[idd] CSettings: fix registry string value handling

Rename the invalid default parameter and make registry string reads
more robust.

Validate the returned value type and byte count after querying the
value data, reject malformed UTF-16 sizes, and explicitly terminate
the string based on the number of bytes actually returned.

Sanity check the parsed mode values

idd/LGIdd/CSettings.cpp

@@ -103,8 +103,11 @@ void CSettings::SetExtraMode(const DisplayMode& mode)
   RegCloseKey(hKey);
 }
 
-std::wstring CSettings::ReadStringValue(const wchar_t* name, const wchar_t* default)
+std::wstring CSettings::ReadStringValue(const wchar_t* name, const wchar_t* defaultValue)
 {
+  if (!defaultValue)
+    defaultValue = L"";
+
   HKEY hKey = nullptr;
   LONG ec = RegOpenKeyExW(
     HKEY_LOCAL_MACHINE,
@@ -115,29 +118,43 @@ std::wstring CSettings::ReadStringValue(const wchar_t* name, const wchar_t* defa
   );
 
   if (ec != ERROR_SUCCESS)
-    return std::wstring(default);
+    return std::wstring(defaultValue);
 
   DWORD type = 0;
   DWORD cb = 0;
 
   ec = RegQueryValueExW(hKey, name, nullptr, &type, nullptr, &cb);
-  if (ec != ERROR_SUCCESS || (type != REG_SZ && type != REG_EXPAND_SZ) || cb == 0)
+  if (ec != ERROR_SUCCESS ||
+    (type != REG_SZ && type != REG_EXPAND_SZ) ||
+    cb == 0 ||
+    (cb % sizeof(wchar_t)) != 0)
   {
     RegCloseKey(hKey);
-    return std::wstring(default);
+    return std::wstring(defaultValue);
   }
 
-  std::vector<wchar_t> buf(cb / sizeof(wchar_t) + 1);
-  ec = RegQueryValueExW(hKey, name, nullptr, &type,
-    reinterpret_cast<LPBYTE>(buf.data()), &cb);
+  std::vector<wchar_t> buf((cb / sizeof(wchar_t)) + 1, L'\0');
+
+  DWORD type2 = 0;
+  DWORD cb2 = cb;
+
+  ec = RegQueryValueExW(
+    hKey,
+    name,
+    nullptr,
+    &type2,
+    reinterpret_cast<LPBYTE>(buf.data()),
+    &cb2
+  );
+
   RegCloseKey(hKey);
-  if (ec != ERROR_SUCCESS)
-    return  std::wstring(default);
 
-  buf.back() = L'\0';
+  if (ec != ERROR_SUCCESS || (type2 != REG_SZ && type2 != REG_EXPAND_SZ) ||
+    cb2 == 0 || (cb2 % sizeof(wchar_t)) != 0 || cb2 > cb)
+    return std::wstring(defaultValue);
 
-  std::wstring s(buf.data());
-  return s;
+  buf[cb2 / sizeof(wchar_t)] = L'\0';
+  return std::wstring(buf.data());
 }
 
 bool CSettings::GetExtraMode(DisplayMode& mode)
@@ -226,5 +243,14 @@ bool CSettings::ParseModeString(const std::wstring& in, DisplayMode& out)
       !toUnsigned(s.substr(atPos + 1), out.refresh))
       return false;
 
+  // sanity check
+  if (out.width   < 640   ||
+      out.height  < 480   ||
+      out.width   > 16384 ||
+      out.height  > 16384 ||
+      out.refresh < 30    ||
+      out.refresh > 1000)
+    return false;
+
   return true;
 }