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TFSA-2021-126: Use after free in boosted trees creation

CVE Number

CVE-2021-37652

Impact

The implementation for tf.raw_ops.BoostedTreesCreateEnsemble can result in a use after free error if an attacker supplies specially crafted arguments:

import tensorflow as tf

v= tf.Variable([0.0])
tf.raw_ops.BoostedTreesCreateEnsemble(
  tree_ensemble_handle=v.handle,
  stamp_token=[0],
  tree_ensemble_serialized=['0'])

The implementation uses a reference counted resource and decrements the refcount if the initialization fails, as it should. However, when the code was written, the resource was represented as a naked pointer but later refactoring has changed it to be a smart pointer. Thus, when the pointer leaves the scope, a subsequent free-ing of the resource occurs, but this fails to take into account that the refcount has already reached 0, thus the resource has been already freed. During this double-free process, members of the resource object are accessed for cleanup but they are invalid as the entire resource has been freed.

Patches

We have patched the issue in GitHub commit 5ecec9c6fbdbc6be03295685190a45e7eee726ab.

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 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.