[FFmpeg-devel] [PATCH 2/2] avcodec/lcldec: Check mthread_inlen instead of cliping

[Michael Niedermayer]

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

diff --git a/libavcodec/lcldec.c b/libavcodec/lcldec.c
index c3787b3cbe..ae7426144d 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);
-- 
2.22.0