Fix out-of-range access in unibrow::Utf8::CalculateValue.

This code should not access bytes out of the permitted range in order to check
the range of a possible UTF-8 value. Instead, the length check should occur
before such checks.

BUG=chromium:667260, chromium:662822

Review-Url: https://codereview.chromium.org/2520053003
Cr-Commit-Position: refs/heads/master@{#41165}

src/unicode-decoder.h

@@ -7,10 +7,11 @@
 
 #include <sys/types.h>
 #include "src/globals.h"
+#include "src/utils.h"
 
 namespace unibrow {
 
-class Utf8DecoderBase {
+class V8_EXPORT_PRIVATE Utf8DecoderBase {
  public:
   // Initialization done in subclass.
   inline Utf8DecoderBase();

src/unicode.cc

@@ -235,35 +235,31 @@ uchar Utf8::CalculateValue(const byte* str, size_t max_length, size_t* cursor) {
   while (count < max_count && IsContinuationCharacter(str[count])) {
     count++;
   }
+  *cursor += count;
+
+  // There must be enough continuation characters.
+  if (count != length) return kBadChar;
 
-  // Check overly long sequences & other conditions. Use length as error
-  // indicator.
+  // Check overly long sequences & other conditions.
   if (length == 3) {
     if (str[0] == 0xE0 && (str[1] < 0xA0 || str[1] > 0xBF)) {
       // Overlong three-byte sequence?
-      length = 0;
+      return kBadChar;
     } else if (str[0] == 0xED && (str[1] < 0x80 || str[1] > 0x9F)) {
       // High and low surrogate halves?
-      length = 0;
+      return kBadChar;
     }
   } else if (length == 4) {
     if (str[0] == 0xF0 && (str[1] < 0x90 || str[1] > 0xBF)) {
       // Overlong four-byte sequence.
-      length = 0;
+      return kBadChar;
     } else if (str[0] == 0xF4 && (str[1] < 0x80 || str[1] > 0x8F)) {
       // Code points outside of the unicode range.
-      length = 0;
-    }
-  }
-
-  if (count != length) {
-    // All invalid encodings should land here.
-    *cursor += count;
       return kBadChar;
     }
+  }
 
   // All errors have been handled, so we only have to assemble the result.
-  *cursor += length;
   switch (length) {
     case 1:
       return str[0];

test/unittests/BUILD.gn

@@ -120,6 +120,7 @@ v8_executable("unittests") {
     "source-position-table-unittest.cc",
     "test-utils.cc",
     "test-utils.h",
+    "unicode-unittest.cc",
     "value-serializer-unittest.cc",
     "wasm/asm-types-unittest.cc",
     "wasm/ast-decoder-unittest.cc",

test/unittests/unicode-unittest.cc

+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include <memory>
+#include <string>
+
+#include "src/unicode-decoder.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace v8 {
+namespace internal {
+
+namespace {
+
+using Utf8Decoder = unibrow::Utf8Decoder<512>;
+
+void Decode(Utf8Decoder* decoder, const std::string& str) {
+  // Put the string in its own buffer on the heap to make sure that
+  // AddressSanitizer's heap-buffer-overflow logic can see what's going on.
+  std::unique_ptr<char[]> buffer(new char[str.length()]);
+  memcpy(buffer.get(), str.data(), str.length());
+  decoder->Reset(buffer.get(), str.length());
+}
+
+}  // namespace
+
+TEST(UnicodeTest, ReadOffEndOfUtf8String) {
+  Utf8Decoder decoder;
+
+  // Not enough continuation bytes before string ends.
+  Decode(&decoder, "\xE0");
+  Decode(&decoder, "\xED");
+  Decode(&decoder, "\xF0");
+  Decode(&decoder, "\xF4");
+}
+
+}  // namespace internal
+}  // namespace v8

test/unittests/unittests.gyp

@@ -118,6 +118,7 @@
       'source-position-table-unittest.cc',
       'test-utils.h',
       'test-utils.cc',
+      'unicode-unittest.cc',
       'value-serializer-unittest.cc',
       'zone/segmentpool-unittest.cc',
       'zone/zone-chunk-list-unittest.cc',