[FFmpeg-devel] [PATCH] avcodec/mobiclip: Check that Motion vectors are within the input frame

[Paul B Mahol]

On Fri, Oct 02, 2020 at 11:07:09PM +0200, Michael Niedermayer wrote:
> The MV checks did not consider the width and height of the block, also they
> had some off by 1 errors. This resulted in undefined behavior and crashes.
> This commit instead errors out on these
> 
> Fixes: out of array read
> Fixes: 26080/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MOBICLIP_fuzzer-5758146355920896
> 
> Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
> Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
> ---
>  libavcodec/mobiclip.c | 30 ++++++------------------------
>  1 file changed, 6 insertions(+), 24 deletions(-)
> 
> diff --git a/libavcodec/mobiclip.c b/libavcodec/mobiclip.c
> index f890cb2599..a03ef8fb7d 100644
> --- a/libavcodec/mobiclip.c
> +++ b/libavcodec/mobiclip.c
> @@ -1189,14 +1189,14 @@ static int predict_motion(AVCodecContext *avctx,
>              dst_linesize = s->pic[s->current_pic]->linesize[i];
>              dst = s->pic[s->current_pic]->data[i] + offsetx + offsety * dst_linesize;
>  
> +            if (offsetx + (mv.x >> 1) < 0 ||
> +                offsety + (mv.y >> 1) < 0 ||
> +                offsetx + width  + (mv.x + 1 >> 1) > fwidth ||
> +                offsety + height + (mv.y + 1 >> 1) > fheight)
> +                return AVERROR_INVALIDDATA;
> +
>              switch (method) {
>              case 0:
> -                if (offsety + (mv.y >> 1) < 0 ||
> -                    offsety + (mv.y >> 1) >= fheight ||
> -                    offsetx + (mv.x >> 1) < 0 ||
> -                    offsetx + (mv.x >> 1) >= fwidth)
> -                    return AVERROR_INVALIDDATA;
> -
>                  src = s->pic[sidx]->data[i] + offsetx + (mv.x >> 1) +
>                                 (offsety + (mv.y >> 1)) * src_linesize;
>                  for (int y = 0; y < height; y++) {
> @@ -1207,12 +1207,6 @@ static int predict_motion(AVCodecContext *avctx,
>                  }
>                  break;
>              case 1:
> -                if (offsety + (mv.y >> 1) < 0 ||
> -                    offsety + (mv.y >> 1) >= fheight ||
> -                    offsetx + (mv.x >> 1) < 0 ||
> -                    offsetx + (mv.x >> 1) >= fwidth)
> -                    return AVERROR_INVALIDDATA;
> -
>                  src = s->pic[sidx]->data[i] + offsetx + (mv.x >> 1) +
>                                 (offsety + (mv.y >> 1)) * src_linesize;
>                  for (int y = 0; y < height; y++) {
> @@ -1225,12 +1219,6 @@ static int predict_motion(AVCodecContext *avctx,
>                  }
>                  break;
>              case 2:
> -                if (offsety + (mv.y >> 1) < 0 ||
> -                    offsety + (mv.y >> 1) >= fheight - 1 ||
> -                    offsetx + (mv.x >> 1) < 0 ||
> -                    offsetx + (mv.x >> 1) >= fwidth)
> -                    return AVERROR_INVALIDDATA;
> -
>                  src = s->pic[sidx]->data[i] + offsetx + (mv.x >> 1) +
>                                 (offsety + (mv.y >> 1)) * src_linesize;
>                  for (int y = 0; y < height; y++) {
> @@ -1243,12 +1231,6 @@ static int predict_motion(AVCodecContext *avctx,
>                  }
>                  break;
>              case 3:
> -                if (offsety + (mv.y >> 1) < 0 ||
> -                    offsety + (mv.y >> 1) >= fheight - 1 ||
> -                    offsetx + (mv.x >> 1) < 0 ||
> -                    offsetx + (mv.x >> 1) >= fwidth)
> -                    return AVERROR_INVALIDDATA;
> -

You completely ignore this case, here it need one scanline extra.

>                  src = s->pic[sidx]->data[i] + offsetx + (mv.x >> 1) +
>                                 (offsety + (mv.y >> 1)) * src_linesize;
>                  for (int y = 0; y < height; y++) {
> -- 
> 2.17.1
> 
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