avcodec/msrledec: restructure msrle_decode_pal4() based on the line number instead of the pixel pointer

Fixes out of array access
Fixes: da14e86d8462be6493eab16bc2d40f88/asan_heap-oob_204cfd2_528_cov_340150052_COMPRESS.BMP

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit f7e1367f58)

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

libavcodec/msrledec.c

@@ -37,16 +37,14 @@ static int msrle_decode_pal4(AVCodecContext *avctx, AVPicture *pic,
     unsigned char extra_byte, odd_pixel;
     unsigned char stream_byte;
     unsigned int pixel_ptr = 0;
-    int row_dec = pic->linesize[0];
-    int row_ptr = (avctx->height - 1) * row_dec;
-    int frame_size = row_dec * avctx->height;
+    int line = avctx->height - 1;
     int i;
 
-    while (row_ptr >= 0) {
+    while (line >= 0 && pixel_ptr <= avctx->width) {
         if (bytestream2_get_bytes_left(gb) <= 0) {
             av_log(avctx, AV_LOG_ERROR,
-                   "MS RLE: bytestream overrun, %d rows left\n",
-                   row_ptr);
+                   "MS RLE: bytestream overrun, %dx%d left\n",
+                   avctx->width - pixel_ptr, line);
             return AVERROR_INVALIDDATA;
         }
         rle_code = stream_byte = bytestream2_get_byteu(gb);
@@ -55,7 +53,7 @@ static int msrle_decode_pal4(AVCodecContext *avctx, AVPicture *pic,
             stream_byte = bytestream2_get_byte(gb);
             if (stream_byte == 0) {
                 /* line is done, goto the next one */
-                row_ptr -= row_dec;
+                line--;
                 pixel_ptr = 0;
             } else if (stream_byte == 1) {
                 /* decode is done */
@@ -65,13 +63,12 @@ static int msrle_decode_pal4(AVCodecContext *avctx, AVPicture *pic,
                 stream_byte = bytestream2_get_byte(gb);
                 pixel_ptr += stream_byte;
                 stream_byte = bytestream2_get_byte(gb);
-                row_ptr -= stream_byte * row_dec;
             } else {
                 // copy pixels from encoded stream
                 odd_pixel =  stream_byte & 1;
                 rle_code = (stream_byte + 1) / 2;
                 extra_byte = rle_code & 0x01;
-                if (row_ptr + pixel_ptr + stream_byte > frame_size ||
+                if (pixel_ptr + 2*rle_code - odd_pixel > avctx->width ||
                     bytestream2_get_bytes_left(gb) < rle_code) {
                     av_log(avctx, AV_LOG_ERROR,
                            "MS RLE: frame/stream ptr just went out of bounds (copy)\n");
@@ -82,13 +79,13 @@ static int msrle_decode_pal4(AVCodecContext *avctx, AVPicture *pic,
                     if (pixel_ptr >= avctx->width)
                         break;
                     stream_byte = bytestream2_get_byteu(gb);
-                    pic->data[0][row_ptr + pixel_ptr] = stream_byte >> 4;
+                    pic->data[0][line * pic->linesize[0] + pixel_ptr] = stream_byte >> 4;
                     pixel_ptr++;
                     if (i + 1 == rle_code && odd_pixel)
                         break;
                     if (pixel_ptr >= avctx->width)
                         break;
-                    pic->data[0][row_ptr + pixel_ptr] = stream_byte & 0x0F;
+                    pic->data[0][line * pic->linesize[0] + pixel_ptr] = stream_byte & 0x0F;
                     pixel_ptr++;
                 }
 
@@ -98,7 +95,7 @@ static int msrle_decode_pal4(AVCodecContext *avctx, AVPicture *pic,
             }
         } else {
             // decode a run of data
-            if (row_ptr + pixel_ptr + stream_byte > frame_size) {
+            if (pixel_ptr + rle_code > avctx->width + 1) {
                 av_log(avctx, AV_LOG_ERROR,
                        "MS RLE: frame ptr just went out of bounds (run)\n");
                 return AVERROR_INVALIDDATA;
@@ -108,9 +105,9 @@ static int msrle_decode_pal4(AVCodecContext *avctx, AVPicture *pic,
                 if (pixel_ptr >= avctx->width)
                     break;
                 if ((i & 1) == 0)
-                    pic->data[0][row_ptr + pixel_ptr] = stream_byte >> 4;
+                    pic->data[0][line * pic->linesize[0] + pixel_ptr] = stream_byte >> 4;
                 else
-                    pic->data[0][row_ptr + pixel_ptr] = stream_byte & 0x0F;
+                    pic->data[0][line * pic->linesize[0] + pixel_ptr] = stream_byte & 0x0F;
                 pixel_ptr++;
             }
         }