Vulnerability: Session operations in eager mode lead to null pointer dereferences ### Impact
In eager mode (default in TF 2.0 and later), session operations are invalid. However, users could still call the raw ops associated with them and trigger a null pointer dereference:

```python
import tensorflow as tf
tf.raw_ops.GetSessionTensor(handle=['\x12\x1a\x07'],dtype=4)
```
```python
import tensorflow as tf
tf.raw_ops.DeleteSessionTensor(handle=['\x12\x1a\x07'])
``` 

The [implementation](https://github.com/tensorflow/tensorflow/blob/eebb96c2830d48597d055d247c0e9aebaea94cd5/tensorflow/core/kernels/session_ops.cc#L104) dereferences the session state pointer without checking if it is valid:
  
```cc
  OP_REQUIRES_OK(ctx, ctx->session_state()->GetTensor(name, &val));
```

Thus, in eager mode, `ctx->session_state()` is nullptr and the call of the member function is undefined behavior.

### Patches
We have patched the issue in GitHub commit [ff70c47a396ef1e3cb73c90513da4f5cb71bebba](https://github.com/tensorflow/tensorflow/commit/ff70c47a396ef1e3cb73c90513da4f5cb71bebba).

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 members of the Aivul Team from Qihoo 360.
Vulnerability ID: GHSA-62gx-355r-9fhg
CWE: CWE-476
Severity score: 2.5
Project: https://github.com/tensorflow/tensorflow
Published: 2021-05-21
File: tensorflow/core/kernels/session_ops.cc
Commit message: Fix `tf.raw_ops.GetSessionTensor` and `tf.raw_ops.DeleteSessionTensor` null pointer dereferences.

PiperOrigin-RevId: 368294154
Change-Id: Ie10f07a0a9a1c2b685e08153d48a0ca4b93f9fc9
Commit author: Amit Patankar

Vulnerable version (before fix):
 TF_CALL_NUMBER_TYPES(REGISTER_GPU_KERNEL); REGISTER_GPU_KERNEL(bool); #undef REGISTER_GPU_KERNEL   class GetSessionTensorOp : public OpKernel {  public:   explicit GetSessionTensorOp(OpKernelConstruction* context) class GetSessionTensorOp : public OpKernel {     const Tensor& handle = ctx->input(0);     const string& name = handle.scalar<tstring>()();     Tensor val;     OP_REQUIRES_OK(ctx, ctx->session_state()->GetTensor(name, &val));     ctx->set_output(0, val);   }  TF_CALL_NUMBER_TYPES(REGISTER_GPU_KERNEL); REGISTER_GPU_KERNEL(bool); #undef REGISTER_GPU_KERNEL   class DeleteSessionTensorOp : public OpKernel {  public:   explicit DeleteSessionTensorOp(OpKernelConstruction* context) class DeleteSessionTensorOp : public OpKernel {   void Compute(OpKernelContext* ctx) override {     const Tensor& handle = ctx->input(0);     const string& name = handle.scalar<tstring>()();     OP_REQUIRES_OK(ctx, ctx->session_state()->DeleteTensor(name));   }    TF_DISALLOW_COPY_AND_ASSIGN(DeleteSessionTensorOp);

Fixed version (after fix):
 TF_CALL_NUMBER_TYPES(REGISTER_GPU_KERNEL); REGISTER_GPU_KERNEL(bool); #undef REGISTER_GPU_KERNEL  class GetSessionTensorOp : public OpKernel {  public:   explicit GetSessionTensorOp(OpKernelConstruction* context) class GetSessionTensorOp : public OpKernel {     const Tensor& handle = ctx->input(0);     const string& name = handle.scalar<tstring>()();     Tensor val;     auto session_state = ctx->session_state();     OP_REQUIRES(ctx, session_state != nullptr,                 errors::FailedPrecondition(                     "GetSessionTensor called on null session state"));     OP_REQUIRES_OK(ctx, session_state->GetTensor(name, &val));     ctx->set_output(0, val);   }  TF_CALL_NUMBER_TYPES(REGISTER_GPU_KERNEL); REGISTER_GPU_KERNEL(bool); #undef REGISTER_GPU_KERNEL  class DeleteSessionTensorOp : public OpKernel {  public:   explicit DeleteSessionTensorOp(OpKernelConstruction* context) class DeleteSessionTensorOp : public OpKernel {   void Compute(OpKernelContext* ctx) override {     const Tensor& handle = ctx->input(0);     const string& name = handle.scalar<tstring>()();     auto session_state = ctx->session_state();     OP_REQUIRES(ctx, session_state != nullptr,                 errors::FailedPrecondition(                     "DeleteSessionTensor called on null session state"));     OP_REQUIRES_OK(ctx, session_state->DeleteTensor(name));   }    TF_DISALLOW_COPY_AND_ASSIGN(DeleteSessionTensorOp);

Patch:
patch: @@ -91,7 +91,6 @@ TF_CALL_NUMBER_TYPES(REGISTER_GPU_KERNEL);
 REGISTER_GPU_KERNEL(bool);
 #undef REGISTER_GPU_KERNEL
 
-
 class GetSessionTensorOp : public OpKernel {
  public:
   explicit GetSessionTensorOp(OpKernelConstruction* context)
@@ -101,7 +100,11 @@ class GetSessionTensorOp : public OpKernel {
     const Tensor& handle = ctx->input(0);
     const string& name = handle.scalar<tstring>()();
     Tensor val;
-    OP_REQUIRES_OK(ctx, ctx->session_state()->GetTensor(name, &val));
+    auto session_state = ctx->session_state();
+    OP_REQUIRES(ctx, session_state != nullptr,
+                errors::FailedPrecondition(
+                    "GetSessionTensor called on null session state"));
+    OP_REQUIRES_OK(ctx, session_state->GetTensor(name, &val));
     ctx->set_output(0, val);
   }
 
@@ -122,7 +125,6 @@ TF_CALL_NUMBER_TYPES(REGISTER_GPU_KERNEL);
 REGISTER_GPU_KERNEL(bool);
 #undef REGISTER_GPU_KERNEL
 
-
 class DeleteSessionTensorOp : public OpKernel {
  public:
   explicit DeleteSessionTensorOp(OpKernelConstruction* context)
@@ -131,7 +133,11 @@ class DeleteSessionTensorOp : public OpKernel {
   void Compute(OpKernelContext* ctx) override {
     const Tensor& handle = ctx->input(0);
     const string& name = handle.scalar<tstring>()();
-    OP_REQUIRES_OK(ctx, ctx->session_state()->DeleteTensor(name));
+    auto session_state = ctx->session_state();
+    OP_REQUIRES(ctx, session_state != nullptr,
+                errors::FailedPrecondition(
+                    "DeleteSessionTensor called on null session state"));
+    OP_REQUIRES_OK(ctx, session_state->DeleteTensor(name));
   }
 
   TF_DISALLOW_COPY_AND_ASSIGN(DeleteSessionTensorOp);
path: tensorflow/core/kernels/session_ops.cc