chore: import upstream snapshot with attribution

tensorflow/security/advisory/tfsa-2021-131.md

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+## TFSA-2021-131: Reference binding to nullptr in `MatrixDiagV*` ops
+
+### CVE Number
+CVE-2021-37657
+
+### Impact
+An attacker can cause undefined behavior via binding a reference to null pointer
+in all operations of type `tf.raw_ops.MatrixDiagV*`:
+
+```python
+import tensorflow as tf
+
+tf.raw_ops.MatrixDiagV3(
+  diagonal=[1,0],
+  k=[],
+  num_rows=[1,2,3],
+  num_cols=[4,5],
+  padding_value=[],
+  align='RIGHT_RIGHT')
+```
+
+The
+[implementation](https://github.com/tensorflow/tensorflow/blob/84d053187cb80d975ef2b9684d4b61981bca0c41/tensorflow/core/kernels/linalg/matrix_diag_op.cc)
+has incomplete validation that the value of `k` is a valid tensor. We have check
+that this value is either a scalar or a vector, but there is no check for the
+number of elements. If this is an empty tensor, then code that accesses the
+first element of the tensor is wrong:
+
+```cc
+  auto& diag_index = context->input(1);
+  ...
+  lower_diag_index = diag_index.flat<int32>()(0);
+```
+
+### Patches
+We have patched the issue in GitHub commit
+[f2a673bd34f0d64b8e40a551ac78989d16daad09](https://github.com/tensorflow/tensorflow/commit/f2a673bd34f0d64b8e40a551ac78989d16daad09).
+
+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.