Re: [PATCH] mm/khugepaged: Fix ->anon_vma race

[Yang Shi <shy828301@gmail.com>]

On Thu, Jan 12, 2023 at 12:56 AM Kirill A. Shutemov
<kirill@shutemov.name> wrote:
>
> On Wed, Jan 11, 2023 at 02:33:51PM +0100, Jann Horn wrote:
> > If an ->anon_vma is attached to the VMA, collapse_and_free_pmd() requires
> > it to be locked. retract_page_tables() bails out if an ->anon_vma is
> > attached, but does this check before holding the mmap lock (as the comment
> > above the check explains).
> >
> > If we racily merge an existing ->anon_vma (shared with a child process)
> > from a neighboring VMA, subsequent rmap traversals on pages belonging to
> > the child will be able to see the page tables that we are concurrently
> > removing while assuming that nothing else can access them.
> >
> > Repeat the ->anon_vma check once we hold the mmap lock to ensure that there
> > really is no concurrent page table access.
> >
> > Reported-by: Zach O'Keefe <zokeefe@google.com>
> > Fixes: f3f0e1d2150b ("khugepaged: add support of collapse for tmpfs/shmem pages")
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Jann Horn <jannh@google.com>
> > ---
> > zokeefe@ pointed out to me that the current code (after my last round of patches)
> > can hit a lockdep assert by racing, and after staring at it a bit I've
> > convinced myself that this is a real, preexisting bug.
> > (I haven't written a reproducer for it though. One way to hit it might be
> > something along the lines of:
> >
> >  - set up a process A with a private-file-mapping VMA V1
> >  - let A fork() to create process B, thereby copying V1 in A to V1' in B
> >  - let B extend the end of V1'
> >  - let B put some anon pages into the extended part of V1'
>
> At this point V1' gets it's own ->anon_vma, not connected to V1, right?

This is what I got confused too.

>
> >  - let A map a new private-file-mapping VMA V2 directly behind V1, without
> >    an anon_vma
> > [race begins here]
> >   - in A's thread 1: begin retract_page_tables() on V2, run through first
> >     ->anon_vma check
> >   - in A's thread 2: run __anon_vma_prepare() on V2 and ensure that it
> >     merges the anon_vma of V1 (which implies V1 and V2 must be mapping the
> >     same file at compatible offsets)
> >   - in B: trigger rmap traversal on anon page in V1'
>
> I don't follow the race. rmap on V1' will not reach V1.
>
> >  mm/khugepaged.c | 14 +++++++++++++-
> >  1 file changed, 13 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 5cb401aa2b9d..0bfed37f3a3b 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -1644,7 +1644,7 @@ static int retract_page_tables(struct address_space *mapping, pgoff_t pgoff,
> >                * has higher cost too. It would also probably require locking
> >                * the anon_vma.
> >                */
> > -             if (vma->anon_vma) {
> > +             if (READ_ONCE(vma->anon_vma)) {
> >                       result = SCAN_PAGE_ANON;
> >                       goto next;
> >               }
>
> This makes perfect sense. At least for readability. But I think
> false-negative should not lead to bad results.
>
> > @@ -1672,6 +1672,18 @@ static int retract_page_tables(struct address_space *mapping, pgoff_t pgoff,
> >               result = SCAN_PTE_MAPPED_HUGEPAGE;
> >               if ((cc->is_khugepaged || is_target) &&
> >                   mmap_write_trylock(mm)) {
> > +                     /*
> > +                      * Re-check whether we have an ->anon_vma, because
> > +                      * collapse_and_free_pmd() requires that either no
> > +                      * ->anon_vma exists or the anon_vma is locked.
> > +                      * We already checked ->anon_vma above, but that check
> > +                      * is racy because ->anon_vma can be populated under the
> > +                      * mmap lock in read mode.
> > +                      */
> > +                     if (vma->anon_vma) {
> > +                             result = SCAN_PAGE_ANON;
> > +                             goto unlock_next;
> > +                     }
>
> This is totally wrong direction. Or I don't understand the race.
>
> At this point we already paid nearly all price of of pagetable retraction.
> I don't see any correctness reason to stop here, except for the assert.

Isn't it possible that collapse_and_free_pmd() clear the pmd which may
point to a PTE which maps the COW'ed anon page if this race happens?

>
> I think lockdep assert in collapse_and_free_pmd() is wrong and has to be
> dropped.
>
> >                       /*
> >                        * When a vma is registered with uffd-wp, we can't
> >                        * recycle the pmd pgtable because there can be pte
> >
> > base-commit: 7dd4b804e08041ff56c88bdd8da742d14b17ed25
> > --
> > 2.39.0.314.g84b9a713c41-goog
> >
>
> --
>   Kiryl Shutsemau / Kirill A. Shutemov