Re: [PATCH] mm/userfaultfd: fix missing PTE unmap for non-migration entries

[Suren Baghdasaryan <surenb@google.com>]

On Tue, Jul 8, 2025 at 8:10 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 01.07.25 02:57, Andrew Morton wrote:
> > On Sun, 29 Jun 2025 23:19:58 -0400 Sasha Levin <sashal@kernel.org> wrote:
> >
> >> When handling non-swap entries in move_pages_pte(), the error handling
> >> for entries that are NOT migration entries fails to unmap the page table
> >> entries before jumping to the error handling label.
> >>
> >> This results in a kmap/kunmap imbalance which on CONFIG_HIGHPTE systems
> >> triggers a WARNING in kunmap_local_indexed() because the kmap stack is
> >> corrupted.
> >>
> >> Example call trace on ARM32 (CONFIG_HIGHPTE enabled):
> >>    WARNING: CPU: 1 PID: 633 at mm/highmem.c:622 kunmap_local_indexed+0x178/0x17c
> >>    Call trace:
> >>      kunmap_local_indexed from move_pages+0x964/0x19f4
> >>      move_pages from userfaultfd_ioctl+0x129c/0x2144
> >>      userfaultfd_ioctl from sys_ioctl+0x558/0xd24
> >>
> >> The issue was introduced with the UFFDIO_MOVE feature but became more
> >> frequent with the addition of guard pages (commit 7c53dfbdb024 ("mm: add
> >> PTE_MARKER_GUARD PTE marker")) which made the non-migration entry code
> >> path more commonly executed during userfaultfd operations.
> >>
> >> Fix this by ensuring PTEs are properly unmapped in all non-swap entry
> >> paths before jumping to the error handling label, not just for migration
> >> entries.
> >
> > I don't get it.
> >
> >> --- a/mm/userfaultfd.c
> >> +++ b/mm/userfaultfd.c
> >> @@ -1384,14 +1384,15 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
> >>
> >>              entry = pte_to_swp_entry(orig_src_pte);
> >>              if (non_swap_entry(entry)) {
> >> +                    pte_unmap(src_pte);
> >> +                    pte_unmap(dst_pte);
> >> +                    src_pte = dst_pte = NULL;
> >>                      if (is_migration_entry(entry)) {
> >> -                            pte_unmap(src_pte);
> >> -                            pte_unmap(dst_pte);
> >> -                            src_pte = dst_pte = NULL;
> >>                              migration_entry_wait(mm, src_pmd, src_addr);
> >>                              err = -EAGAIN;
> >> -                    } else
> >> +                    } else {
> >>                              err = -EFAULT;
> >> +                    }
> >>                      goto out;
> >
> > where we have
> >
> > out:
> >       ...
> >       if (dst_pte)
> >               pte_unmap(dst_pte);
> >       if (src_pte)
> >               pte_unmap(src_pte);
>
> AI slop?

Hmm, but there is even a Call trace in the report. I wonder if the
issue is somewhere else?

>
> --
> Cheers,
>
> David / dhildenb
>