Re: [PATCH v8 15/16] mm: make vma cache SLAB_TYPESAFE_BY_RCU

[Suren Baghdasaryan <surenb@google.com>]

On Fri, Jan 10, 2025 at 9:48 AM Liam R. Howlett <Liam.Howlett@oracle.com> wrote:
>
> * Suren Baghdasaryan <surenb@google.com> [250108 21:31]:
> > To enable SLAB_TYPESAFE_BY_RCU for vma cache we need to ensure that
> > object reuse before RCU grace period is over will be detected by
> > lock_vma_under_rcu().
> > Current checks are sufficient as long as vma is detached before it is
> > freed. The only place this is not currently happening is in exit_mmap().
> > Add the missing vma_mark_detached() in exit_mmap().
> > Another issue which might trick lock_vma_under_rcu() during vma reuse
> > is vm_area_dup(), which copies the entire content of the vma into a new
> > one, overriding new vma's vm_refcnt and temporarily making it appear as
> > attached. This might trick a racing lock_vma_under_rcu() to operate on
> > a reused vma if it found the vma before it got reused. To prevent this
> > situation, we should ensure that vm_refcnt stays at detached state (0)
> > when it is copied and advances to attached state only after it is added
> > into the vma tree. Introduce vma_copy() which preserves new vma's
> > vm_refcnt and use it in vm_area_dup(). Since all vmas are in detached
> > state with no current readers when they are freed, lock_vma_under_rcu()
> > will not be able to take vm_refcnt after vma got detached even if vma
> > is reused.
> > Finally, make vm_area_cachep SLAB_TYPESAFE_BY_RCU. This will facilitate
> > vm_area_struct reuse and will minimize the number of call_rcu() calls.
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > ---
> >  include/linux/mm.h               |  2 -
> >  include/linux/mm_types.h         | 10 +++--
> >  include/linux/slab.h             |  6 ---
> >  kernel/fork.c                    | 72 ++++++++++++++++++++------------
> >  mm/mmap.c                        |  3 +-
> >  mm/vma.c                         | 11 ++---
> >  mm/vma.h                         |  2 +-
> >  tools/testing/vma/vma_internal.h |  7 +---
> >  8 files changed, 59 insertions(+), 54 deletions(-)
> >
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 1d6b1563b956..a674558e4c05 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -258,8 +258,6 @@ void setup_initial_init_mm(void *start_code, void *end_code,
> >  struct vm_area_struct *vm_area_alloc(struct mm_struct *);
> >  struct vm_area_struct *vm_area_dup(struct vm_area_struct *);
> >  void vm_area_free(struct vm_area_struct *);
> > -/* Use only if VMA has no other users */
> > -void __vm_area_free(struct vm_area_struct *vma);
> >
> >  #ifndef CONFIG_MMU
> >  extern struct rb_root nommu_region_tree;
> > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > index 2d83d79d1899..93bfcd0c1fde 100644
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -582,6 +582,12 @@ static inline void *folio_get_private(struct folio *folio)
> >
> >  typedef unsigned long vm_flags_t;
> >
> > +/*
> > + * freeptr_t represents a SLUB freelist pointer, which might be encoded
> > + * and not dereferenceable if CONFIG_SLAB_FREELIST_HARDENED is enabled.
> > + */
> > +typedef struct { unsigned long v; } freeptr_t;
> > +
> >  /*
> >   * A region containing a mapping of a non-memory backed file under NOMMU
> >   * conditions.  These are held in a global tree and are pinned by the VMAs that
> > @@ -695,9 +701,7 @@ struct vm_area_struct {
> >                       unsigned long vm_start;
> >                       unsigned long vm_end;
> >               };
> > -#ifdef CONFIG_PER_VMA_LOCK
> > -             struct rcu_head vm_rcu; /* Used for deferred freeing. */
> > -#endif
> > +             freeptr_t vm_freeptr; /* Pointer used by SLAB_TYPESAFE_BY_RCU */
> >       };
> >
> >       /*
> > diff --git a/include/linux/slab.h b/include/linux/slab.h
> > index 10a971c2bde3..681b685b6c4e 100644
> > --- a/include/linux/slab.h
> > +++ b/include/linux/slab.h
> > @@ -234,12 +234,6 @@ enum _slab_flag_bits {
> >  #define SLAB_NO_OBJ_EXT              __SLAB_FLAG_UNUSED
> >  #endif
> >
> > -/*
> > - * freeptr_t represents a SLUB freelist pointer, which might be encoded
> > - * and not dereferenceable if CONFIG_SLAB_FREELIST_HARDENED is enabled.
> > - */
> > -typedef struct { unsigned long v; } freeptr_t;
> > -
> >  /*
> >   * ZERO_SIZE_PTR will be returned for zero sized kmalloc requests.
> >   *
> > diff --git a/kernel/fork.c b/kernel/fork.c
> > index 9d9275783cf8..770b973a099c 100644
> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -449,6 +449,41 @@ struct vm_area_struct *vm_area_alloc(struct mm_struct *mm)
> >       return vma;
> >  }
> >
>
> There exists a copy_vma() which copies the vma to a new area in the mm
> in rmap.  Naming this vma_copy() is confusing :)
>
> It might be better to just put this code in the vm_area_dup() or call it
> __vm_area_dup(), or __vma_dup() ?

