prc: fix possible stack overflow in StreamReader

Fixes #754

dtool/src/prc/streamReader.cxx

@@ -26,14 +26,20 @@ get_string() {
 
   // First, get the length of the string
   size_t size = get_uint16();
+
+  std::string result(size, 0);
   if (size == 0) {
-    return string();
+    return result;
   }
 
-  char *buffer = (char *)alloca(size);
-  _in->read(buffer, size);
+  _in->read(&result[0], size);
   size_t read_bytes = _in->gcount();
-  return string(buffer, read_bytes);
+
+  if (read_bytes == size) {
+    return result;
+  } else {
+    return result.substr(0, read_bytes);
+  }
 }
 
 /**
@@ -82,17 +88,17 @@ string StreamReader::
 get_fixed_string(size_t size) {
   nassertr(!_in->eof() && !_in->fail(), string());
 
+  std::string result(size, 0);
   if (size == 0) {
-    return string();
+    return result;
   }
 
-  char *buffer = (char *)alloca(size);
-  _in->read(buffer, size);
+  _in->read(&result[0], size);
   size_t read_bytes = _in->gcount();
-  string result(buffer, read_bytes);
+  result.resize(read_bytes);
 
   size_t zero_byte = result.find('\0');
-  return result.substr(0, zero_byte);
+  return result.substr(0, std::min(zero_byte, read_bytes));
 }
 
 /**

dtool/src/prc/streamReader_ext.cxx

@@ -21,14 +21,20 @@
  */
 PyObject *Extension<StreamReader>::
 extract_bytes(size_t size) {
-  unsigned char *buffer = (unsigned char *)alloca(size);
-  size_t read_bytes = _this->extract_bytes(buffer, size);
+  std::istream *in = _this->get_istream();
+  if (in->eof() || in->fail() || size == 0) {
+    return PyBytes_FromStringAndSize(nullptr, 0);
+  }
 
-#if PY_MAJOR_VERSION >= 3
-  return PyBytes_FromStringAndSize((char *)buffer, read_bytes);
-#else
-  return PyString_FromStringAndSize((char *)buffer, read_bytes);
-#endif
+  PyObject *bytes = PyBytes_FromStringAndSize(nullptr, size);
+  in->read(PyBytes_AS_STRING(bytes), size);
+  size_t read_bytes = in->gcount();
+
+  if (read_bytes == size || _PyBytes_Resize(&bytes, read_bytes) == 0) {
+    return bytes;
+  } else {
+    return nullptr;
+  }
 }
 
 /**

tests/prc/test_stream_reader.py

@@ -155,3 +155,38 @@ def test_streamreader_readline():
     stream = StringStream(b'\x00\x00')
     reader = StreamReader(stream, False)
     assert reader.readline() == b'\x00\x00'
+
+
+def test_streamreader_extract_bytes():
+    # Empty bytes
+    stream = StringStream(b'')
+    reader = StreamReader(stream, False)
+    assert reader.extract_bytes(10) == b''
+
+    # Small bytes object, small reads
+    stream = StringStream(b'abcd')
+    reader = StreamReader(stream, False)
+    assert reader.extract_bytes(2) == b'ab'
+    assert reader.extract_bytes(2) == b'cd'
+    assert reader.extract_bytes(2) == b''
+
+    # Embedded null bytes
+    stream = StringStream(b'a\x00b\x00c')
+    reader = StreamReader(stream, False)
+    assert reader.extract_bytes(5) == b'a\x00b\x00c'
+
+    # Not enough data in stream to fill buffer
+    stream = StringStream(b'abcdefghijklmnop')
+    reader = StreamReader(stream, False)
+    assert reader.extract_bytes(10) == b'abcdefghij'
+    assert reader.extract_bytes(10) == b'klmnop'
+    assert reader.extract_bytes(10) == b''
+
+    # Read of 0 bytes
+    stream = StringStream(b'abcd')
+    reader = StreamReader(stream, False)
+    assert reader.extract_bytes(0) == b''
+    assert reader.extract_bytes(0) == b''
+
+    # Very large read (8 MiB buffer allocation)
+    assert reader.extract_bytes(8 * 1024 * 1024) == b'abcd'