Commit message: Fix segfault/heap buffer overflow in `{Experimental,}DatasetToTFRecord` where dataset is numeric.

Code assumes only strings inputs and then interprets numbers as valid `tstring`s. Then, when trying to compute the CRC of the record this results in heap buffer overflow.

PiperOrigin-RevId: 387675909
Change-Id: I7396b9b8afc1ac744112af7c0b1cd7bb41e0f556
CVE: CVE-2021-37650
CWE: CWE-787
Vulnerability description: TensorFlow is an end-to-end open source platform for machine learning. In affected versions the implementation for `tf.raw_ops.ExperimentalDatasetToTFRecord` and `tf.raw_ops.DatasetToTFRecord` can trigger heap buffer overflow and segmentation fault. The [implementation](https://github.com/tensorflow/tensorflow/blob/f24faa153ad31a4b51578f8181d3aaab77a1ddeb/tensorflow/core/kernels/data/experimental/to_tf_record_op.cc#L93-L102) assumes that all records in the dataset are of string type. However, there is no check for that, and the example given above uses numeric types. We have patched the issue in GitHub commit e0b6e58c328059829c3eb968136f17aa72b6c876. The fix will be included in TensorFlow 2.6.0. We will also cherrypick this commit on TensorFlow 2.5.1, TensorFlow 2.4.3, and TensorFlow 2.3.4, as these are also affected and still in supported range.

Project: tensorflow
File: to_tf_record_op.cc

  Status DoCompute(OpKernelContext* ctx) {
    tensorflow::ResourceTagger tag(kTFDataResourceTag,
                                   ctx->op_kernel().type_string());
    tstring filename;
    TF_RETURN_IF_ERROR(
        ParseScalarArgument<tstring>(ctx, "filename", &filename));
    tstring compression_type;
    TF_RETURN_IF_ERROR(ParseScalarArgument<tstring>(ctx, "compression_type",
                                                    &compression_type));
    std::unique_ptr<WritableFile> file;
    TF_RETURN_IF_ERROR(ctx->env()->NewWritableFile(filename, &file));
    auto writer = absl::make_unique<io::RecordWriter>(
        file.get(),
        io::RecordWriterOptions::CreateRecordWriterOptions(compression_type));

    DatasetBase* dataset;
    TF_RETURN_IF_ERROR(GetDatasetFromVariantTensor(ctx->input(0), &dataset));

    IteratorContext::Params params(ctx);
    FunctionHandleCache function_handle_cache(params.flr);
    params.function_handle_cache = &function_handle_cache;
    ResourceMgr resource_mgr;
    params.resource_mgr = &resource_mgr;
    CancellationManager cancellation_manager(ctx->cancellation_manager());
    params.cancellation_manager = &cancellation_manager;

    IteratorContext iter_ctx(std::move(params));
    DatasetBase* finalized_dataset;
    TF_RETURN_IF_ERROR(FinalizeDataset(ctx, dataset, &finalized_dataset));

    std::unique_ptr<IteratorBase> iterator;
    TF_RETURN_IF_ERROR(finalized_dataset->MakeIterator(
        &iter_ctx, /*parent=*/nullptr, "ToTFRecordOpIterator", &iterator));

    std::vector<Tensor> components;
    components.reserve(finalized_dataset->output_dtypes().size());
    bool end_of_sequence;
    do {
      TF_RETURN_IF_ERROR(
          iterator->GetNext(&iter_ctx, &components, &end_of_sequence));

      if (!end_of_sequence) {
        TF_RETURN_IF_ERROR(
            writer->WriteRecord(components[0].scalar<tstring>()()));
      }
      components.clear();
    } while (!end_of_sequence);
    return Status::OK();
  }