wma: fix invalid buffer size assumptions causing random overreads.

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

libavcodec/wma.h

@@ -124,7 +124,7 @@ typedef struct WMACodecContext {
     /* output buffer for one frame and the last for IMDCT windowing */
     DECLARE_ALIGNED(32, float, frame_out)[MAX_CHANNELS][BLOCK_MAX_SIZE * 2];
     /* last frame info */
-    uint8_t last_superframe[MAX_CODED_SUPERFRAME_SIZE + 4]; /* padding added */
+    uint8_t last_superframe[MAX_CODED_SUPERFRAME_SIZE + FF_INPUT_BUFFER_PADDING_SIZE]; /* padding added */
     int last_bitoffset;
     int last_superframe_len;
     float noise_table[NOISE_TAB_SIZE];

libavcodec/wmadec.c

@@ -845,6 +845,12 @@ static int wma_decode_superframe(AVCodecContext *avctx, void *data,
 
     if (s->use_bit_reservoir) {
         bit_offset = get_bits(&s->gb, s->byte_offset_bits + 3);
+        if (bit_offset > get_bits_left(&s->gb)) {
+            av_log(avctx, AV_LOG_ERROR,
+                   "Invalid last frame bit offset %d > buf size %d (%d)\n",
+                   bit_offset, get_bits_left(&s->gb), buf_size);
+            goto fail;
+        }
 
         if (s->last_superframe_len > 0) {
             //        printf("skip=%d\n", s->last_bitoffset);
@@ -861,9 +867,10 @@ static int wma_decode_superframe(AVCodecContext *avctx, void *data,
             if (len > 0) {
                 *q++ = (get_bits)(&s->gb, len) << (8 - len);
             }
+            memset(q, 0, FF_INPUT_BUFFER_PADDING_SIZE);
 
             /* XXX: bit_offset bits into last frame */
-            init_get_bits(&s->gb, s->last_superframe, MAX_CODED_SUPERFRAME_SIZE*8);
+            init_get_bits(&s->gb, s->last_superframe, s->last_superframe_len * 8 + bit_offset);
             /* skip unused bits */
             if (s->last_bitoffset > 0)
                 skip_bits(&s->gb, s->last_bitoffset);
@@ -877,9 +884,9 @@ static int wma_decode_superframe(AVCodecContext *avctx, void *data,
 
         /* read each frame starting from bit_offset */
         pos = bit_offset + 4 + 4 + s->byte_offset_bits + 3;
-        if (pos >= MAX_CODED_SUPERFRAME_SIZE * 8)
+        if (pos >= MAX_CODED_SUPERFRAME_SIZE * 8 || pos > buf_size * 8)
             return AVERROR_INVALIDDATA;
-        init_get_bits(&s->gb, buf + (pos >> 3), (MAX_CODED_SUPERFRAME_SIZE - (pos >> 3))*8);
+        init_get_bits(&s->gb, buf + (pos >> 3), (buf_size - (pos >> 3))*8);
         len = pos & 7;
         if (len > 0)
             skip_bits(&s->gb, len);