Division by 0 in `Reverse`
### Impact
An attacker can cause a denial of service via a FPE runtime error in `tf.raw_ops.Reverse`:
```python
import tensorflow as tf
tensor_input = tf.constant([], shape=[0, 1, 1], dtype=tf.int32)
dims = tf.constant([False, True, False], shape=[3], dtype=tf.bool)
tf.raw_ops.Reverse(tensor=tensor_input, dims=dims)
```
This is because the implementation
https://github.com/tensorflow/tensorflow/blob/36229ea9e9451dac14a8b1f4711c435a1d84a594/tensorflow/core/kernels/reverse_op.cc#L75-L76 performs a division based on the first dimension of the tensor argument:
```cc
const int64 N = input.dim_size(0);
const int64 cost_per_unit = input.NumElements() / N;
```
Since this is controlled by the user, an attacker can trigger a denial of service.
### Patches
We have patched the issue in GitHub commit 4071d8e2f6c45c1955a811fee757ca2adbe462c1
https://github.com/tensorflow/tensorflow/commit/4071d8e2f6c45c1955a811fee757ca2adbe462c1.
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 Ying Wang and Yakun Zhang of Baidu X-Team.