On Thu, Sep 10, 2020 at 02:22:37PM -0700, John Hubbard wrote:

Or am I way off here, and it really is possible (aside from the current
s390 situation) to observe something that "is no longer a page table"?

Yes, that is the issue. Remember there is no locking for GUP
fast. While a page table cannot be freed there is nothing preventing
the page table entry from being concurrently modified.

Without the stack variable it looks like this:

       pud_t pud = READ_ONCE(*pudp);
       if (!pud_present(pud))
            return
       pmd_offset(pudp, address);

And pmd_offset() expands to

    return (pmd_t *)pud_page_vaddr(*pud) + pmd_index(address);

Between the READ_ONCE(*pudp) and (*pud) inside pmd_offset() the value
of *pud can change, eg to !pud_present.

Then pud_page_vaddr(*pud) will crash. It is not use after free, it
is using data that has not been validated.

Jason