Reference binding to nullptr in shape inference

### Impact An attacker can cause undefined behavior via binding a reference to null pointer in `tf.raw_ops.SparseFillEmptyRows`: ```python import tensorflow as tf tf.compat.v1.disable_v2_behavior() tf.raw_ops.SparseFillEmptyRows( indices = tf.constant([], shape=[0, 0], dtype=tf.int64), values = tf.constant([], shape=[0], dtype=tf.int64), dense_shape = tf.constant([], shape=[0], dtype=tf.int64), default_value = 0) ``` The shape inference [implementation](https://github.com/tensorflow/tensorflow/blob/460e000de3a83278fb00b61a16d161b1964f15f4/tensorflow/core/ops/sparse_ops.cc#L608-L634) does not validate that the input arguments are not empty tensors. ### Patches We have patched the issue in GitHub commit [578e634b4f1c1c684d4b4294f9e5281b2133b3ed](https://github.com/tensorflow/tensorflow/commit/578e634b4f1c1c684d4b4294f9e5281b2133b3ed). 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 Yakun Zhang of Baidu Security

CVSS 3.1

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

Severity from

GitHub (reviewed advisory)

Weakness

CWE-824

Also known as

BIT-tensorflow-2021-37676, CVE-2021-37676, PYSEC-2021-298, PYSEC-2021-589, PYSEC-2021-787

• github.com/tensorflow/tensorflow/security/advisories/GHSA-v768-w7m9-2vmm
• nvd.nist.gov/vuln/detail/CVE-2021-37676
• github.com/tensorflow/tensorflow/commit/578e634b4f1c1c684d4b4294f9e5281b2133b3ed
• github.com/pypa/advisory-database/tree/main/vulns/tensorflow-cpu/PYSEC-2021-589.yaml
• github.com/pypa/advisory-database/tree/main/vulns/tensorflow-gpu/PYSEC-2021-787.yaml
• github.com/pypa/advisory-database/tree/main/vulns/tensorflow/PYSEC-2021-298.yaml
• github.com/tensorflow/tensorflow