[FFmpeg-cvslog] avcodec/pngdec: Check length in fdAT

[Michael Niedermayer]

ffmpeg | branch: release/2.8 | Michael Niedermayer <michael at niedermayer.cc> | Tue Apr  7 19:29:44 2020 +0200| [8a359e077f6b01add8a809239a98642f6f338df5] | committer: Michael Niedermayer

avcodec/pngdec: Check length in fdAT

Fixes: 21089/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_APNG_fuzzer-5135981419429888
Fixes: out of array read

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 79e5c2ee2bbdf462cabd2113c723dfb613d735c2)
Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=8a359e077f6b01add8a809239a98642f6f338df5
---

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

diff --git a/libavcodec/pngdec.c b/libavcodec/pngdec.c
index 78cf274128..3fa4e4e4c0 100644
--- a/libavcodec/pngdec.c
+++ b/libavcodec/pngdec.c
@@ -1161,7 +1161,7 @@ static int decode_frame_common(AVCodecContext *avctx, PNGDecContext *s,
         case MKTAG('f', 'd', 'A', 'T'):
             if (!CONFIG_APNG_DECODER || avctx->codec_id != AV_CODEC_ID_APNG)
                 goto skip_tag;
-            if (!decode_next_dat) {
+            if (!decode_next_dat || length < 4) {
                 ret = AVERROR_INVALIDDATA;
                 goto fail;
             }