Re: [PATCH v3 mm 08/13] kasan, mm: optimize krealloc poisoning

[Andrey Konovalov <andreyknvl@google.com>]

On Mon, Feb 8, 2021 at 11:48 AM Marco Elver <elver@google.com> wrote:
>
> On Fri, Feb 05, 2021 at 06:34PM +0100, Andrey Konovalov wrote:
> > Currently, krealloc() always calls ksize(), which unpoisons the whole
> > object including the redzone. This is inefficient, as kasan_krealloc()
> > repoisons the redzone for objects that fit into the same buffer.
> >
> > This patch changes krealloc() instrumentation to use uninstrumented
> > __ksize() that doesn't unpoison the memory. Instead, kasan_kreallos()
> > is changed to unpoison the memory excluding the redzone.
> >
> > For objects that don't fit into the old allocation, this patch disables
> > KASAN accessibility checks when copying memory into a new object instead
> > of unpoisoning it.
> >
> > Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
>
> Reviewed-by: Marco Elver <elver@google.com>
>
> Clarification below.
>
> > ---
> >  mm/kasan/common.c | 12 ++++++++++--
> >  mm/slab_common.c  | 20 ++++++++++++++------
> >  2 files changed, 24 insertions(+), 8 deletions(-)
> >
> > diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> > index 7ea643f7e69c..a8a67dca5e55 100644
> > --- a/mm/kasan/common.c
> > +++ b/mm/kasan/common.c
> > @@ -476,7 +476,7 @@ static void *____kasan_kmalloc(struct kmem_cache *cache, const void *object,
> >
> >       /*
> >        * The object has already been unpoisoned by kasan_slab_alloc() for
> > -      * kmalloc() or by ksize() for krealloc().
> > +      * kmalloc() or by kasan_krealloc() for krealloc().
> >        */
> >
> >       /*
> > @@ -526,7 +526,7 @@ void * __must_check __kasan_kmalloc_large(const void *ptr, size_t size,
> >
> >       /*
> >        * The object has already been unpoisoned by kasan_alloc_pages() for
> > -      * alloc_pages() or by ksize() for krealloc().
> > +      * alloc_pages() or by kasan_krealloc() for krealloc().
> >        */
> >
> >       /*
> > @@ -554,8 +554,16 @@ void * __must_check __kasan_krealloc(const void *object, size_t size, gfp_t flag
> >       if (unlikely(object == ZERO_SIZE_PTR))
> >               return (void *)object;
> >
> > +     /*
> > +      * Unpoison the object's data.
> > +      * Part of it might already have been unpoisoned, but it's unknown
> > +      * how big that part is.
> > +      */
> > +     kasan_unpoison(object, size);
> > +
> >       page = virt_to_head_page(object);
> >
> > +     /* Piggy-back on kmalloc() instrumentation to poison the redzone. */
> >       if (unlikely(!PageSlab(page)))
> >               return __kasan_kmalloc_large(object, size, flags);
> >       else
> > diff --git a/mm/slab_common.c b/mm/slab_common.c
> > index dad70239b54c..60a2f49df6ce 100644
> > --- a/mm/slab_common.c
> > +++ b/mm/slab_common.c
> > @@ -1140,19 +1140,27 @@ static __always_inline void *__do_krealloc(const void *p, size_t new_size,
> >       void *ret;
> >       size_t ks;
> >
> > -     if (likely(!ZERO_OR_NULL_PTR(p)) && !kasan_check_byte(p))
> > -             return NULL;
> > -
> > -     ks = ksize(p);
> > +     /* Don't use instrumented ksize to allow precise KASAN poisoning. */
> > +     if (likely(!ZERO_OR_NULL_PTR(p))) {
> > +             if (!kasan_check_byte(p))
> > +                     return NULL;
>
> Just checking: Check byte returns true if the object is not tracked by KASAN, right? I.e. if it's a KFENCE object, kasan_check_byte() always returns true.

kasan_check_byte() still performs the check, but since KFENCE objects
are never poisoned, the check always passes.

Thanks!