avcodec/gifdec: factorize interleave end handling out

also change it to a loop
Fixes out of array access
Fixes: asan_heap-oob_ca5410_8_asan_heap-oob_ca5410_97_ID_LSD_Size_Less_Then_Data_Inter_3.gif

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 8f1457864be8fb9653643519dea1c6492f1dde57)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

diff --git a/libavcodec/gifdec.c b/libavcodec/gifdec.c
index 39d0db9..1c9358f 100644 (file)
--- a/libavcodec/gifdec.c
+++ b/libavcodec/gifdec.c
@@ -125,26 +125,21 @@ static int gif_read_image(GifState *s)
             case 1:
                 y1 += 8;
                 ptr += linesize * 8;
-                if (y1 >= height) {
-                    y1 = pass ? 2 : 4;
-                    ptr = ptr1 + linesize * y1;
-                    pass++;
-                }
                 break;
             case 2:
                 y1 += 4;
                 ptr += linesize * 4;
-                if (y1 >= height) {
-                    y1 = 1;
-                    ptr = ptr1 + linesize;
-                    pass++;
-                }
                 break;
             case 3:
                 y1 += 2;
                 ptr += linesize * 2;
                 break;
             }
+            while (y1 >= height) {
+                y1 = 4 >> pass;
+                ptr = ptr1 + linesize * y1;
+                pass++;
+            }
         } else {
             ptr += linesize;
         }