Reference binding to null pointer in `MatrixDiag*` ops

### Impact The implementation of [`MatrixDiag*` operations](https://github.com/tensorflow/tensorflow/blob/4c4f420e68f1cfaf8f4b6e8e3eb857e9e4c3ff33/tensorflow/core/kernels/linalg/matrix_diag_op.cc#L195-L197) does not validate that the tensor arguments are non-empty: ```cc num_rows = context->input(2).flat<int32>()(0); num_cols = context->input(3).flat<int32>()(0); padding_value = context->input(4).flat<T>()(0); ``` Thus, users can trigger null pointer dereferences if any of the above tensors are null: ```python import tensorflow as tf d = tf.convert_to_tensor([],dtype=tf.float32) p = tf.convert_to_tensor([],dtype=tf.float32) tf.raw_ops.MatrixDiagV2(diagonal=d, k=0, num_rows=0, num_cols=0, padding_value=p) ``` Changing from `tf.raw_ops.MatrixDiagV2` to `tf.raw_ops.MatrixDiagV3` still reproduces the issue. ### Patches We have patched the issue in GitHub commit [a7116dd3913c4a4afd2a3a938573aa7c785fdfc6](https://github.com/tensorflow/tensorflow/commit/a7116dd3913c4a4afd2a3a938573aa7c785fdfc6). 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 Ye Zhang and Yakun Zhang of Baidu X-Team.

CVSS 3.1

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

Severity from

GitHub (reviewed advisory)

Weakness

CWE-476

Also known as

BIT-tensorflow-2021-29515, CVE-2021-29515, PYSEC-2021-152, PYSEC-2021-443, PYSEC-2021-641

• github.com/tensorflow/tensorflow/security/advisories/GHSA-hc6c-75p4-hmq4
• nvd.nist.gov/vuln/detail/CVE-2021-29515
• github.com/tensorflow/tensorflow/commit/a7116dd3913c4a4afd2a3a938573aa7c785fdfc6
• github.com/pypa/advisory-database/tree/main/vulns/tensorflow-cpu/PYSEC-2021-443.yaml
• github.com/pypa/advisory-database/tree/main/vulns/tensorflow-gpu/PYSEC-2021-641.yaml
• github.com/pypa/advisory-database/tree/main/vulns/tensorflow/PYSEC-2021-152.yaml
• github.com/tensorflow/tensorflow