[FFmpeg-cvslog] avcodec/shorten: Fix bitstream end check in read_header()

[Michael Niedermayer]

ffmpeg | branch: release/3.4 | Michael Niedermayer <michael at niedermayer.cc> | Sat Sep 15 02:08:20 2018 +0200| [98709a12448283bff56a03f436c735d2f58e7b3e] | committer: Michael Niedermayer

avcodec/shorten: Fix bitstream end check in read_header()

Fixes: Timeout
Fixes: 9961/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_SHORTEN_fuzzer-5687856176562176

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Paul B Mahol <onemda at gmail.com>
Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
(cherry picked from commit 28b80c2d52d82eb4f73af5f818dab60946bcf299)
Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=98709a12448283bff56a03f436c735d2f58e7b3e
---

 libavcodec/shorten.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libavcodec/shorten.c b/libavcodec/shorten.c
index 1ffb7d8d79..4b45e6d6dc 100644
--- a/libavcodec/shorten.c
+++ b/libavcodec/shorten.c
@@ -456,7 +456,7 @@ static int read_header(ShortenContext *s)
         }
 
         skip_bytes = get_uint(s, NSKIPSIZE);
-        if ((unsigned)skip_bytes > get_bits_left(&s->gb)/8) {
+        if ((unsigned)skip_bytes > FFMAX(get_bits_left(&s->gb), 0)/8) {
             av_log(s->avctx, AV_LOG_ERROR, "invalid skip_bytes: %d\n", skip_bytes);
             return AVERROR_INVALIDDATA;
         }