The white matter mask after topology correction is stored in subject.dewisp.mask.nii.gz files.
The WM GM and CSF tissue types are stored in subject.pvc.label.nii.gz. Since at MR resolution, most voxels contains a mixture of tissues. The partial tissue fractrions are stored in subject.pvc.frac.nii.gz. These are before topology correction, but depending on your application, these might serve the purpose. Please refer to the following link for more information.