero may occur in rare constellation of conditions if:
> 1. not TrueColor mode on the client side
>    tight_detect_smooth_image16() and tight_detect_smooth_image32(),
>    defined by macro DEFINE_DETECT_FUNCTION()2, are affected.
> 2. if all pixels on the screen are equal, then pixels == stats[0]
>
> Found by Linux Verification Center (linuxtesting.org) with SVACE.
>
> Signed-off-by: Dmitry Frolov <frolov@swemel.ru>

What about the tight_detect_smooth_image24() division?
    errors /= (pixels * 3 - stats[0]);

It should probably have a similar safety check.

The code is originally from libvncserver, but they completely changed
their implementation in:
https://github.com/LibVNC/libvncserver/commit/7124b5fbcf0df8db4d3f73023d77af6ea56409e7

otherwise,
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>

> ---
>  ui/vnc-enc-tight.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/ui/vnc-enc-tight.c b/ui/vnc-enc-tight.c
> index 41f559eb83..f1249ab136 100644
> --- a/ui/vnc-enc-tight.c
> +++ b/ui/vnc-enc-tight.c
> @@ -284,6 +284,9 @@ tight_detect_smooth_image24(VncState *vs, int w, int h)
>          for (; c < 256; c++) {                                          \
>              errors += stats[c] * (c * c);                               \
>          }                                                               \
> +        if (pixels == stats[0]) {                                       \
> +            return 0;                                                   \
> +        }                                                               \
>          errors /= (pixels - stats[0]);                                  \
>                                                                          \
>          return errors;                                                  \
> --
> 2.34.1
>