[FFmpeg-devel] [PATCH 2/2] avcodec/siren: Check available bits at frame start

[James Almer]

On 9/27/2021 7:37 PM, Michael Niedermayer wrote:
> Fixes: Timeout
> Fixes: 39089/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MSNSIREN_fuzzer-6677219854909440
> 
> 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/siren.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/libavcodec/siren.c b/libavcodec/siren.c
> index 7f2b4678608..708990d6654 100644
> --- a/libavcodec/siren.c
> +++ b/libavcodec/siren.c
> @@ -718,6 +718,9 @@ static int siren_decode(AVCodecContext *avctx, void *data,
>       if ((ret = init_get_bits8(gb, avpkt->data, avpkt->size)) < 0)
>           return ret;
>   
> +    if (s->sample_rate_bits + 4 > get_bits_left(gb))
> +        return AVERROR_INVALIDDATA;

This is not enough. You'll inevitably get another timeout in the future 
unless you add a get_bits_left(gb) > 0 condition to the loop in 
decode_envelope().
And there should be at least four bits left after decode_envelope() 
returns, so check for that too.

> +
>       skip_bits(gb, s->sample_rate_bits);
>   
>       decode_envelope(s, gb, s->number_of_regions,
>