Re: [PATCH] mm/gup: don't permit users to call get_user_pages with FOLL_LONGTERM

[Souptick Joarder]

On Wed, Aug 19, 2020 at 11:45 PM John Hubbard <jhubbard@xxxxxxxxxx> wrote:
>
> On 8/19/20 4:01 AM, Barry Song wrote:
> > gug prohibits users from calling get_user_pages() with FOLL_PIN. But it
>
> Maybe Andrew can fix the typo above: gug --> gup.
>
>
> > allows users to call get_user_pages() with FOLL_LONGTERM only. It seems
> > insensible.
> >
> > since FOLL_LONGTERM is a stricter case of FOLL_PIN, we should prohibit
> > users from calling get_user_pages() with FOLL_LONGTERM while not with
> > FOLL_PIN.
> >
> > mm/gup_benchmark.c used to be the only user who did this improperly.
> > But it has been fixed by moving to use pin_user_pages().
>
> For future patches, you don't have to write everything in the
> commit log. Some things are better placed in a cover letter or after
> the "---" line, because they don't need to be recorded forever.
>
> Anyway, the diffs seem fine, assuming that you've audited the call sites.

We can use is_valid_gup_flags() inside ->
get_user_pages_locked(),
get_user_pages_unlocked(),
pin_user_pages_locked() as well.

Are you planning to add it in future patches ?

>
> thanks,
> --
> John Hubbard
> NVIDIA
>
> >
> > Cc: John Hubbard <jhubbard@xxxxxxxxxx>
> > Cc: Jan Kara <jack@xxxxxxx>
> > Cc: Jérôme Glisse <jglisse@xxxxxxxxxx>
> > Cc: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
> > Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> > Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
> > Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> > Cc: Dave Chinner <david@xxxxxxxxxxxxx>
> > Cc: Jason Gunthorpe <jgg@xxxxxxxx>
> > Cc: Jonathan Corbet <corbet@xxxxxxx>
> > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> > Cc: Shuah Khan <shuah@xxxxxxxxxx>
> > Cc: Vlastimil Babka <vbabka@xxxxxxx>
> > Signed-off-by: Barry Song <song.bao.hua@xxxxxxxxxxxxx>
> > ---
> >   mm/gup.c | 37 ++++++++++++++++++++++---------------
> >   1 file changed, 22 insertions(+), 15 deletions(-)
> >
> > diff --git a/mm/gup.c b/mm/gup.c
> > index ae096ea7583f..4da669f79566 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -1789,6 +1789,25 @@ static long __get_user_pages_remote(struct mm_struct *mm,
> >                                      gup_flags | FOLL_TOUCH | FOLL_REMOTE);
> >   }
> >
> > +static bool is_valid_gup_flags(unsigned int gup_flags)
> > +{
> > +     /*
> > +      * FOLL_PIN must only be set internally by the pin_user_pages*() APIs,
> > +      * never directly by the caller, so enforce that with an assertion:
> > +      */
> > +     if (WARN_ON_ONCE(gup_flags & FOLL_PIN))
> > +             return false;
> > +     /*
> > +      * FOLL_PIN is a prerequisite to FOLL_LONGTERM. Another way of saying
> > +      * that is, FOLL_LONGTERM is a specific case, more restrictive case of
> > +      * FOLL_PIN.
> > +      */
> > +     if (WARN_ON_ONCE(gup_flags & FOLL_LONGTERM))
> > +             return false;
> > +
> > +     return true;
> > +}
> > +
> >   /**
> >    * get_user_pages_remote() - pin user pages in memory
> >    * @mm:             mm_struct of target mm
> > @@ -1854,11 +1873,7 @@ long get_user_pages_remote(struct mm_struct *mm,
> >               unsigned int gup_flags, struct page **pages,
> >               struct vm_area_struct **vmas, int *locked)
> >   {
> > -     /*
> > -      * FOLL_PIN must only be set internally by the pin_user_pages*() APIs,
> > -      * never directly by the caller, so enforce that with an assertion:
> > -      */
> > -     if (WARN_ON_ONCE(gup_flags & FOLL_PIN))
> > +     if (!is_valid_gup_flags(gup_flags))
> >               return -EINVAL;
> >
> >       return __get_user_pages_remote(mm, start, nr_pages, gup_flags,
> > @@ -1904,11 +1919,7 @@ long get_user_pages(unsigned long start, unsigned long nr_pages,
> >               unsigned int gup_flags, struct page **pages,
> >               struct vm_area_struct **vmas)
> >   {
> > -     /*
> > -      * FOLL_PIN must only be set internally by the pin_user_pages*() APIs,
> > -      * never directly by the caller, so enforce that with an assertion:
> > -      */
> > -     if (WARN_ON_ONCE(gup_flags & FOLL_PIN))
> > +     if (!is_valid_gup_flags(gup_flags))
> >               return -EINVAL;
> >
> >       return __gup_longterm_locked(current->mm, start, nr_pages,
> > @@ -2810,11 +2821,7 @@ EXPORT_SYMBOL_GPL(get_user_pages_fast_only);
> >   int get_user_pages_fast(unsigned long start, int nr_pages,
> >                       unsigned int gup_flags, struct page **pages)
> >   {
> > -     /*
> > -      * FOLL_PIN must only be set internally by the pin_user_pages*() APIs,
> > -      * never directly by the caller, so enforce that:
> > -      */
> > -     if (WARN_ON_ONCE(gup_flags & FOLL_PIN))
> > +     if (!is_valid_gup_flags(gup_flags))
> >               return -EINVAL;
> >
> >       /*
> >
>
>