chore: import upstream snapshot with attribution

tensorflow/security/advisory/tfsa-2021-177.md

@@ -0,0 +1,48 @@
+## TFSA-2021-177: Use after free / memory leak in `CollectiveReduceV2`
+
+### CVE Number
+CVE-2021-41220
+
+### Impact
+The [async implementation](https://github.com/tensorflow/tensorflow/blob/8d72537c6abf5a44103b57b9c2e22c14f5f49698/tensorflow/core/kernels/collective_ops.cc#L604-L615) of `CollectiveReduceV2` suffers from a memory leak and a use after free:
+
+```python
+import tensorflow as tf
+
+tf.raw_ops.CollectiveReduceV2(
+  input=[],
+  group_size=[-10, -10, -10],
+  group_key=[-10, -10],
+  instance_key=[-10],
+  ordering_token=[],
+  merge_op='Mul',
+  final_op='Div')
+```
+
+This occurs due to the asynchronous computation and the fact that objects that have been `std::move()`d from are still accessed:
+
+```cc
+auto done_with_cleanup = [col_params, done = std::move(done)]() {
+  done();
+  col_params->Unref();
+};
+OP_REQUIRES_OK_ASYNC(c,
+                     FillCollectiveParams(col_params, REDUCTION_COLLECTIVE,
+                                          /*group_size*/ c->input(1),
+                                          /*group_key*/ c->input(2),
+                                          /*instance_key*/ c->input(3)),
+                     done);
+```
+
+Here, `done` is already moved from by the time `OP_REQUIRES_OK_ASYNC` macro needs to invoke it in case of errors. In this case, we get an undefined behavior, which can manifest via crashes, `std::bad_alloc` throws or just memory leaks.
+
+### Patches
+We have patched the issue in GitHub commit [ca38dab9d3ee66c5de06f11af9a4b1200da5ef75](https://github.com/tensorflow/tensorflow/commit/ca38dab9d3ee66c5de06f11af9a4b1200da5ef75).
+
+The fix will be included in TensorFlow 2.7.0. We will also cherrypick this commit on TensorFlow 2.6.1, as this version is the only one that is also affected.
+
+### 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.