[FFmpeg-cvslog] avcodec/lcldec: Check mthread_inlen instead of cliping

[Michael Niedermayer]

ffmpeg | branch: master | Michael Niedermayer <michael at niedermayer.cc> | Sat Jul 27 23:24:25 2019 +0200| [4d4734bdc881de3af0ebe5935890a81423c80fdf] | committer: Michael Niedermayer

avcodec/lcldec: Check mthread_inlen instead of cliping

Clipping was added in 2009 to avoid crashes.
The clipped case would produce a 2nd slice with 0 input
thus also producing 0 output.
Subsequent checks will cause decoder failure unless both
slices have the same output length. thus the only way this
would not already fail is if the output from both slices
was 0 bytes.

Fixes: Timeout (134sec -> 241ms)
Fixes: 15599/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MSZH_fuzzer-5658127116009472

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

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=4d4734bdc881de3af0ebe5935890a81423c80fdf
---

 libavcodec/lcldec.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/libavcodec/lcldec.c b/libavcodec/lcldec.c
index 104defa5f5..046cdc4f8e 100644
--- a/libavcodec/lcldec.c
+++ b/libavcodec/lcldec.c
@@ -190,11 +190,10 @@ static int decode_frame(AVCodecContext *avctx, void *data, int *got_frame, AVPac
                 ;
             } else if (c->flags & FLAG_MULTITHREAD) {
                 mthread_inlen = AV_RL32(buf);
-                if (len < 8) {
+                if (len < 8 || len - 8 < mthread_inlen) {
                     av_log(avctx, AV_LOG_ERROR, "len %d is too small\n", len);
                     return AVERROR_INVALIDDATA;
                 }
-                mthread_inlen = FFMIN(mthread_inlen, len - 8);
                 mthread_outlen = AV_RL32(buf + 4);
                 mthread_outlen = FFMIN(mthread_outlen, c->decomp_size);
                 mszh_dlen = mszh_decomp(buf + 8, mthread_inlen, c->decomp_buf, c->decomp_size);