[FFmpeg-cvslog] avcodec/msmpeg4dec: Skip frame if its smaller than 1/8 of the minimal size

[Michael Niedermayer]

ffmpeg | branch: release/3.4 | Michael Niedermayer <michael at niedermayer.cc> | Thu Nov 29 02:32:10 2018 +0100| [f06485063df089145ad91f6e53d17587bcaf0fb6] | committer: Michael Niedermayer

avcodec/msmpeg4dec: Skip frame if its smaller than 1/8 of the minimal size

Frames that small are not valid and of limited use for error concealment, while
being very computationally intensive to process.

Fixes: Timeout
Fixes: 11318/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MSMPEG4V1_fuzzer-5710884555456512

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
(cherry picked from commit 09ec182864d41c990bc18f620eabb77444aeff57)
Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=f06485063df089145ad91f6e53d17587bcaf0fb6
---

 libavcodec/msmpeg4dec.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/libavcodec/msmpeg4dec.c b/libavcodec/msmpeg4dec.c
index 4105d4ba7d..2de308f608 100644
--- a/libavcodec/msmpeg4dec.c
+++ b/libavcodec/msmpeg4dec.c
@@ -409,6 +409,14 @@ int ff_msmpeg4_decode_picture_header(MpegEncContext * s)
 {
     int code;
 
+    // at minimum one bit per macroblock is required at least in a valid frame,
+    // we discard frames much smaller than this. Frames smaller than 1/8 of the
+    // smallest "black/skip" frame generally contain not much recoverable content
+    // while at the same time they have the highest computational requirements
+    // per byte
+    if (get_bits_left(&s->gb) * 8LL < (s->width+15)/16 * ((s->height+15)/16))
+        return AVERROR_INVALIDDATA;
+
     if(s->msmpeg4_version==1){
         int start_code = get_bits_long(&s->gb, 32);
         if(start_code!=0x00000100){