Vulnerability: Null pointer dereference in `SparseFillEmptyRows` ### Impact
An attacker can trigger a null pointer dereference in the implementation of `tf.raw_ops.SparseFillEmptyRows`:

```python
import tensorflow as tf

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
    
tf.raw_ops.SparseFillEmptyRows(
    indices=indices, values=values, dense_shape=dense_shape,
    default_value=default_value)
```

This is because of missing [validation](https://github.com/tensorflow/tensorflow/blob/fdc82089d206e281c628a93771336bf87863d5e8/tensorflow/core/kernels/sparse_fill_empty_rows_op.cc#L230-L231) that was covered under a `TODO`. If the `dense_shape` tensor is empty, then `dense_shape_t.vec<>()` would cause a null pointer dereference in the implementation of the op:

```cc
template <typename T, typename Tindex>
struct SparseFillEmptyRows<CPUDevice, T, Tindex> {
  Status operator()(OpKernelContext* context, const Tensor& default_value_t,
                    const Tensor& indices_t, const Tensor& values_t,
                    const Tensor& dense_shape_t,
                    typename AsyncOpKernel::DoneCallback done) {
    ...
    const auto dense_shape = dense_shape_t.vec<Tindex>();
    ... 
  }
}
``` 

### Patches
We have patched the issue in GitHub commit [faa76f39014ed3b5e2c158593b1335522e573c7f](https://github.com/tensorflow/tensorflow/commit/faa76f39014ed3b5e2c158593b1335522e573c7f).

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.
Vulnerability ID: GHSA-r6pg-pjwc-j585
CWE: CWE-476
Severity score: 2.5
Project: https://github.com/tensorflow/tensorflow
Published: 2021-05-21
File: tensorflow/core/kernels/sparse_fill_empty_rows_op.cc
Commit message: Fix heap-buffer-overflow issue with `tf.raw_ops.SparseFillEmptyRows`.

PiperOrigin-RevId: 372009178
Change-Id: Ia1a9e9691ecaa072f32fb39a0887b2aabd399210
Commit author: Amit Patankar

Vulnerable version (before fix):
 void SparseFillEmptyRowsOpImpl(OpKernelContext* context,                               default_value_t.shape().DebugString()),       done);   // TODO(ebrevdo): add shape checks between values, indices,   // dense_shape.  Also add check that dense rank > 0.    using FunctorType = functor::SparseFillEmptyRows<Device, T, Tindex>;   OP_REQUIRES_OK_ASYNC(context,

Fixed version (after fix):
 void SparseFillEmptyRowsOpImpl(OpKernelContext* context,                               default_value_t.shape().DebugString()),       done);   // TODO(ebrevdo): add shape checks between values, indices,   // Also add check that dense rank > 0.   OP_REQUIRES_ASYNC(context, dense_shape_t.NumElements() != 0,                     errors::InvalidArgument("Dense shape cannot be empty."),                     done);    using FunctorType = functor::SparseFillEmptyRows<Device, T, Tindex>;   OP_REQUIRES_OK_ASYNC(context,

Patch:
patch: @@ -228,7 +228,10 @@ void SparseFillEmptyRowsOpImpl(OpKernelContext* context,
                               default_value_t.shape().DebugString()),
       done);
   // TODO(ebrevdo): add shape checks between values, indices,
-  // dense_shape.  Also add check that dense rank > 0.
+  // Also add check that dense rank > 0.
+  OP_REQUIRES_ASYNC(context, dense_shape_t.NumElements() != 0,
+                    errors::InvalidArgument("Dense shape cannot be empty."),
+                    done);
 
   using FunctorType = functor::SparseFillEmptyRows<Device, T, Tindex>;
   OP_REQUIRES_OK_ASYNC(context,
path: tensorflow/core/kernels/sparse_fill_empty_rows_op.cc