[FFmpeg-cvslog] avcodec/mpegvideo_dec: Fix lowres=3 field select interlaced mpeg4 frame

[Michael Niedermayer]

ffmpeg | branch: release/6.1 | Michael Niedermayer <michael at niedermayer.cc> | Sun Jul 13 01:17:02 2025 +0200| [e8a2d5fca3c00c9ff6f7a6f14804c2013d25dc82] | committer: Michael Niedermayer

avcodec/mpegvideo_dec: Fix lowres=3 field select interlaced mpeg4 frame

Fixes: out of array read in the chroma plane
Fixes: 428034092/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MPEG4_DEC_fuzzer-5582608941776896.test

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 b849ac006b667dbd494a28de2f8b059fec308ac2)
Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=e8a2d5fca3c00c9ff6f7a6f14804c2013d25dc82
---

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

diff --git a/libavcodec/mpegvideo_dec.c b/libavcodec/mpegvideo_dec.c
index f4b7fc367c..c8011bc0b3 100644
--- a/libavcodec/mpegvideo_dec.c
+++ b/libavcodec/mpegvideo_dec.c
@@ -705,7 +705,7 @@ static av_always_inline void mpeg_motion_lowres(MpegEncContext *s,
     ptr_cr = ref_picture[2] + uvsrc_y * uvlinesize + uvsrc_x;
 
     if ((unsigned) src_x > FFMAX( h_edge_pos - (!!sx) - 2 * block_s,       0) || uvsrc_y<0 ||
-        (unsigned) src_y > FFMAX((v_edge_pos >> field_based) - (!!sy) - FFMAX(h, hc<<s->chroma_y_shift), 0)) {
+        (unsigned) src_y > FFMAX((v_edge_pos >> field_based) - (!!sy) - FFMAX(h, field_select + hc<<s->chroma_y_shift), 0)) {
         s->vdsp.emulated_edge_mc(s->sc.edge_emu_buffer, ptr_y,
                                  linesize >> field_based, linesize >> field_based,
                                  17, 17 + field_based,