Division by 0 in `Conv2DBackpropInput`
### Impact
An attacker can trigger a division by 0 in `tf.raw_ops.Conv2DBackpropInput`:
```python
import tensorflow as tf
input_tensor = tf.constant([52, 1, 1, 5], shape=[4], dtype=tf.int32)
filter_tensor = tf.constant([], shape=[0, 1, 5, 0], dtype=tf.float32)
out_backprop = tf.constant([], shape=[52, 1, 1, 0], dtype=tf.float32)
tf.raw_ops.Conv2DBackpropInput(input_sizes=input_tensor, filter=filter_tensor,
out_backprop=out_backprop, strides=[1, 1, 1, 1],
use_cudnn_on_gpu=True, padding='SAME',
explicit_paddings=[], data_format='NHWC',
dilations=[1, 1, 1, 1])
```
This is because the implementation
https://github.com/tensorflow/tensorflow/blob/b40060c9f697b044e3107917c797ba052f4506ab/tensorflow/core/kernels/conv_grad_input_ops.h#L625-L655 does a division by a quantity that is controlled by the caller:
```cc
const size_t size_A = output_image_size * dims.out_depth;
const size_t size_B = filter_total_size * dims.out_depth;
const size_t size_C = output_image_size * filter_total_size;
const size_t work_unit_size = size_A + size_B + size_C;
...
const size_t shard_size =
use_parallel_contraction ? 1 :
(target_working_set_size + work_unit_size - 1) / work_unit_size;
```
### Patches
We have patched the issue in GitHub commit 2be2cdf3a123e231b16f766aa0e27d56b4606535
https://github.com/tensorflow/tensorflow/commit/2be2cdf3a123e231b16f766aa0e27d56b4606535.
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 Yakun Zhang and Ying Wang of Baidu X-Team.