Vulnerability: Heap buffer overflow in `MaxPoolGrad` ### Impact
The implementation of `tf.raw_ops.MaxPoolGrad` is vulnerable to a heap buffer overflow:
  
```python
import tensorflow as tf

orig_input = tf.constant([0.0], shape=[1, 1, 1, 1], dtype=tf.float32)
orig_output = tf.constant([0.0], shape=[1, 1, 1, 1], dtype=tf.float32)
grad = tf.constant([], shape=[0, 0, 0, 0], dtype=tf.float32)
ksize = [1, 1, 1, 1] 
strides = [1, 1, 1, 1]
padding = "SAME"

tf.raw_ops.MaxPoolGrad(
  orig_input=orig_input, orig_output=orig_output, grad=grad, ksize=ksize,
  strides=strides, padding=padding, explicit_paddings=[])
```

The [implementation](https://github.com/tensorflow/tensorflow/blob/ab1e644b48c82cb71493f4362b4dd38f4577a1cf/tensorflow/core/kernels/maxpooling_op.cc#L194-L203) fails to validate that indices used to access elements of input/output arrays are valid:

```cc
for (int index = out_start; index < out_end; ++index) {
  int input_backprop_index = out_arg_max_flat(index);
  FastBoundsCheck(input_backprop_index - in_start, in_end - in_start);
  input_backprop_flat(input_backprop_index) += out_backprop_flat(index);
}
```

Whereas accesses to `input_backprop_flat` are guarded by `FastBoundsCheck`, the indexing in `out_backprop_flat` can result in OOB access.

### Patches
We have patched the issue in GitHub commit [a74768f8e4efbda4def9f16ee7e13cf3922ac5f7](https://github.com/tensorflow/tensorflow/commit/a74768f8e4efbda4def9f16ee7e13cf3922ac5f7).

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-79fv-9865-4qcv
CWE: CWE-787
Severity score: 2.5
Project: https://github.com/tensorflow/tensorflow
Published: 2021-05-21
File: tensorflow/core/kernels/maxpooling_op.cc
Commit message: Prevent heap OOB error in `MaxPoolGrad`

PiperOrigin-RevId: 372424854
Change-Id: Idac0f23867ad8b0601cafbaaa52d5e64269e63a7
Commit author: Mihai Maruseac

Vulnerable version (before fix):
 static void SpatialMaxPoolWithArgMaxHelper(         // CHECK(input_backprop_index >= in_start && input_backprop_index <         // in_end)         FastBoundsCheck(input_backprop_index - in_start, in_end - in_start);         input_backprop_flat(input_backprop_index) += out_backprop_flat(index);       }     }   };

Fixed version (after fix):
 static void SpatialMaxPoolWithArgMaxHelper(         // CHECK(input_backprop_index >= in_start && input_backprop_index <         // in_end)         FastBoundsCheck(input_backprop_index - in_start, in_end - in_start);         if (index < out_backprop.NumElements()) {           input_backprop_flat(input_backprop_index) += out_backprop_flat(index);         }       }     }   };

Patch:
patch: @@ -199,7 +199,9 @@ static void SpatialMaxPoolWithArgMaxHelper(
         // CHECK(input_backprop_index >= in_start && input_backprop_index <
         // in_end)
         FastBoundsCheck(input_backprop_index - in_start, in_end - in_start);
-        input_backprop_flat(input_backprop_index) += out_backprop_flat(index);
+        if (index < out_backprop.NumElements()) {
+          input_backprop_flat(input_backprop_index) += out_backprop_flat(index);
+        }
       }
     }
   };
path: tensorflow/core/kernels/maxpooling_op.cc