png: convert to bytestream2 API.

Protects against overreads in the input buffer.

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

libavcodec/pngdec.c

@@ -35,9 +35,7 @@
 typedef struct PNGDecContext {
     PNGDSPContext dsp;
 
-    const uint8_t *bytestream;
-    const uint8_t *bytestream_start;
-    const uint8_t *bytestream_end;
+    GetByteContext gb;
     AVFrame picture1, picture2;
     AVFrame *current_picture, *last_picture;
 
@@ -362,12 +360,9 @@ static void png_handle_row(PNGDecContext *s)
 static int png_decode_idat(PNGDecContext *s, int length)
 {
     int ret;
-    s->zstream.avail_in = length;
-    s->zstream.next_in = s->bytestream;
-    s->bytestream += length;
-
-    if(s->bytestream > s->bytestream_end)
-        return -1;
+    s->zstream.avail_in = FFMIN(length, bytestream2_get_bytes_left(&s->gb));
+    s->zstream.next_in = s->gb.buffer;
+    bytestream2_skip(&s->gb, length);
 
     /* decode one line if possible */
     while (s->zstream.avail_in > 0) {
@@ -403,15 +398,13 @@ static int decode_frame(AVCodecContext *avctx,
     avctx->coded_frame= s->current_picture;
     p = s->current_picture;
 
-    s->bytestream_start=
-    s->bytestream= buf;
-    s->bytestream_end= buf + buf_size;
-
     /* check signature */
-    if (memcmp(s->bytestream, ff_pngsig, 8) != 0 &&
-        memcmp(s->bytestream, ff_mngsig, 8) != 0)
+    if (buf_size < 8 ||
+        memcmp(buf, ff_pngsig, 8) != 0 &&
+        memcmp(buf, ff_mngsig, 8) != 0)
         return -1;
-    s->bytestream+= 8;
+
+    bytestream2_init(&s->gb, buf + 8, buf_size - 8);
     s->y=
     s->state=0;
 //    memset(s, 0, sizeof(PNGDecContext));
@@ -423,14 +416,12 @@ static int decode_frame(AVCodecContext *avctx,
     if (ret != Z_OK)
         return -1;
     for(;;) {
-        int tag32;
-        if (s->bytestream >= s->bytestream_end)
+        if (bytestream2_get_bytes_left(&s->gb) <= 0)
             goto fail;
-        length = bytestream_get_be32(&s->bytestream);
+        length = bytestream2_get_be32(&s->gb);
         if (length > 0x7fffffff)
             goto fail;
-        tag32 = bytestream_get_be32(&s->bytestream);
-        tag = av_bswap32(tag32);
+        tag = bytestream2_get_le32(&s->gb);
         av_dlog(avctx, "png: tag=%c%c%c%c length=%u\n",
                 (tag & 0xff),
                 ((tag >> 8) & 0xff),
@@ -440,18 +431,18 @@ static int decode_frame(AVCodecContext *avctx,
         case MKTAG('I', 'H', 'D', 'R'):
             if (length != 13)
                 goto fail;
-            s->width = bytestream_get_be32(&s->bytestream);
-            s->height = bytestream_get_be32(&s->bytestream);
+            s->width  = bytestream2_get_be32(&s->gb);
+            s->height = bytestream2_get_be32(&s->gb);
             if(av_image_check_size(s->width, s->height, 0, avctx)){
                 s->width= s->height= 0;
                 goto fail;
             }
-            s->bit_depth = *s->bytestream++;
-            s->color_type = *s->bytestream++;
-            s->compression_type = *s->bytestream++;
-            s->filter_type = *s->bytestream++;
-            s->interlace_type = *s->bytestream++;
-            s->bytestream += 4; /* crc */
+            s->bit_depth        = bytestream2_get_byte(&s->gb);
+            s->color_type       = bytestream2_get_byte(&s->gb);
+            s->compression_type = bytestream2_get_byte(&s->gb);
+            s->filter_type      = bytestream2_get_byte(&s->gb);
+            s->interlace_type   = bytestream2_get_byte(&s->gb);
+            bytestream2_skip(&s->gb, 4); /* crc */
             s->state |= PNG_IHDR;
             av_dlog(avctx, "width=%d height=%d depth=%d color_type=%d compression_type=%d filter_type=%d interlace_type=%d\n",
                     s->width, s->height, s->bit_depth, s->color_type,
@@ -547,7 +538,7 @@ static int decode_frame(AVCodecContext *avctx,
             s->state |= PNG_IDAT;
             if (png_decode_idat(s, length) < 0)
                 goto fail;
-            s->bytestream += 4; /* crc */
+            bytestream2_skip(&s->gb, 4); /* crc */
             break;
         case MKTAG('P', 'L', 'T', 'E'):
             {
@@ -558,16 +549,16 @@ static int decode_frame(AVCodecContext *avctx,
                 /* read the palette */
                 n = length / 3;
                 for(i=0;i<n;i++) {
-                    r = *s->bytestream++;
-                    g = *s->bytestream++;
-                    b = *s->bytestream++;
+                    r = bytestream2_get_byte(&s->gb);
+                    g = bytestream2_get_byte(&s->gb);
+                    b = bytestream2_get_byte(&s->gb);
                     s->palette[i] = (0xff << 24) | (r << 16) | (g << 8) | b;
                 }
                 for(;i<256;i++) {
                     s->palette[i] = (0xff << 24);
                 }
                 s->state |= PNG_PLTE;
-                s->bytestream += 4; /* crc */
+                bytestream2_skip(&s->gb, 4); /* crc */
             }
             break;
         case MKTAG('t', 'R', 'N', 'S'):
@@ -580,21 +571,21 @@ static int decode_frame(AVCodecContext *avctx,
                     !(s->state & PNG_PLTE))
                     goto skip_tag;
                 for(i=0;i<length;i++) {
-                    v = *s->bytestream++;
+                    v = bytestream2_get_byte(&s->gb);
                     s->palette[i] = (s->palette[i] & 0x00ffffff) | (v << 24);
                 }
-                s->bytestream += 4; /* crc */
+                bytestream2_skip(&s->gb, 4); /* crc */
             }
             break;
         case MKTAG('I', 'E', 'N', 'D'):
             if (!(s->state & PNG_ALLIMAGE))
                 goto fail;
-            s->bytestream += 4; /* crc */
+            bytestream2_skip(&s->gb, 4); /* crc */
             goto exit_loop;
         default:
             /* skip tag */
         skip_tag:
-            s->bytestream += length + 4;
+            bytestream2_skip(&s->gb, length + 4);
             break;
         }
     }
@@ -619,7 +610,7 @@ static int decode_frame(AVCodecContext *avctx,
     *picture= *s->current_picture;
     *data_size = sizeof(AVFrame);
 
-    ret = s->bytestream - s->bytestream_start;
+    ret = bytestream2_tell(&s->gb);
  the_end:
     inflateEnd(&s->zstream);
     av_free(crow_buf_base);