The address space size limitations doesn't mean that only the least significants bits are used, the memory hole is in the middle of the address space[1].

I don't know what Linux does specifically (or under what configurations), but one some other operating systems the user space stack is in the higher half[2].

[1] https://en.wikipedia.org/wiki/X86-64#Canonical_form_addresse...

[2] https://github.com/illumos/illumos-gate/blob/master/usr/src/...