vc1: check that slices have a positive height

Fixes possible invalid reads.

CC: libav-stable@libav.org
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind

diff --git a/libavcodec/vc1dec.c b/libavcodec/vc1dec.c
index 3d042d0b0d5502fccfe500e258ce05016f3068f5..b26fbf2efffce74e09a4027d12afc3ac12e4bd99 100644 (file)
--- a/libavcodec/vc1dec.c
+++ b/libavcodec/vc1dec.c
@@ -881,6 +881,12 @@ static int vc1_decode_frame(AVCodecContext *avctx, void *data,
                 s->end_mb_y = (i == n_slices     ) ? mb_height : FFMIN(mb_height, slices[i].mby_start % mb_height);
             else
                 s->end_mb_y = (i <= n_slices1 + 1) ? mb_height : FFMIN(mb_height, slices[i].mby_start % mb_height);
+
+            if (s->end_mb_y <= s->start_mb_y) {
+                av_log(v->s.avctx, AV_LOG_ERROR, "Invalid slice size\n");
+                goto err;
+            }
+
             ff_vc1_decode_blocks(v);
             if (i != n_slices)
                 s->gb = slices[i].gb;