Vulnerability: Null pointer dereference in `MatrixDiagPartOp` ### Impact
If a user does not provide a valid padding value to `tf.raw_ops.MatrixDiagPartOp`, then the code triggers a null pointer dereference (if input is empty) or produces invalid behavior, ignoring all values after the first:

```python
import tensorflow as tf

tf.raw_ops.MatrixDiagPartV2(
  input=tf.ones(2,dtype=tf.int32),
  k=tf.ones(2,dtype=tf.int32),
  padding_value=[])
```

Although this example is given for `MatrixDiagPartV2`, all versions of the operation are affected.

The [implementation](https://github.com/tensorflow/tensorflow/blob/8d72537c6abf5a44103b57b9c2e22c14f5f49698/tensorflow/core/kernels/linalg/matrix_diag_op.cc#L89) reads the first value from a tensor buffer without first checking that the tensor has values to read from.

### Patches
We have patched the issue in GitHub commit [482da92095c4d48f8784b1f00dda4f81c28d2988](https://github.com/tensorflow/tensorflow/commit/482da92095c4d48f8784b1f00dda4f81c28d2988).

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.
Vulnerability ID: GHSA-fcwc-p4fc-c5cc
CWE: CWE-476
Severity score: 7.7
Project: https://github.com/tensorflow/tensorflow
Published: 2021-08-25
File: tensorflow/core/kernels/linalg/matrix_diag_op.cc
Commit message: Ensure non-empty padding_value input to tf.raw_ops.MatrixDiagPartV2, if a padding_value is input

PiperOrigin-RevId: 388314614
Change-Id: If0b51ad58d5d8543a6be6ce8f42ae4755c80d55f
Commit author: Laura Pak

Vulnerable version (before fix):
 class MatrixDiagPartOp : public OpKernel {           upper_diag_index = diag_index.flat<int32>()(1);         }       }       padding_value = context->input(2).flat<T>()(0);     }     const TensorShape& input_shape = input.shape(); 

Fixed version (after fix):
 class MatrixDiagPartOp : public OpKernel {           upper_diag_index = diag_index.flat<int32>()(1);         }       }       const Tensor& padding_in = context->input(2);       OP_REQUIRES(context, padding_in.NumElements() == 1,                   errors::InvalidArgument("Padding must be scalar."));       padding_value = padding_in.flat<T>()(0);     }     const TensorShape& input_shape = input.shape(); 

Patch:
patch: @@ -89,7 +89,10 @@ class MatrixDiagPartOp : public OpKernel {
           upper_diag_index = diag_index.flat<int32>()(1);
         }
       }
-      padding_value = context->input(2).flat<T>()(0);
+      const Tensor& padding_in = context->input(2);
+      OP_REQUIRES(context, padding_in.NumElements() == 1,
+                  errors::InvalidArgument("Padding must be scalar."));
+      padding_value = padding_in.flat<T>()(0);
     }
     const TensorShape& input_shape = input.shape();
path: tensorflow/core/kernels/linalg/matrix_diag_op.cc