Division by zero in TFLite's implementation of `GatherNd`

Impact

The reference implementation of the GatherNd TFLite operator is [vulnerable to a division by zero error](https://github.com/tensorflow/tensorflow/blob/0d45ea1ca641b21b73bcf9c00e0179cda284e7e7/tensorflow/lite/kernels/internal/reference/reference_ops.h#L966):

```cc

ret.dims_to_count[i] = remain_flat_size / params_shape.Dims(i);

`

An attacker can craft a model such that params input would be an empty tensor. In turn, params_shape.Dims(.) would be zero, in at least one dimension.

Patches

We have patched the issue in GitHub commit [8e45822aa0b9f5df4b4c64f221e64dc930a70a9d](https://github.com/tensorflow/tensorflow/commit/8e45822aa0b9f5df4b4c64f221e64dc930a70a9d).

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 members of the Aivul Team from Qihoo 360.