[FFmpeg-cvslog] avcodec/pngdec: Check deloco index more exactly

[Michael Niedermayer]

ffmpeg | branch: release/4.2 | Michael Niedermayer <michael at niedermayer.cc> | Wed Jan 11 23:26:06 2023 +0100| [6875531f44d7bb7e0c03f6d26173b1aff46e027a] | committer: Michael Niedermayer

avcodec/pngdec: Check deloco index more exactly

Fixes: out of array access:
Fixes: 48567/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_PNG_fuzzer-6716193709096960

Alternatively it should be possible to limit this to 3 plane RGB 8 /16bit to ensure the size is what it should be

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 d5bae704068dc37191280e024eecb8d02b762b28)
Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=6875531f44d7bb7e0c03f6d26173b1aff46e027a
---

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

diff --git a/libavcodec/pngdec.c b/libavcodec/pngdec.c
index dd349a4a50..d80d0f5908 100644
--- a/libavcodec/pngdec.c
+++ b/libavcodec/pngdec.c
@@ -309,7 +309,7 @@ static void png_filter_row(PNGDSPContext *dsp, uint8_t *dst, int filter_type,
 static void deloco_ ## NAME(TYPE *dst, int size, int alpha) \
 { \
     int i; \
-    for (i = 0; i < size; i += 3 + alpha) { \
+    for (i = 0; i < size - 2; i += 3 + alpha) { \
         int g = dst [i + 1]; \
         dst[i + 0] += g; \
         dst[i + 2] += g; \