Unitialized access in `EinsumHelper::ParseEquation`
### Impact
During execution, `EinsumHelper::ParseEquation()`
https://github.com/tensorflow/tensorflow/blob/e0b6e58c328059829c3eb968136f17aa72b6c876/tensorflow/core/kernels/linalg/einsum_op_impl.h#L126-L181 is supposed to set the flags in `input_has_ellipsis` vector and `*output_has_ellipsis` boolean to indicate whether there is ellipsis in the corresponding inputs and output.
However, the code only changes these flags to `true` and never assigns `false`.
```cc
for (int i = 0; i < num_inputs; ++i) {
input_label_counts->at(i).resize(num_labels);
for (const int label : input_labels->at(i)) {
if (label != kEllipsisLabel)
input_label_counts->at(i)[label] += 1;
else
input_has_ellipsis->at(i) = true;
}
}
output_label_counts->resize(num_labels);
for (const int label : *output_labels) {
if (label != kEllipsisLabel)
output_label_counts->at(label) += 1;
else
*output_has_ellipsis = true;
}
```
This results in unitialized variable access if callers assume that `EinsumHelper::ParseEquation()` always sets these flags.
### Patches
We have patched the issue in GitHub commit f09caa532b6e1ac8d2aa61b7832c78c5b79300c6
https://github.com/tensorflow/tensorflow/commit/f09caa532b6e1ac8d2aa61b7832c78c5b79300c6.
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.