Fix out of bound read in PDF_DecodeText().

Add unit test for PDF_DecodeText() with basic test cases and test cases
for handling escape sequences. Also use size_t consistently.

Bug: chromium:1001159
Change-Id: I6e8750ce90fdd6cf6e5b2eff35bcdc89f28fc6c5
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/60310
Reviewed-by: Chris Palmer <palmer@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>

core/fpdfapi/parser/fpdf_parser_decode.cpp

diff --git a/core/fpdfapi/parser/fpdf_parser_decode.cpp b/core/fpdfapi/parser/fpdf_parser_decode.cpp
index 232b6ea..7074061 100644
--- a/core/fpdfapi/parser/fpdf_parser_decode.cpp
+++ b/core/fpdfapi/parser/fpdf_parser_decode.cpp
@@ -472,7 +472,7 @@
   WideString result;
   if (span.size() >= 2 && ((span[0] == 0xfe && span[1] == 0xff) ||
                            (span[0] == 0xff && span[1] == 0xfe))) {
-    uint32_t max_chars = (span.size() - 2) / 2;
+    size_t max_chars = (span.size() - 2) / 2;
     if (!max_chars)
       return result;
 
@@ -481,7 +481,7 @@
         span[0] == 0xfe ? GetUnicodeFromBigEndianBytes
                         : GetUnicodeFromLittleEndianBytes;
     const uint8_t* unicode_str = &span[2];
-    for (uint32_t i = 0; i < max_chars * 2; i += 2) {
+    for (size_t i = 0; i < max_chars * 2; i += 2) {
       uint16_t unicode = GetUnicodeFromBytes(unicode_str + i);
 
       // 0x001B is a begin/end marker for language metadata region that
@@ -492,7 +492,8 @@
           unicode = GetUnicodeFromBytes(unicode_str + i);
           if (unicode == 0x001B) {
             i += 2;
-            unicode = GetUnicodeFromBytes(unicode_str + i);
+            if (i < max_chars * 2)
+              unicode = GetUnicodeFromBytes(unicode_str + i);
             break;
           }
         }
@@ -504,7 +505,7 @@
     }
   } else {
     pdfium::span<wchar_t> dest_buf = result.GetBuffer(span.size());
-    for (uint32_t i = 0; i < span.size(); ++i)
+    for (size_t i = 0; i < span.size(); ++i)
       dest_buf[i] = PDFDocEncoding[span[i]];
     dest_pos = span.size();
   }

core/fpdfapi/parser/fpdf_parser_decode_unittest.cpp

diff --git a/core/fpdfapi/parser/fpdf_parser_decode_unittest.cpp b/core/fpdfapi/parser/fpdf_parser_decode_unittest.cpp
index 171482e..0ddf9ed 100644
--- a/core/fpdfapi/parser/fpdf_parser_decode_unittest.cpp
+++ b/core/fpdfapi/parser/fpdf_parser_decode_unittest.cpp
@@ -181,6 +181,55 @@
   }
 }
 
+TEST(fpdf_parser_decode, DecodeText) {
+  const struct DecodeTestData {
+    const char* input;
+    size_t input_length;
+    const wchar_t* expected_output;
+    size_t expected_length;
+  } kTestData[] = {
+      // Empty src string.
+      {"", 0, L"", 0},
+      // ASCII text.
+      {"the quick\tfox", 13, L"the quick\tfox", 13},
+      // Unicode text.
+      {"\xFE\xFF\x03\x30\x03\x31", 6, L"\x0330\x0331", 2},
+      // More Unicode text.
+      {"\xFE\xFF\x7F\x51\x98\x75\x00\x20\x56\xFE\x72\x47\x00"
+       "\x20\x8D\x44\x8B\xAF\x66\xF4\x59\x1A\x00\x20\x00\xBB",
+       26,
+       L"\x7F51\x9875\x0020\x56FE\x7247\x0020"
+       L"\x8D44\x8BAF\x66F4\x591A\x0020\x00BB",
+       12},
+      // Unicode escape sequence. https://crbug.com/pdfium/182
+      {"\xFE\xFF\x00\x1B\x6A\x61\x00\x1B\x00\x20\x53\x70\x52\x37", 14,
+       L"\x0020\x5370\x5237", 3},
+      {"\xFE\xFF\x00\x1B\x6A\x61\x00\x1B\x00\x20\x53\x70\x52\x37\x29", 15,
+       L"\x0020\x5370\x5237", 3},
+      {"\xFE\xFF\x00\x1B\x6A\x61\x4A\x50\x00\x1B\x00\x20\x53\x70\x52\x37", 16,
+       L"\x0020\x5370\x5237", 3},
+      {"\xFE\xFF\x00\x20\x00\x1B\x6A\x61\x4A\x50\x00\x1B\x52\x37", 14,
+       L"\x0020\x5237", 2},
+      // https://crbug.com/1001159
+      {"\xFE\xFF\x00\x1B\x00\x1B", 6, L"", 0},
+      {"\xFE\xFF\x00\x1B\x00\x1B\x20", 7, L"", 0},
+      {"\xFE\xFF\x00\x1B\x00\x1B\x00\x20", 8, L"\x0020", 1},
+  };
+
+  for (const auto& test_case : kTestData) {
+    WideString output = PDF_DecodeText(
+        pdfium::make_span(reinterpret_cast<const uint8_t*>(test_case.input),
+                          test_case.input_length));
+    ASSERT_EQ(test_case.expected_length, output.GetLength())
+        << "for case " << test_case.input;
+    const wchar_t* str_ptr = output.c_str();
+    for (size_t i = 0; i < test_case.expected_length; ++i) {
+      EXPECT_EQ(test_case.expected_output[i], str_ptr[i])
+          << "for case " << test_case.input << " char " << i;
+    }
+  }
+}
+
 TEST(fpdf_parser_decode, EncodeText) {
   const struct EncodeTestData {
     const wchar_t* input;