Vulnerability: Null pointer dereference and heap OOB read in operations restoring tensors ### Impact
When restoring tensors via raw APIs, if the tensor name is not provided, TensorFlow can be tricked into dereferencing a null pointer:

```python
import tensorflow as tf

tf.raw_ops.Restore(
  file_pattern=['/tmp'],
  tensor_name=[], 
  default_value=21,
  dt=tf.int,
  preferred_shard=1)
```
  
The same undefined behavior can be triggered by `tf.raw_ops.RestoreSlice`:
  
```python
import tensorflow as tf

tf.raw_ops.RestoreSlice(
  file_pattern=['/tmp'],
  tensor_name=[], 
  shape_and_slice='2',
  dt=inp.array([tf.int]),
  preferred_shard=1)
```

Alternatively, attackers can read memory outside the bounds of heap allocated data by providing some tensor names but not enough for a successful restoration:

```python
import tensorflow as tf

tf.raw_ops.Restore(
  file_pattern=['/tmp'],
  tensor_name=['x'], 
  default_value=21,
  dt=tf.int,
  preferred_shard=42)
```
  
The [implementation](https://github.com/tensorflow/tensorflow/blob/47a06f40411a69c99f381495f490536972152ac0/tensorflow/core/kernels/save_restore_tensor.cc#L158-L159) retrieves the tensor list corresponding to the `tensor_name` user controlled input and immediately retrieves the tensor at the restoration index (controlled via `preferred_shard` argument). This occurs without validating that the provided list has enough values.

If the list is empty this results in dereferencing a null pointer (undefined behavior). If, however, the list has some elements, if the restoration index is outside the bounds this results in heap OOB read.

### Patches 
We have patched the issue in GitHub commit [9e82dce6e6bd1f36a57e08fa85af213e2b2f2622](https://github.com/tensorflow/tensorflow/commit/9e82dce6e6bd1f36a57e08fa85af213e2b2f2622).

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-gh6x-4whr-2qv4
CWE: CWE-125
Severity score: 8.4
Project: https://github.com/tensorflow/tensorflow
Published: 2021-08-25
File: tensorflow/core/kernels/save_restore_tensor.cc
Commit message: Fix NPE in restoring code.

PiperOrigin-RevId: 388303253
Change-Id: Ia8c68568cb854bca538909a182b31a618d68ce55
Commit author: Mihai Maruseac

Vulnerable version (before fix):
 void RestoreTensor(OpKernelContext* context,         context, size == 1,         errors::InvalidArgument(             "Input 0 (file_pattern) must be a string scalar; got a tensor of ",             size, "elements"));   }   const string& file_pattern = file_pattern_t.flat<tstring>()(0);    const Tensor& tensor_name_t = context->input(1);   const string& tensor_name = tensor_name_t.flat<tstring>()(restore_index);    // If we cannot find a cached reader we will allocate our own.

Fixed version (after fix):
 void RestoreTensor(OpKernelContext* context,         context, size == 1,         errors::InvalidArgument(             "Input 0 (file_pattern) must be a string scalar; got a tensor of ",             size, " elements"));   }   const string& file_pattern = file_pattern_t.flat<tstring>()(0);    const Tensor& tensor_name_t = context->input(1);   {     const int64_t size = tensor_name_t.NumElements();     OP_REQUIRES(context, size > restore_index,                 errors::InvalidArgument(                     "Input 1 (file_pattern) must be a have at least ",                     restore_index + 1, " elements"));   }   const string& tensor_name = tensor_name_t.flat<tstring>()(restore_index);    // If we cannot find a cached reader we will allocate our own.

Patch:
patch: @@ -151,11 +151,18 @@ void RestoreTensor(OpKernelContext* context,
         context, size == 1,
         errors::InvalidArgument(
             "Input 0 (file_pattern) must be a string scalar; got a tensor of ",
-            size, "elements"));
+            size, " elements"));
   }
   const string& file_pattern = file_pattern_t.flat<tstring>()(0);
 
   const Tensor& tensor_name_t = context->input(1);
+  {
+    const int64_t size = tensor_name_t.NumElements();
+    OP_REQUIRES(context, size > restore_index,
+                errors::InvalidArgument(
+                    "Input 1 (file_pattern) must be a have at least ",
+                    restore_index + 1, " elements"));
+  }
   const string& tensor_name = tensor_name_t.flat<tstring>()(restore_index);
 
   // If we cannot find a cached reader we will allocate our own.
path: tensorflow/core/kernels/save_restore_tensor.cc