[FFmpeg-devel] [PATCH 2/2] avcodec/cbs_h2645: Avoid more cases of 0 size slices

[Andreas Rheinhardt]

Michael Niedermayer:
> Fixes: Assertion failure
> Fixes: 19629/clusterfuzz-testcase-minimized-ffmpeg_BSF_H264_METADATA_fuzzer-5676822528524288
> 
> 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/cbs_h2645.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/libavcodec/cbs_h2645.c b/libavcodec/cbs_h2645.c
> index c8347ba5fa..7427eefa30 100644
> --- a/libavcodec/cbs_h2645.c
> +++ b/libavcodec/cbs_h2645.c
> @@ -870,7 +870,8 @@ static int cbs_h264_read_nal_unit(CodedBitstreamContext *ctx,
>                         "from slice data.\n", z);
>                  len -= z;
>              }
> -
> +            if (len <= pos / 8)
> +                return AVERROR_INVALIDDATA;
>              slice->data_size = len - pos / 8;
>              slice->data_ref  = av_buffer_ref(unit->data_ref);
>              if (!slice->data_ref)
> @@ -1052,7 +1053,8 @@ static int cbs_h265_read_nal_unit(CodedBitstreamContext *ctx,
>                         "from slice data.\n", z);
>                  len -= z;
>              }
> -
> +            if (len <= pos / 8)
> +                return AVERROR_INVALIDDATA;
>              slice->data_size = len - pos / 8;
>              slice->data_ref  = av_buffer_ref(unit->data_ref);
>              if (!slice->data_ref)
> 

Imagine you have CAVLC encoded H.264 where there is nothing after the
header except zero bits and where the slice header is not
byte-aligned. Then av_assert0(temp) in cbs_h2645_write_slice_data()
will be triggered both now as well as with your patch, because your
check is too crude.

I have already sent a patch [2] for this. It presupposes [1].

- Andreas

[1]:
https://patchwork.ffmpeg.org/project/ffmpeg/patch/20191209222604.28920-5-andreas.rheinhardt@gmail.com/
[2]:
https://patchwork.ffmpeg.org/project/ffmpeg/patch/20191209222604.28920-6-andreas.rheinhardt@gmail.com/