[FFmpeg-devel] [PATCH 1/4] avcodec/mpc8: Fixes invalid shift in mpc8_decode_frame()

[James Almer]

On 7/11/2019 6:49 PM, Michael Niedermayer wrote:
> Fixes: left shift of negative value -456
> Fixes: 15561/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MPC8_fuzzer-5758130404720640
> 
> 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/mpc8.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/libavcodec/mpc8.c b/libavcodec/mpc8.c
> index 3be2f79a5a..75943064da 100644
> --- a/libavcodec/mpc8.c
> +++ b/libavcodec/mpc8.c
> @@ -364,7 +364,7 @@ static int mpc8_decode_frame(AVCodecContext * avctx, void *data,
>                  for(j = 0; j < SAMPLES_PER_BAND; j += SAMPLES_PER_BAND / 2){
>                      cnt = get_vlc2(gb, q1_vlc.table, MPC8_Q1_BITS, 2);
>                      t = mpc8_get_mask(gb, 18, cnt);
> -                    for(k = 0; k < SAMPLES_PER_BAND / 2; k++, t <<= 1)
> +                    for(k = 0; k < SAMPLES_PER_BAND / 2; k++, t += (unsigned)t)

How about something like this instead (Untested)

for(k = 0; k < SAMPLES_PER_BAND / 2; k++)
    c->Q[ch][off + j + k] = t & (1 << (SAMPLES_PER_BAND / 2 - k - 1))
                            ? (get_bits1(gb) << 1) - 1 : 0;

>                          c->Q[ch][off + j + k] = (t & 0x20000) ? (get_bits1(gb) << 1) - 1 : 0;
>                  }
>                  break;
>