libwebp: cherry-pick lossless signature check improvement

third_party/libwebp/dec/vp8l.c

 // Copyright 2012 Google Inc. All Rights Reserved.
 //
 // Use of this source code is governed by a BSD-style license
 // that can be found in the COPYING file in the root of the source
 // tree. An additional intellectual property rights grant can be found
 // in the file PATENTS. All contributing project authors may
 // be found in the AUTHORS file in the root of the source tree.
 // -----------------------------------------------------------------------------
 //
 // main entry for the decoder
@@ skipping 64 matching lines @@
 };
 
 static int DecodeImageStream(int xsize, int ysize,
                              int is_level0,
                              VP8LDecoder* const dec,
                              uint32_t** const decoded_data);
 
 //------------------------------------------------------------------------------
 
 int VP8LCheckSignature(const uint8_t* const data, size_t size) {
-  return (size >= 1) && (data[0] == VP8L_MAGIC_BYTE);
+  return (size >= VP8L_FRAME_HEADER_SIZE &&
+          data[0] == VP8L_MAGIC_BYTE &&
+          (data[4] >> 5) == 0);  // version
 }
 
 static int ReadImageInfo(VP8LBitReader* const br,
                          int* const width, int* const height,
                          int* const has_alpha) {
-  const uint8_t signature = VP8LReadBits(br, 8);
-  if (!VP8LCheckSignature(&signature, 1)) {
-    return 0;
-  }
+  if (VP8LReadBits(br, 8) != VP8L_MAGIC_BYTE) return 0;
   *width = VP8LReadBits(br, VP8L_IMAGE_SIZE_BITS) + 1;
   *height = VP8LReadBits(br, VP8L_IMAGE_SIZE_BITS) + 1;
   *has_alpha = VP8LReadBits(br, 1);
-  VP8LReadBits(br, VP8L_VERSION_BITS);  // Read/ignore the version number.
+  if (VP8LReadBits(br, VP8L_VERSION_BITS) != 0) return 0;
   return 1;
 }
 
 int VP8LGetInfo(const uint8_t* data, size_t data_size,
                 int* const width, int* const height, int* const has_alpha) {
   if (data == NULL || data_size < VP8L_FRAME_HEADER_SIZE) {
     return 0;         // not enough data
+  } else if (!VP8LCheckSignature(data, data_size)) {
+    return 0;         // bad signature
   } else {
     int w, h, a;
     VP8LBitReader br;
     VP8LInitBitReader(&br, data, data_size);
     if (!ReadImageInfo(&br, &w, &h, &a)) {
       return 0;
     }
     if (width != NULL) *width = w;
     if (height != NULL) *height = h;
     if (has_alpha != NULL) *has_alpha = a;
@@ skipping 1124 matching lines @@
   VP8LClear(dec);
   assert(dec->status_ != VP8_STATUS_OK);
   return 0;
 }
 
 //------------------------------------------------------------------------------
 
 #if defined(__cplusplus) || defined(c_plusplus)
 }    // extern "C"
 #endif