SERVER-84125: Check fieldname size in BSONColumn validation (#17688)

src/mongo/bson/bson_validate.cpp

@@ -436,29 +436,32 @@ class ValidateBuffer {
 
     /* Assumes the root level is a single literal element (which may contain nested objects).
      * Only validates up to the termination of that first literal, more data is permitted to
-       remain in the buffer after that and is not validated. Throws exception on invalid data*/
+     * remain in the buffer after that and is not validated. Throws exception on invalid data.
+     * Confirm field names for literals in BSONColumn have empty field names.
+     */
     int validateAndMeasureElem() {
         setupValidation();
         uassert(InvalidBSON,
                 "BSON literal is not followed by fieldname",
                 _maxLength > 1);  // must at least have a 0-terminator after control
-        size_t fieldNameSize = strnlen(_data + 1 /* skip type */, _maxLength - 1) + 1;
+        // Confirm fieldName is just a null terminator
+        uassert(NonConformantBSON,
+                "BSON literal content does not have an empty fieldname",
+                _maxLength > 1 && _data[1] == 0);
 
         // Handle one element without using iterative loop, and without expecting
         // multiple instances or an EOO.  Only resume with the iterative loop if
         // the frame stack has been incremented, meaning we have nested objects
 
         // Save pointer to currFrame->end so we can fill it in once we know the size
         const char** preEnd = &(_currFrame->end);
-        const char* ptr =
-            _validateElem(Cursor{_data + 1 + fieldNameSize, _data + _maxLength}, *_data);
-        _validator.checkNonConformantElem(_data, 1 + fieldNameSize, *_data);
+        const char* ptr = _validateElem(Cursor{_data + 2, _data + _maxLength}, *_data);
+        _validator.checkNonConformantElem(_data, 2, *_data);
 
         if (_currFrame != _frames.begin()) {
             // We know that type was kObject or kArray, so size is fieldname, type,
             // and a stored int
-            int size = 1 + fieldNameSize +
-                ConstDataView(_data + fieldNameSize + 1).read<LittleEndian<int32_t>>();
+            int size = 2 + ConstDataView(_data + 2).read<LittleEndian<int32_t>>();
             uassert(InvalidBSON,
                     "BSON literal content exceeds buffer size",
                     (size_t)size <= _maxLength);

src/mongo/bson/bson_validate_test.cpp

@@ -977,6 +977,24 @@ TEST(BSONValidateColumn, BSONColumnMissingEOO) {
               ErrorCodes::InvalidBSON);
 }
 
+TEST(BSONValidateColumn, BSONColumnFieldnameNotEmpty) {
+    BSONColumnBuilder cb;
+    cb.append(BSON("a"
+                   << "deadbeef")
+                  .getField("a"));
+    BSONBinData columnData = cb.finalize();
+    ASSERT_OK(validateBSONColumn((char*)columnData.data, columnData.length));
+
+    char buf[1024];
+    buf[0] = ((const char*)columnData.data)[0];
+    buf[1] = 'f';
+    buf[2] = 'o';
+    buf[3] = 'o';
+    memcpy(buf + 4, ((const char*)columnData.data) + 1, columnData.length - 1);
+
+    ASSERT_EQ(validateBSONColumn(buf, columnData.length + 3).code(), ErrorCodes::NonConformantBSON);
+}
+
 TEST(BSONValidateColumn, BSONColumnNoOverflowMissingAllEOOInColumn) {
     BSONColumnBuilder cb;
     cb.append(BSON("a"