Vulnerability: Undefined behavior via `nullptr` reference binding in sparse matrix multiplication ### Impact
The [code for sparse matrix multiplication](https://github.com/tensorflow/tensorflow/blob/8d72537c6abf5a44103b57b9c2e22c14f5f49698/tensorflow/core/kernels/sparse_matmul_op.cc#L954-L1086) is vulnerable to undefined behavior via binding a reference to `nullptr`:

```python
import tensorflow as tf
  
tf.raw_ops.SparseMatMul(
  a=[[1.0,1.0,1.0]],
  b=[[],[],[]],
  transpose_a=False,
  transpose_b=False,
  a_is_sparse=False, 
  b_is_sparse=True)
```

This occurs whenever the dimensions of `a` or `b` are 0 or less. In the case on one of these is 0, an empty output tensor should be allocated (to conserve the invariant that output tensors are always allocated when the operation is successful) but nothing should be written to it (that is, we should return early from the kernel implementation). Otherwise, attempts to write to this empty tensor would result in heap OOB access.

### Patches
We have patched the issue in GitHub commit [e6cf28c72ba2eb949ca950d834dd6d66bb01cfae](https://github.com/tensorflow/tensorflow/commit/e6cf28c72ba2eb949ca950d834dd6d66bb01cfae).

The fix will be included in TensorFlow 2.7.0. We will also cherrypick this commit on TensorFlow 2.6.1, TensorFlow 2.5.2, and TensorFlow 2.4.4, 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-4f99-p9c2-3j8x
CWE: CWE-125
Severity score: 7.8
Project: https://github.com/tensorflow/tensorflow
Published: 2021-11-10
File: tensorflow/core/kernels/sparse_matmul_op.cc
Commit message: Validate that matrix dimension sizes in SparseMatMul are positive.

PiperOrigin-RevId: 401149683
Change-Id: Ib33eafc561a39c8741ece80b2edce6d4aae9a57d
Commit author: Penporn Koanantakool

Vulnerable version (before fix):
 limitations under the License. #include "tensorflow/core/kernels/fill_functor.h" #include "tensorflow/core/lib/core/blocking_counter.h" #include "tensorflow/core/lib/core/threadpool.h" #include "tensorflow/core/platform/logging.h" #include "tensorflow/core/platform/macros.h" #include "tensorflow/core/platform/mutex.h" class SparseMatMulOp : public OpKernel {                 errors::InvalidArgument(                     "Matrix size incompatible: a: ", a.shape().DebugString(),                     ", b: ", b.shape().DebugString()));     Tensor* output = nullptr;     OP_REQUIRES_OK(ctx, ctx->allocate_output(0, TensorShape({m, n}), &output));      if (k == 0) {       // If the inner dimension k in the matrix multiplication is zero, we fill       // the output with zeros.

Fixed version (after fix):
 limitations under the License. #include "tensorflow/core/kernels/fill_functor.h" #include "tensorflow/core/lib/core/blocking_counter.h" #include "tensorflow/core/lib/core/threadpool.h" #include "tensorflow/core/platform/errors.h" #include "tensorflow/core/platform/logging.h" #include "tensorflow/core/platform/macros.h" #include "tensorflow/core/platform/mutex.h" class SparseMatMulOp : public OpKernel {                 errors::InvalidArgument(                     "Matrix size incompatible: a: ", a.shape().DebugString(),                     ", b: ", b.shape().DebugString()));     OP_REQUIRES(ctx, m >= 0 && n >= 0 && k >= 0,                 errors::InvalidArgument(                     "Matrix dimensions cannot be negative: a: ",                     a.shape().DebugString(), ", b: ", b.shape().DebugString()));     Tensor* output = nullptr;     OP_REQUIRES_OK(ctx, ctx->allocate_output(0, TensorShape({m, n}), &output));      // Return early if at least one of the output dimension size is 0.     if (m == 0 || n == 0) {       return;     }      if (k == 0) {       // If the inner dimension k in the matrix multiplication is zero, we fill       // the output with zeros.

Patch:
patch: @@ -32,6 +32,7 @@ limitations under the License.
 #include "tensorflow/core/kernels/fill_functor.h"
 #include "tensorflow/core/lib/core/blocking_counter.h"
 #include "tensorflow/core/lib/core/threadpool.h"
+#include "tensorflow/core/platform/errors.h"
 #include "tensorflow/core/platform/logging.h"
 #include "tensorflow/core/platform/macros.h"
 #include "tensorflow/core/platform/mutex.h"
@@ -980,9 +981,18 @@ class SparseMatMulOp : public OpKernel {
                 errors::InvalidArgument(
                     "Matrix size incompatible: a: ", a.shape().DebugString(),
                     ", b: ", b.shape().DebugString()));
+    OP_REQUIRES(ctx, m >= 0 && n >= 0 && k >= 0,
+                errors::InvalidArgument(
+                    "Matrix dimensions cannot be negative: a: ",
+                    a.shape().DebugString(), ", b: ", b.shape().DebugString()));
     Tensor* output = nullptr;
     OP_REQUIRES_OK(ctx, ctx->allocate_output(0, TensorShape({m, n}), &output));
 
+    // Return early if at least one of the output dimension size is 0.
+    if (m == 0 || n == 0) {
+      return;
+    }
+
     if (k == 0) {
       // If the inner dimension k in the matrix multiplication is zero, we fill
       // the output with zeros.
path: tensorflow/core/kernels/sparse_matmul_op.cc