Re: [PATCH 1/3] mm/hugetlb: Fix race condition of uffd missing/minor handling

[Mike Kravetz <mike.kravetz@oracle.com>]

On 10/03/22 17:27, Peter Xu wrote:
> On Mon, Oct 03, 2022 at 10:20:29AM -0700, Mike Kravetz wrote:
> > On 10/03/22 11:56, Peter Xu wrote:
> > > After the recent rework patchset of hugetlb locking on pmd sharing,
> > > kselftest for userfaultfd sometimes fails on hugetlb private tests with
> > > unexpected write fault checks.
> > > 
> > > It turns out there's nothing wrong within the locking series regarding this
> > > matter, but it could have changed the timing of threads so it can trigger
> > > an old bug.
> > > 
> > > The real bug is when we call hugetlb_no_page() we're not with the pgtable
> > > lock.  It means we're reading the pte values lockless.  It's perfectly fine
> > > in most cases because before we do normal page allocations we'll take the
> > > lock and check pte_same() again.  However before that, there are actually
> > > two paths on userfaultfd missing/minor handling that may directly move on
> > > with the fault process without checking the pte values.
> > > 
> > > It means for these two paths we may be generating an uffd message based on
> > > an unstable pte, while an unstable pte can legally be anything as long as
> > > the modifier holds the pgtable lock.
> > > 
> > > One example, which is also what happened in the failing kselftest and
> > > caused the test failure, is that for private mappings CoW can happen on one
> > > page.  CoW requires pte being cleared before being replaced with a new page
> > > for TLB coherency, but then there can be a race condition:
> > > 
> > >         thread 1                              thread 2
> > >         --------                              --------
> > > 
> > >       hugetlb_fault                         hugetlb_fault
> > >         private pte RO
> > >         hugetlb_wp
> > >           pgtable_lock()
> > >           huge_ptep_clear_flush
> > >                                               pte=NULL
> > >                                               hugetlb_no_page
> > >                                                 generate uffd missing event
> > >                                                 even if page existed!!
> > >           set_huge_pte_at
> > >           pgtable_unlock()
> > 
> > Thanks for working on this Peter!
> > 
> > I agree with this patch, but I suspect the above race is not possible.  Why?
> > In both cases, we take the hugetlb fault mutex when processing a huegtlb
> > page fault.  This means only one thread can execute the fault code for
> > a specific mapping/index at a time.  This is why I was so confused, and may
> > remain a bit confused about how we end up with userfault processing a write
> > fault.  It was part of the reason for my 'unclear' wording about this being
> > more about cpus not seeing updated values.  Note that we do drop the fault
> > mutex before calling handle_usefault, but by then we have already made the
> > 'missing' determination.
> > 
> > Thoughts?  Perhaps, I am still confused.
> 
> It's my fault to have the commit message wrong, sorry. :) And thanks for
> raising this question, I could have overlooked that.
> 
> It turns out it's not the CoW that's clearing the pte... it's the
> wr-protect with huge_ptep_modify_prot_start().  So the race is with
> UFFDIO_WRITEPROTECT, not CoW.

Thank you!  Now I understand.

This also explains why the new locking exposes the race.
hugetlb_change_protection needs to take the i_mmap_sema in write mode because
it could unshare pmds.  Previously, hugetlb page faults took i_mmap_sema in
read mode so this race could not happen.
-- 
Mike Kravetz