NPE in TFLite
### Impact
The implementation of SVDF in TFLite is vulnerable to a null pointer error
https://github.com/tensorflow/tensorflow/blob/460e000de3a83278fb00b61a16d161b1964f15f4/tensorflow/lite/kernels/svdf.cc#L300-L313:
```cc
TfLiteTensor* state = GetVariableInput(context, node, kStateTensor);
// ...
GetTensorData<float>(state)
```
The `GetVariableInput` function
https://github.com/tensorflow/tensorflow/blob/460e000de3a83278fb00b61a16d161b1964f15f4/tensorflow/lite/kernels/kernel_util.cc#L115-L119 can return a null pointer but `GetTensorData` assumes that the argument is always a valid tensor.
```cc
TfLiteTensor* GetVariableInput(TfLiteContext* context, const TfLiteNode* node,
int index) {
TfLiteTensor* tensor = GetMutableInput(context, node, index);
return tensor->is_variable ? tensor : nullptr;
}
```
Furthermore, because `GetVariableInput` calls `GetMutableInput`
https://github.com/tensorflow/tensorflow/blob/460e000de3a83278fb00b61a16d161b1964f15f4/tensorflow/lite/kernels/kernel_util.cc#L82-L90 which might return `nullptr`, the `tensor->is_variable` expression can also trigger a null pointer exception.
### Patches
We have patched the issue in GitHub commit 5b048e87e4e55990dae6b547add4dae59f4e1c76
https://github.com/tensorflow/tensorflow/commit/5b048e87e4e55990dae6b547add4dae59f4e1c76.
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.