Vulnerability: Division by 0 in `SparseMatMul` ### Impact
An attacker can cause a denial of service via a FPE runtime error in `tf.raw_ops.SparseMatMul`:

```python
import tensorflow as tf

a = tf.constant([100.0, 100.0, 100.0, 100.0], shape=[2, 2], dtype=tf.float32)
b = tf.constant([], shape=[0, 2], dtype=tf.float32)

tf.raw_ops.SparseMatMul(
    a=a, b=b, transpose_a=True, transpose_b=True,
    a_is_sparse=True, b_is_sparse=True)
``` 
    
The division by 0 occurs deep in Eigen code because the `b` tensor is empty.
    
### Patches
We have patched the issue in GitHub commit [7f283ff806b2031f407db64c4d3edcda8fb9f9f5](https://github.com/tensorflow/tensorflow/commit/7f283ff806b2031f407db64c4d3edcda8fb9f9f5).

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 Ying Wang and Yakun Zhang of Baidu X-Team.
Vulnerability ID: GHSA-xw93-v57j-fcgh
CWE: CWE-369
Severity score: 2.5
Project: https://github.com/tensorflow/tensorflow
Published: 2021-05-21
File: tensorflow/core/kernels/sparse_matmul_op.cc
Commit message: Fix FPE issue in external Eigen source code issue with `tf.raw_ops.SparseMatMul`.

PiperOrigin-RevId: 370992919
Change-Id: Icfb276fef5fb40928b27c3e44608d2aca72c9fd7
Commit author: Amit Patankar

Vulnerable version (before fix):
 class SparseMatMulOp : public OpKernel {     if (transpose_b) {       // TODO(agarwal): avoid transposing the matrix here and directly handle       // transpose in CreateDenseSlices.       right_tr.reset(           new Tensor(right->dtype(),                      TensorShape({right->dim_size(1), right->dim_size(0)})));

Fixed version (after fix):
 class SparseMatMulOp : public OpKernel {     if (transpose_b) {       // TODO(agarwal): avoid transposing the matrix here and directly handle       // transpose in CreateDenseSlices.       OP_REQUIRES(ctx, right->dim_size(0) != 0,                   errors::InvalidArgument("b has an entry 0 in it's shape."));       OP_REQUIRES(ctx, right->dim_size(1) != 0,                   errors::InvalidArgument("b has an entry 0 in it's shape."));       right_tr.reset(           new Tensor(right->dtype(),                      TensorShape({right->dim_size(1), right->dim_size(0)})));

Patch:
patch: @@ -1039,6 +1039,10 @@ class SparseMatMulOp : public OpKernel {
     if (transpose_b) {
       // TODO(agarwal): avoid transposing the matrix here and directly handle
       // transpose in CreateDenseSlices.
+      OP_REQUIRES(ctx, right->dim_size(0) != 0,
+                  errors::InvalidArgument("b has an entry 0 in it's shape."));
+      OP_REQUIRES(ctx, right->dim_size(1) != 0,
+                  errors::InvalidArgument("b has an entry 0 in it's shape."));
       right_tr.reset(
           new Tensor(right->dtype(),
                      TensorShape({right->dim_size(1), right->dim_size(0)})));
path: tensorflow/core/kernels/sparse_matmul_op.cc