Vulnerability: Heap OOB in `RaggedGather` ### Impact
If the arguments to `tf.raw_ops.RaggedGather` don't determine a valid ragged tensor code can trigger a read from outside of bounds of heap allocated buffers.
                                                                                                                                                                                                                                                                                          
```python
import tensorflow as tf

tf.raw_ops.RaggedGather(
  params_nested_splits = [0,0,0],
  params_dense_values = [1,1],
  indices = [0,0,9,0,0],
  OUTPUT_RAGGED_RANK=0)
```

In debug mode, the same code triggers a `CHECK` failure.

The [implementation](https://github.com/tensorflow/tensorflow/blob/8d72537c6abf5a44103b57b9c2e22c14f5f49698/tensorflow/core/kernels/ragged_gather_op.cc#L70) directly reads the first dimension of a tensor shape before checking that said tensor has rank of at least 1 (i.e., it is not a scalar). Furthermore, the implementation does not check that the list given by `params_nested_splits` is not an empty list of tensors.

### Patches
We have patched the issue in GitHub commit [a2b743f6017d7b97af1fe49087ae15f0ac634373](https://github.com/tensorflow/tensorflow/commit/a2b743f6017d7b97af1fe49087ae15f0ac634373).

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-9c8h-vvrj-w2p8
CWE: CWE-125
Severity score: 7.1
Project: https://github.com/tensorflow/tensorflow
Published: 2021-08-25
File: tensorflow/core/kernels/ragged_gather_op.cc
Commit message: Fix heap OOB in `tf.raw_ops.RaggedGather`

PiperOrigin-RevId: 388355464
Change-Id: If14d96231d1cd7aad7c4d1c22c1bab1576b75717
Commit author: Mihai Maruseac

Vulnerable version (before fix):
 class RaggedGatherOpBase : public OpKernel {    void Compute(OpKernelContext* context) override {     // Get the input Tensors.     OpInputList params_nested_splits_in;     OP_REQUIRES_OK(context, context->input_list("params_nested_splits",                                                 &params_nested_splits_in));     const Tensor& params_dense_values_in =         context->input(params_nested_splits_in.size());     const Tensor& indices_in =         context->input(params_nested_splits_in.size() + 1);      DCHECK_GT(params_nested_splits_in.size(), 0);  // Enforced by REGISTER_OP.     SPLITS_TYPE num_params = params_nested_splits_in[0].dim_size(0) - 1;     OP_REQUIRES_OK(context, ValidateIndices(indices_in, num_params)); 

Fixed version (after fix):
 class RaggedGatherOpBase : public OpKernel {    void Compute(OpKernelContext* context) override {     // Get the input Tensors.      OpInputList params_nested_splits_in;     OP_REQUIRES_OK(context, context->input_list("params_nested_splits",                                                 &params_nested_splits_in));     OP_REQUIRES(         context, params_nested_splits_in.size() > 0,         errors::InvalidArgument("params_nested_splits must be non empty"));      const Tensor& params_dense_values_in =         context->input(params_nested_splits_in.size());     const Tensor& indices_in =         context->input(params_nested_splits_in.size() + 1);      OP_REQUIRES(context, params_nested_splits_in[0].dims() > 0,                 errors::InvalidArgument("Split tensors must not be scalars"));     SPLITS_TYPE num_params = params_nested_splits_in[0].dim_size(0) - 1;     OP_REQUIRES_OK(context, ValidateIndices(indices_in, num_params)); 

Patch:
patch: @@ -58,15 +58,21 @@ class RaggedGatherOpBase : public OpKernel {
 
   void Compute(OpKernelContext* context) override {
     // Get the input Tensors.
+
     OpInputList params_nested_splits_in;
     OP_REQUIRES_OK(context, context->input_list("params_nested_splits",
                                                 &params_nested_splits_in));
+    OP_REQUIRES(
+        context, params_nested_splits_in.size() > 0,
+        errors::InvalidArgument("params_nested_splits must be non empty"));
+
     const Tensor& params_dense_values_in =
         context->input(params_nested_splits_in.size());
     const Tensor& indices_in =
         context->input(params_nested_splits_in.size() + 1);
 
-    DCHECK_GT(params_nested_splits_in.size(), 0);  // Enforced by REGISTER_OP.
+    OP_REQUIRES(context, params_nested_splits_in[0].dims() > 0,
+                errors::InvalidArgument("Split tensors must not be scalars"));
     SPLITS_TYPE num_params = params_nested_splits_in[0].dim_size(0) - 1;
     OP_REQUIRES_OK(context, ValidateIndices(indices_in, num_params));
path: tensorflow/core/kernels/ragged_gather_op.cc