Hmm. It's not really duplicating a vma but copying its content (no
allocation). How about __vm_area_copy() to indicate it is copying
vm_area_struct content?

>
> > +static void vma_copy(const struct vm_area_struct *src, struct vm_area_struct *dest)
> > +{
> > +     dest->vm_mm = src->vm_mm;
> > +     dest->vm_ops = src->vm_ops;
> > +     dest->vm_start = src->vm_start;
> > +     dest->vm_end = src->vm_end;
> > +     dest->anon_vma = src->anon_vma;
> > +     dest->vm_pgoff = src->vm_pgoff;
> > +     dest->vm_file = src->vm_file;
> > +     dest->vm_private_data = src->vm_private_data;
> > +     vm_flags_init(dest, src->vm_flags);
> > +     memcpy(&dest->vm_page_prot, &src->vm_page_prot,
> > +            sizeof(dest->vm_page_prot));
> > +     /*
> > +      * src->shared.rb may be modified concurrently, but the clone
> > +      * will be reinitialized.
> > +      */
> > +     data_race(memcpy(&dest->shared, &src->shared, sizeof(dest->shared)));
> > +     memcpy(&dest->vm_userfaultfd_ctx, &src->vm_userfaultfd_ctx,
> > +            sizeof(dest->vm_userfaultfd_ctx));
> > +#ifdef CONFIG_ANON_VMA_NAME
> > +     dest->anon_name = src->anon_name;
> > +#endif
> > +#ifdef CONFIG_SWAP
> > +     memcpy(&dest->swap_readahead_info, &src->swap_readahead_info,
> > +            sizeof(dest->swap_readahead_info));
> > +#endif
> > +#ifndef CONFIG_MMU
> > +     dest->vm_region = src->vm_region;
> > +#endif
> > +#ifdef CONFIG_NUMA
> > +     dest->vm_policy = src->vm_policy;
> > +#endif
> > +}
> > +
> >  struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig)
> >  {
> >       struct vm_area_struct *new = kmem_cache_alloc(vm_area_cachep, GFP_KERNEL);
> > @@ -458,11 +493,7 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig)
> >
> >       ASSERT_EXCLUSIVE_WRITER(orig->vm_flags);
> >       ASSERT_EXCLUSIVE_WRITER(orig->vm_file);
> > -     /*
> > -      * orig->shared.rb may be modified concurrently, but the clone
> > -      * will be reinitialized.
> > -      */
> > -     data_race(memcpy(new, orig, sizeof(*new)));
> > +     vma_copy(orig, new);
> >       vma_lock_init(new, true);
>
> I think this suffers from a race still?
>
> That is, we can still race between vm_lock_seq == mm_lock_seq and the
> lock acquire, where a free and reuse happens.  In the even that the
> reader is caught between the sequence and lock taking, the
> vma->vmlock_dep_map may not be replaced and it could see the old lock
> (or zero?) and things go bad:
>
> It could try to take vmlock_dep_map == 0 in read mode.
>
> It can take the old lock, detect the refcnt is wrong and release the new
> lock.

I don't think this race can happen. Notice a call to
vma_assert_detached() inside vm_area_free(), so before vma is freed
and possibly reused, it has to be detached. vma_mark_detached()
ensures that there are no current or future readers by executing the
__vma_enter_locked() + __vma_exit_locked() sequence if vm_refcnt is
not already at 0. Once __vma_exit_locked() is done, vm_refcnt is at 0
and any new reader will be rejected on
__refcount_inc_not_zero_limited(), before even checking vm_lock_seq ==
mm_lock_seq. Even if a reader tries to sneak in between
__vma_enter_locked() and __vma_exit_locked() calls,
__refcount_inc_not_zero_limited() will reject it because
VMA_LOCK_OFFSET is set and VMA_REF_LIMIT will be violated.
IOW, when VMA is freed, it's guaranteed to be detached with no current
or future readers, therefore "race between vm_lock_seq == mm_lock_seq
and the lock acquire, where a free and reuse happens" should not be
possible.

Did I understand your concern correctly and does my explanation make
sense to you?

>
> Thanks,
> Liam