[FFmpeg-devel] [PATCH] apng: use correct size for output buffer

[wm4]

On Fri, 6 Nov 2015 22:18:04 +0100
Andreas Cadhalpun <andreas.cadhalpun at googlemail.com> wrote:

> This fixes a stack buffer overflow.
> 
> Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun at googlemail.com>
> ---
>  libavcodec/pngdec.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/libavcodec/pngdec.c b/libavcodec/pngdec.c
> index 689aa2b..c974654 100644
> --- a/libavcodec/pngdec.c
> +++ b/libavcodec/pngdec.c
> @@ -1010,13 +1010,13 @@ static int handle_p_frame_apng(AVCodecContext *avctx, PNGDecContext *s,
>              memcpy(buffer + row_start, p->data[0] + row_start, s->bpp * s->cur_w);
>          }
>      } else { // APNG_BLEND_OP_OVER
> +        uint8_t *output = av_malloc(s->bpp);
>          for (y = s->y_offset; y < s->y_offset + s->cur_h; ++y) {
>              uint8_t *foreground = p->data[0] + s->image_linesize * y + s->bpp * s->x_offset;
>              uint8_t *background = buffer + s->image_linesize * y + s->bpp * s->x_offset;
>              for (x = s->x_offset; x < s->x_offset + s->cur_w; ++x, foreground += s->bpp, background += s->bpp) {
>                  size_t b;
>                  uint8_t foreground_alpha, background_alpha, output_alpha;
> -                uint8_t output[4];
>  
>                  // Since we might be blending alpha onto alpha, we use the following equations:
>                  // output_alpha = foreground_alpha + (1 - foreground_alpha) * background_alpha
> @@ -1069,6 +1069,7 @@ static int handle_p_frame_apng(AVCodecContext *avctx, PNGDecContext *s,
>                  memcpy(background, output, s->bpp);
>              }
>          }
> +        av_freep(&output);
>      }
>  
>      // Copy blended buffer into the frame and free

This seems wasteful, can't it just be output[8]?

It also adds a bug (unchecked malloc).