Vulnerability: Null pointer dereference via invalid Ragged Tensors ### Impact
Calling `tf.raw_ops.RaggedTensorToVariant` with arguments specifying an invalid ragged tensor results in a null pointer dereference:

```python
import tensorflow as tf

input_tensor = tf.constant([], shape=[0, 0, 0, 0, 0], dtype=tf.float32)
filter_tensor = tf.constant([], shape=[0, 0, 0, 0, 0], dtype=tf.float32)

tf.raw_ops.Conv3D(input=input_tensor, filter=filter_tensor, strides=[1, 56, 56, 56, 1], padding='VALID', data_format='NDHWC', dilations=[1, 1, 1, 23, 1])
```

```python
import tensorflow as tf

input_tensor = tf.constant([], shape=[2, 2, 2, 2, 0], dtype=tf.float32)
filter_tensor = tf.constant([], shape=[0, 0, 2, 6, 2], dtype=tf.float32)

tf.raw_ops.Conv3D(input=input_tensor, filter=filter_tensor, strides=[1, 56, 39, 34, 1], padding='VALID', data_format='NDHWC', dilations=[1, 1, 1, 1, 1])
```

The implementation of [`RaggedTensorToVariant` operations](https://github.com/tensorflow/tensorflow/blob/904b3926ed1c6c70380d5313d282d248a776baa1/tensorflow/core/kernels/ragged_tensor_to_variant_op.cc#L39-L40) does not validate that the ragged tensor argument is non-empty:

```cc
  int ragged_rank = batched_ragged.ragged_rank();
  auto batched_splits_top_vec = batched_ragged.splits(0).vec<SPLIT_TYPE>();
```

Since `batched_ragged` contains no elements, `batched_ragged.splits` is a null vector, thus `batched_ragged.splits(0)` will result in  dereferencing `nullptr`.

### Patches
We have patched the issue in GitHub commit [b055b9c474cd376259dde8779908f9eeaf097d93](https://github.com/tensorflow/tensorflow/commit/b055b9c474cd376259dde8779908f9eeaf097d93).

The fix will be included in TensorFlow 2.5.0. We will also cherrypick this commit on TensorFlow 2.4.2, TensorFlow 2.3.3, TensorFlow 2.2.3 and TensorFlow 2.1.4, as these are also affected and still in supported range.

### For more information
Please consult [our security guide](https://github.com/tensorflow/tensorflow/blob/master/SECURITY.md) for more information regarding the security model and how to contact us with issues and questions.

### Attribution
This vulnerability has been reported by Yakun Zhang and Ying Wang of Baidu X-Team.
Vulnerability ID: GHSA-84mw-34w6-2q43
CWE: CWE-476
Severity score: 2.5
Project: https://github.com/tensorflow/tensorflow
Published: 2021-05-21
File: tensorflow/core/kernels/ragged_tensor_to_variant_op.cc
Commit message: Fix `tf.raw_ops.RaggedTensorToVariant` invalid resize.

PiperOrigin-RevId: 368299574
Change-Id: I751c186325aa0bab397928845e790e60c2d90918
Commit author: Amit Patankar

Vulnerable version (before fix):
 class RaggedTensorToVariantOp : public OpKernel {      // Unbatch the Ragged Tensor and encode the components.     std::vector<RaggedTensorVariant> unbatched_ragged_input;     OP_REQUIRES_OK(context, UnbatchRaggedZerothDim<VALUE_TYPE, SPLIT_TYPE>(                                 batched_ragged_input, &unbatched_ragged_input)); 

Fixed version (after fix):
 class RaggedTensorToVariantOp : public OpKernel {      // Unbatch the Ragged Tensor and encode the components.     std::vector<RaggedTensorVariant> unbatched_ragged_input;     auto batched_splits_top_vec =         batched_ragged_input.splits(0).vec<SPLIT_TYPE>();     int num_components = batched_splits_top_vec.size() - 1;     OP_REQUIRES(context, num_components >= 0,                 errors::Internal("Invalid split argument."));     OP_REQUIRES_OK(context, UnbatchRaggedZerothDim<VALUE_TYPE, SPLIT_TYPE>(                                 batched_ragged_input, &unbatched_ragged_input)); 

Patch:
patch: @@ -159,6 +159,11 @@ class RaggedTensorToVariantOp : public OpKernel {
 
     // Unbatch the Ragged Tensor and encode the components.
     std::vector<RaggedTensorVariant> unbatched_ragged_input;
+    auto batched_splits_top_vec =
+        batched_ragged_input.splits(0).vec<SPLIT_TYPE>();
+    int num_components = batched_splits_top_vec.size() - 1;
+    OP_REQUIRES(context, num_components >= 0,
+                errors::Internal("Invalid split argument."));
     OP_REQUIRES_OK(context, UnbatchRaggedZerothDim<VALUE_TYPE, SPLIT_TYPE>(
                                 batched_ragged_input, &unbatched_ragged_input));
path: tensorflow/core/kernels/ragged_tensor_to_variant_op.cc