Re: [RFC PATCH v2 1/3] mm/gup: fix gup_fast with dynamic page table folding

[Alexander Gordeev <agordeev@linux.ibm.com>]

On Thu, Sep 10, 2020 at 07:11:16PM -0300, Jason Gunthorpe wrote:
> 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.

One thing I ask myself and it is probably a good moment to wonder.

What if the entry is still pud_present, but got remapped after
READ_ONCE(*pudp)? IOW, it is still valid, but points elsewhere?

> Jason