Crash in `tf.strings.substr` due to `CHECK`-fail

Impact

An attacker can cause a denial of service via CHECK-fail in tf.strings.substr with invalid arguments:

```python

import tensorflow as tf

tf.strings.substr(input='abc', len=1, pos=[1,-1])

`

```python

import tensorflow as tf

tf.strings.substr(input='abc', len=1, pos=[1,2])

`

Patches

We have received a patch for the issue in GitHub commit [890f7164b70354c57d40eda52dcdd7658677c09f](https://github.com/tensorflow/tensorflow/commit/890f7164b70354c57d40eda52dcdd7658677c09f).

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 in [#46900](https://github.com/tensorflow/issues/46900) and fixed in [#46974](https://github.com/tensorflow/issues/46974).