Re: [PATCH v3 06/10] kfence, kasan: make KFENCE compatible with KASAN

[Alexander Potapenko <glider@google.com>]

On Tue, Sep 29, 2020 at 2:21 PM Andrey Konovalov <andreyknvl@google.com> wrote:
>
> On Mon, Sep 21, 2020 at 3:26 PM Marco Elver <elver@google.com> wrote:
> >
> > From: Alexander Potapenko <glider@google.com>
> >
> > We make KFENCE compatible with KASAN for testing KFENCE itself. In
> > particular, KASAN helps to catch any potential corruptions to KFENCE
> > state, or other corruptions that may be a result of freepointer
> > corruptions in the main allocators.
> >
> > To indicate that the combination of the two is generally discouraged,
> > CONFIG_EXPERT=y should be set. It also gives us the nice property that
> > KFENCE will be build-tested by allyesconfig builds.
> >
> > Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
> > Co-developed-by: Marco Elver <elver@google.com>
> > Signed-off-by: Marco Elver <elver@google.com>
> > Signed-off-by: Alexander Potapenko <glider@google.com>
> > ---
> >  lib/Kconfig.kfence | 2 +-
> >  mm/kasan/common.c  | 7 +++++++
> >  2 files changed, 8 insertions(+), 1 deletion(-)
> >
> > diff --git a/lib/Kconfig.kfence b/lib/Kconfig.kfence
> > index 4c2ea1c722de..6825c1c07a10 100644
> > --- a/lib/Kconfig.kfence
> > +++ b/lib/Kconfig.kfence
> > @@ -10,7 +10,7 @@ config HAVE_ARCH_KFENCE_STATIC_POOL
> >
> >  menuconfig KFENCE
> >         bool "KFENCE: low-overhead sampling-based memory safety error detector"
> > -       depends on HAVE_ARCH_KFENCE && !KASAN && (SLAB || SLUB)
> > +       depends on HAVE_ARCH_KFENCE && (!KASAN || EXPERT) && (SLAB || SLUB)
> >         depends on JUMP_LABEL # To ensure performance, require jump labels
> >         select STACKTRACE
> >         help
> > diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> > index 950fd372a07e..f5c49f0fdeff 100644
> > --- a/mm/kasan/common.c
> > +++ b/mm/kasan/common.c
> > @@ -18,6 +18,7 @@
> >  #include <linux/init.h>
> >  #include <linux/kasan.h>
> >  #include <linux/kernel.h>
> > +#include <linux/kfence.h>
> >  #include <linux/kmemleak.h>
> >  #include <linux/linkage.h>
> >  #include <linux/memblock.h>
> > @@ -396,6 +397,9 @@ static bool __kasan_slab_free(struct kmem_cache *cache, void *object,
> >         tagged_object = object;
> >         object = reset_tag(object);
> >
> > +       if (is_kfence_address(object))
> > +               return false;
> > +
> >         if (unlikely(nearest_obj(cache, virt_to_head_page(object), object) !=
> >             object)) {
> >                 kasan_report_invalid_free(tagged_object, ip);
> > @@ -444,6 +448,9 @@ static void *__kasan_kmalloc(struct kmem_cache *cache, const void *object,
> >         if (unlikely(object == NULL))
> >                 return NULL;
> >
> > +       if (is_kfence_address(object))
> > +               return (void *)object;
> > +
> >         redzone_start = round_up((unsigned long)(object + size),
> >                                 KASAN_SHADOW_SCALE_SIZE);
> >         redzone_end = round_up((unsigned long)object + cache->object_size,
> > --
> > 2.28.0.681.g6f77f65b4e-goog
> >
>
> With KFENCE + KASAN both enabled we need to bail out in all KASAN
> hooks that get called from the allocator, right? Do I understand
> correctly that these two are the only ones that are called for
> KFENCE-allocated objects due to the way KFENCE is integrated into the
> allocator?

Yes, these two places were sufficient; we've checked that KFENCE and
KASAN work together.