Vulnerability: Null pointer dereference in `UncompressElement` ### Impact
The code for `tf.raw_ops.UncompressElement` can be made to trigger a null pointer dereference: 

```python
import tensorflow as tf

data = tf.data.Dataset.from_tensors([0.0])
tf.raw_ops.UncompressElement(
  compressed=tf.data.experimental.to_variant(data),
  output_types=[tf.int64],
  output_shapes=[2])
```
  
The [implementation](https://github.com/tensorflow/tensorflow/blob/f24faa153ad31a4b51578f8181d3aaab77a1ddeb/tensorflow/core/kernels/data/experimental/compression_ops.cc#L50-L53) obtains a pointer to a `CompressedElement` from a `Variant` tensor and then proceeds to dereference it for decompressing. There is no check that the `Variant` tensor contained a `CompressedElement`, so the pointer is actually `nullptr`.

### Patches
We have patched the issue in GitHub commit [7bdf50bb4f5c54a4997c379092888546c97c3ebd](https://github.com/tensorflow/tensorflow/commit/7bdf50bb4f5c54a4997c379092888546c97c3ebd).

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-6gv8-p3vj-pxvr
CWE: CWE-476
Severity score: 7.7
Project: https://github.com/tensorflow/tensorflow
Published: 2021-08-25
File: tensorflow/core/kernels/data/experimental/compression_ops.cc
Commit message: Ensure non-empty compressed input in tf.raw_ops.UncompressElement

PiperOrigin-RevId: 383955815
Change-Id: I072a84fd02738dd2f51b3f42836ed80067dba4a8
Commit author: Laura Pak

Vulnerable version (before fix):
 void UncompressElementOp::Compute(OpKernelContext* ctx) {   Tensor tensor = ctx->input(0);   const Variant& variant = tensor.scalar<Variant>()();   const CompressedElement* compressed = variant.get<CompressedElement>();    std::vector<Tensor> components;   OP_REQUIRES_OK(ctx, UncompressElement(*compressed, &components));

Fixed version (after fix):
 void UncompressElementOp::Compute(OpKernelContext* ctx) {   Tensor tensor = ctx->input(0);   const Variant& variant = tensor.scalar<Variant>()();   const CompressedElement* compressed = variant.get<CompressedElement>();   OP_REQUIRES(       ctx, compressed != nullptr,       errors::InvalidArgument(           "Input does not contain a compressed element. Instead got tensor ",           tensor.DebugString()));    std::vector<Tensor> components;   OP_REQUIRES_OK(ctx, UncompressElement(*compressed, &components));

Patch:
patch: @@ -48,6 +48,11 @@ void UncompressElementOp::Compute(OpKernelContext* ctx) {
   Tensor tensor = ctx->input(0);
   const Variant& variant = tensor.scalar<Variant>()();
   const CompressedElement* compressed = variant.get<CompressedElement>();
+  OP_REQUIRES(
+      ctx, compressed != nullptr,
+      errors::InvalidArgument(
+          "Input does not contain a compressed element. Instead got tensor ",
+          tensor.DebugString()));
 
   std::vector<Tensor> components;
   OP_REQUIRES_OK(ctx, UncompressElement(*compressed, &components));
path: tensorflow/core/kernels/data/experimental/compression_ops.cc