Re: [PATCH 1/2] kasan, mm: fix conflicts with init_on_alloc/free

[Andrey Konovalov <andreyknvl@google.com>]

On Wed, Jan 13, 2021 at 6:25 PM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 1/13/21 5:03 PM, Andrey Konovalov wrote:
> > A few places where SLUB accesses object's data or metadata were missed in
> > a previous patch. This leads to false positives with hardware tag-based
> > KASAN when bulk allocations are used with init_on_alloc/free.
> >
> > Fix the false-positives by resetting pointer tags during these accesses.
> >
> > Link: https://linux-review.googlesource.com/id/I50dd32838a666e173fe06c3c5c766f2c36aae901
> > Fixes: aa1ef4d7b3f67 ("kasan, mm: reset tags when accessing metadata")
> > Reported-by: Dmitry Vyukov <dvyukov@google.com>
> > Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
>
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
>
> > ---
> >  mm/slub.c | 7 ++++---
> >  1 file changed, 4 insertions(+), 3 deletions(-)
> >
> > diff --git a/mm/slub.c b/mm/slub.c
> > index dc5b42e700b8..75fb097d990d 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -2791,7 +2791,8 @@ static __always_inline void maybe_wipe_obj_freeptr(struct kmem_cache *s,
> >                                                  void *obj)
> >  {
> >       if (unlikely(slab_want_init_on_free(s)) && obj)
> > -             memset((void *)((char *)obj + s->offset), 0, sizeof(void *));
> > +             memset((void *)((char *)kasan_reset_tag(obj) + s->offset),
> > +                     0, sizeof(void *));
> >  }
> >
> >  /*
> > @@ -2883,7 +2884,7 @@ static __always_inline void *slab_alloc_node(struct kmem_cache *s,
> >               stat(s, ALLOC_FASTPATH);
> >       }
> >
> > -     maybe_wipe_obj_freeptr(s, kasan_reset_tag(object));
> > +     maybe_wipe_obj_freeptr(s, object);
>
> And in that case the reset was unnecessary, right. (commit log only mentions
> adding missing resets).

The reset has been moved into maybe_wipe_obj_freeptr(). I'll mention
it in the changelog in v2.

Thanks!