[FFmpeg-cvslog] avcodec/alac: Check decorr_shift to avoid invalid shift

[Michael Niedermayer]

ffmpeg | branch: master | Michael Niedermayer <michael at niedermayer.cc> | Thu Jul 23 23:41:27 2020 +0200| [4333718b357a9ad195031e5d0ea080d37677b795] | committer: Michael Niedermayer

avcodec/alac: Check decorr_shift to avoid invalid shift

Later the decorrelate_stereo call is guarded by channels == 2
and non-zero decorr_left_weight. Make sure decorr_shift is in
the expected shift range for that case.

Fixes: shift exponent 128 is too large for 32-bit type 'int'
Fixes: 23860/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ALAC_fuzzer-5751138914402304

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Alexander Strasser <eclipse7 at gmx.net>
Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=4333718b357a9ad195031e5d0ea080d37677b795
---

 libavcodec/alac.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/libavcodec/alac.c b/libavcodec/alac.c
index bf05406230..9040673528 100644
--- a/libavcodec/alac.c
+++ b/libavcodec/alac.c
@@ -302,6 +302,9 @@ static int decode_element(AVCodecContext *avctx, AVFrame *frame, int ch_index,
         decorr_shift       = get_bits(&alac->gb, 8);
         decorr_left_weight = get_bits(&alac->gb, 8);
 
+        if (channels == 2 && decorr_left_weight && decorr_shift > 31)
+            return AVERROR_INVALIDDATA;
+
         for (ch = 0; ch < channels; ch++) {
             prediction_type[ch]   = get_bits(&alac->gb, 4);
             lpc_quant[ch]         = get_bits(&alac->gb, 4);