chore: import upstream snapshot with attribution

tensorflow/security/advisory/tfsa-2021-068.md

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+## TFSA-2021-068: Heap buffer overflow in `MaxPoolGrad`
+
+### CVE Number
+CVE-2021-29579
+
+### Impact
+The implementation of `tf.raw_ops.MaxPoolGrad` is vulnerable to a heap buffer
+overflow:
+
+```python
+import tensorflow as tf
+
+orig_input = tf.constant([0.0], shape=[1, 1, 1, 1], dtype=tf.float32)
+orig_output = tf.constant([0.0], shape=[1, 1, 1, 1], dtype=tf.float32)
+grad = tf.constant([], shape=[0, 0, 0, 0], dtype=tf.float32)
+ksize = [1, 1, 1, 1]
+strides = [1, 1, 1, 1]
+padding = "SAME"
+
+tf.raw_ops.MaxPoolGrad(
+  orig_input=orig_input, orig_output=orig_output, grad=grad, ksize=ksize,
+  strides=strides, padding=padding, explicit_paddings=[])
+```
+
+The
+[implementation](https://github.com/tensorflow/tensorflow/blob/ab1e644b48c82cb71493f4362b4dd38f4577a1cf/tensorflow/core/kernels/maxpooling_op.cc#L194-L203)
+fails to validate that indices used to access elements of input/output arrays
+are valid:
+
+```cc
+for (int index = out_start; index < out_end; ++index) {
+  int input_backprop_index = out_arg_max_flat(index);
+  FastBoundsCheck(input_backprop_index - in_start, in_end - in_start);
+  input_backprop_flat(input_backprop_index) += out_backprop_flat(index);
+}
+```
+
+Whereas accesses to `input_backprop_flat` are guarded by `FastBoundsCheck`, the
+indexing in `out_backprop_flat` can result in OOB access.
+
+### Patches
+We have patched the issue in GitHub commit
+[a74768f8e4efbda4def9f16ee7e13cf3922ac5f7](https://github.com/tensorflow/tensorflow/commit/a74768f8e4efbda4def9f16ee7e13cf3922ac5f7).
+
+The fix will be included in TensorFlow 2.5.0. We will also cherrypick this
+commit on TensorFlow 2.4.2, TensorFlow 2.3.3, TensorFlow 2.2.3 and TensorFlow
+2.1.4, 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 Ying Wang and Yakun Zhang of Baidu
+X-Team.