avcodec/av1dec: Check tiles sizes, fix assert, don't read bytes bitwise

Tiles have a size field with a length from one to four bytes. As such it
is not possible to read it all at once with a call to get_bits() as this
only allows to read up to 25 bits; this is guarded by an av_assert2. Yet
this is done by the AV1 decoder in get_tiles_info(). It has been done
despite said size fields being byte-aligned. This commit fixes this by
using the bytestream2 API instead.

Furthermore, it is now explicitly checked whether the data is
consistent, i.e. whether the data that is supposed to be there extends
beyond the end of the data actually present.
Reviewed-by: Wang, Fei W <fei.w.wang@intel.com>
Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavcodec/av1dec.c

@@ -21,7 +21,7 @@
 #include "libavutil/pixdesc.h"
 #include "avcodec.h"
 #include "av1dec.h"
-#include "get_bits.h"
+#include "bytestream.h"
 #include "hwconfig.h"
 #include "internal.h"
 #include "profiles.h"
@@ -205,18 +205,12 @@ static int init_tile_data(AV1DecContext *s)
 static int get_tiles_info(AVCodecContext *avctx, const AV1RawTileGroup *tile_group)
 {
     AV1DecContext *s = avctx->priv_data;
-    GetBitContext gb;
+    GetByteContext gb;
     uint16_t tile_num, tile_row, tile_col;
-    uint32_t size = 0, size_bytes = 0, offset = 0;
-    int ret;
-
-    if ((ret = init_get_bits8(&gb,
-                              tile_group->tile_data.data,
-                              tile_group->tile_data.data_size)) < 0) {
-        av_log(avctx, AV_LOG_ERROR, "Failed to initialize bitstream reader.\n");
-        return ret;
-    }
+    uint32_t size = 0, size_bytes = 0;
 
+    bytestream2_init(&gb, tile_group->tile_data.data,
+                     tile_group->tile_data.data_size);
     s->tg_start = tile_group->tg_start;
     s->tg_end = tile_group->tg_end;
 
@@ -225,24 +219,28 @@ static int get_tiles_info(AVCodecContext *avctx, const AV1RawTileGroup *tile_gro
         tile_col = tile_num % s->raw_frame_header->tile_cols;
 
         if (tile_num == tile_group->tg_end) {
-            s->tile_group_info[tile_num].tile_size = get_bits_left(&gb) / 8;
-            s->tile_group_info[tile_num].tile_offset = offset;
+            s->tile_group_info[tile_num].tile_size = bytestream2_get_bytes_left(&gb);
+            s->tile_group_info[tile_num].tile_offset = bytestream2_tell(&gb);
             s->tile_group_info[tile_num].tile_row = tile_row;
             s->tile_group_info[tile_num].tile_column = tile_col;
             return 0;
         }
         size_bytes = s->raw_frame_header->tile_size_bytes_minus1 + 1;
-        size = get_bits_le(&gb, size_bytes * 8) + 1;
-        skip_bits(&gb, size * 8);
-
-        offset += size_bytes;
+        if (bytestream2_get_bytes_left(&gb) < size_bytes)
+            return AVERROR_INVALIDDATA;
+        size = 0;
+        for (int i = 0; i < size_bytes; i++)
+            size |= bytestream2_get_byteu(&gb) << 8 * i;
+        if (bytestream2_get_bytes_left(&gb) <= size)
+            return AVERROR_INVALIDDATA;
+        size++;
 
         s->tile_group_info[tile_num].tile_size = size;
-        s->tile_group_info[tile_num].tile_offset = offset;
+        s->tile_group_info[tile_num].tile_offset = bytestream2_tell(&gb);
         s->tile_group_info[tile_num].tile_row = tile_row;
         s->tile_group_info[tile_num].tile_column = tile_col;
 
-        offset += size;
+        bytestream2_skipu(&gb, size);
     }
 
     return 0;
@@ -778,7 +776,9 @@ static int av1_decode_frame(AVCodecContext *avctx, void *frame,
             else
                 raw_tile_group = &obu->obu.tile_group;
 
-            get_tiles_info(avctx, raw_tile_group);
+            ret = get_tiles_info(avctx, raw_tile_group);
+            if (ret < 0)
+                goto end;
 
             if (avctx->hwaccel) {
                 ret = avctx->hwaccel->decode_slice(avctx,