cook: use the bytestream2 API for reading extradata

Fixes possible invalid reads in corrupted files.

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

libavcodec/cook.c

@@ -1041,9 +1041,7 @@ static void dump_cook_context(COOKContext *q)
 static av_cold int cook_decode_init(AVCodecContext *avctx)
 {
     COOKContext *q = avctx->priv_data;
-    const uint8_t *edata_ptr = avctx->extradata;
-    const uint8_t *edata_ptr_end = edata_ptr + avctx->extradata_size;
-    int extradata_size = avctx->extradata_size;
+    GetByteContext gb;
     int s = 0;
     unsigned int channel_mask = 0;
     int samples_per_frame;
@@ -1051,12 +1049,14 @@ static av_cold int cook_decode_init(AVCodecContext *avctx)
     q->avctx = avctx;
 
     /* Take care of the codec specific extradata. */
-    if (extradata_size < 8) {
+    if (avctx->extradata_size < 8) {
         av_log(avctx, AV_LOG_ERROR, "Necessary extradata missing!\n");
         return AVERROR_INVALIDDATA;
     }
     av_log(avctx, AV_LOG_DEBUG, "codecdata_length=%d\n", avctx->extradata_size);
 
+    bytestream2_init(&gb, avctx->extradata, avctx->extradata_size);
+
     /* Take data from the AVCodecContext (RM container). */
     if (!avctx->channels) {
         av_log(avctx, AV_LOG_ERROR, "Invalid number of channels\n");
@@ -1068,21 +1068,15 @@ static av_cold int cook_decode_init(AVCodecContext *avctx)
 
     ff_audiodsp_init(&q->adsp);
 
-    while (edata_ptr < edata_ptr_end) {
+    while (bytestream2_get_bytes_left(&gb)) {
         /* 8 for mono, 16 for stereo, ? for multichannel
            Swap to right endianness so we don't need to care later on. */
-        if (extradata_size >= 8) {
-            q->subpacket[s].cookversion = bytestream_get_be32(&edata_ptr);
-            samples_per_frame           = bytestream_get_be16(&edata_ptr);
-            q->subpacket[s].subbands = bytestream_get_be16(&edata_ptr);
-            extradata_size -= 8;
-        }
-        if (extradata_size >= 8) {
-            bytestream_get_be32(&edata_ptr);    // Unknown unused
-            q->subpacket[s].js_subband_start = bytestream_get_be16(&edata_ptr);
-            q->subpacket[s].js_vlc_bits = bytestream_get_be16(&edata_ptr);
-            extradata_size -= 8;
-        }
+        q->subpacket[s].cookversion      = bytestream2_get_be32(&gb);
+        samples_per_frame                = bytestream2_get_be16(&gb);
+        q->subpacket[s].subbands         = bytestream2_get_be16(&gb);
+        bytestream2_get_be32(&gb);    // Unknown unused
+        q->subpacket[s].js_subband_start = bytestream2_get_be16(&gb);
+        q->subpacket[s].js_vlc_bits      = bytestream2_get_be16(&gb);
 
         /* Initialize extradata related variables. */
         q->subpacket[s].samples_per_channel = samples_per_frame / avctx->channels;
@@ -1134,8 +1128,7 @@ static av_cold int cook_decode_init(AVCodecContext *avctx)
             break;
         case MC_COOK:
             av_log(avctx, AV_LOG_DEBUG, "MULTI_CHANNEL\n");
-            if (extradata_size >= 4)
-                channel_mask |= q->subpacket[s].channel_mask = bytestream_get_be32(&edata_ptr);
+            channel_mask |= q->subpacket[s].channel_mask = bytestream2_get_be32(&gb);
 
             if (av_get_channel_layout_nb_channels(q->subpacket[s].channel_mask) > 1) {
                 q->subpacket[s].total_subbands = q->subpacket[s].subbands +