Re: [PATCH] mm/gup: fix omission of check on FOLL_LONGTERM in get_user_pages_fast()

[Pingfan Liu]

On Fri, May 31, 2019 at 5:46 AM Ira Weiny <ira.weiny@xxxxxxxxx> wrote:
>
> On Thu, May 30, 2019 at 06:54:04AM +0800, Pingfan Liu wrote:
> > As for FOLL_LONGTERM, it is checked in the slow path
> > __gup_longterm_unlocked(). But it is not checked in the fast path, which
> > means a possible leak of CMA page to longterm pinned requirement through
> > this crack.
> >
> > Place a check in the fast path.
> >
> > Signed-off-by: Pingfan Liu <kernelfans@xxxxxxxxx>
> > Cc: Ira Weiny <ira.weiny@xxxxxxxxx>
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: Mike Rapoport <rppt@xxxxxxxxxxxxx>
> > Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> > Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> > Cc: John Hubbard <jhubbard@xxxxxxxxxx>
> > Cc: "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxx>
> > Cc: Keith Busch <keith.busch@xxxxxxxxx>
> > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > ---
> >  mm/gup.c | 12 ++++++++++++
> >  1 file changed, 12 insertions(+)
> >
> > diff --git a/mm/gup.c b/mm/gup.c
> > index f173fcb..00feab3 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -2235,6 +2235,18 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
> >               local_irq_enable();
> >               ret = nr;
> >       }
> > +#if defined(CONFIG_CMA)
> > +     if (unlikely(gup_flags & FOLL_LONGTERM)) {
> > +             int i, j;
> > +
> > +             for (i = 0; i < nr; i++)
> > +                     if (is_migrate_cma_page(pages[i])) {
> > +                             for (j = i; j < nr; j++)
> > +                                     put_page(pages[j]);
>
> Should be put_user_page() now.  For now that just calls put_page() but it is
> slated to change soon.
>
Not aware of these changes. And get your point now.

> I also wonder if this would be more efficient as a check as we are walking the
> page tables and bail early.
>
> Perhaps the code complexity is not worth it?
>
Yes. That will spread such logic in huge page and normal page.
> > +                             nr = i;
>
> Why not just break from the loop here?
>
A mistake.

Thanks,
  Pingfan
> Or better yet just use 'i' in the inner loop...
>
> Ira
>
> > +                     }
> > +     }
> > +#endif
> >
> >       if (nr < nr_pages) {
> >               /* Try to get the remaining pages with get_user_pages */
> > --
> > 2.7.5
> >