Re: [PATCH 2/3] mm: prevent vm_area_struct::anon_name refcount saturation

[Suren Baghdasaryan <surenb@google.com>]

On Tue, Feb 22, 2022 at 1:17 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Mon 21-02-22 21:40:24, Suren Baghdasaryan wrote:
> > A deep process chain with many vmas could grow really high.
>
> This would really benefit from some numbers. With default
> sysctl_max_map_count (64k) and default pid_max (32k) the INT_MAX could
> be theoretically reached but I find it impractical because not all vmas
> can be anonymous same as all available pids can be consumed for a
> theoretical attack (if my counting is proper).
> On the other hand any non-default configuration with any of the values
> increased could hit this theoretically.

re: This would really benefit from some numbers
Should I just add the details you provided above into the description?
Would that suffice?

>
> > kref
> > refcounting interface used in anon_vma_name structure will detect
> > a counter overflow when it reaches REFCOUNT_SATURATED value but will
> > only generate a warning about broken refcounting.
> > To ensure anon_vma_name refcount does not overflow, stop anon_vma_name
> > sharing when the refcount reaches INT_MAX, which still leaves INT_MAX/2
> > values before the counter reaches REFCOUNT_SATURATED. This should provide
> > enough headroom for raising the refcounts temporarily.
> >
> > Suggested-by: Michal Hocko <mhocko@suse.com>
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > ---
> >  include/linux/mm_inline.h | 18 ++++++++++++++----
> >  mm/madvise.c              |  3 +--
> >  2 files changed, 15 insertions(+), 6 deletions(-)
> >
> > diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
> > index 70b619442d56..b189e2638843 100644
> > --- a/include/linux/mm_inline.h
> > +++ b/include/linux/mm_inline.h
> > @@ -156,15 +156,25 @@ static inline void anon_vma_name_get(struct anon_vma_name *anon_name)
> >
> >  extern void anon_vma_name_put(struct anon_vma_name *anon_name);
> >
> > +static inline
> > +struct anon_vma_name *anon_vma_name_reuse(struct anon_vma_name *anon_name)
> > +{
> > +     /* Prevent anon_name refcount saturation early on */
> > +     if (kref_read(&anon_name->kref) < INT_MAX) {
>
> REFCOUNT_MAX seems to be defined by the kref framework.

Ah, indeed. I missed that. Will change to use it.

>
> Other than that looks good to me.

Thanks for the review!

>
> > +             anon_vma_name_get(anon_name);
> > +             return anon_name;
> > +
> > +     }
> > +     return anon_vma_name_alloc(anon_name->name);
> > +}
> > +
> >  static inline void dup_vma_anon_name(struct vm_area_struct *orig_vma,
> >                                    struct vm_area_struct *new_vma)
> >  {
> >       struct anon_vma_name *anon_name = vma_anon_name(orig_vma);
> >
> > -     if (anon_name) {
> > -             anon_vma_name_get(anon_name);
> > -             new_vma->anon_name = anon_name;
> > -     }
> > +     if (anon_name)
> > +             new_vma->anon_name = anon_vma_name_reuse(anon_name);
> >  }
> >
> >  static inline void free_vma_anon_name(struct vm_area_struct *vma)
> > diff --git a/mm/madvise.c b/mm/madvise.c
> > index f81d62d8ce9b..a395884aeecb 100644
> > --- a/mm/madvise.c
> > +++ b/mm/madvise.c
> > @@ -122,8 +122,7 @@ static int replace_vma_anon_name(struct vm_area_struct *vma,
> >       if (anon_vma_name_eq(orig_name, anon_name))
> >               return 0;
> >
> > -     anon_vma_name_get(anon_name);
> > -     vma->anon_name = anon_name;
> > +     vma->anon_name = anon_vma_name_reuse(anon_name);
> >       anon_vma_name_put(orig_name);
> >
> >       return 0;
> > --
> > 2.35.1.473.g83b2b277ed-goog
>
> --
> Michal Hocko
> SUSE Labs