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

[Marco Elver <elver@google.com>]

On Fri, 11 Sep 2020 at 09:05, Dmitry Vyukov <dvyukov@google.com> wrote:
>
> On Mon, Sep 7, 2020 at 3:41 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.
> >
> > 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 7ac91162edb0..b080e49e15d4 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;
>
> Is this needed?
> At least in the slab patch I see that we do :
>
> if (kfence_free(objp)) {
>   kmemleak_free_recursive(objp, cachep->flags);
>   return;
> }
>
> before:
>
> /* Put the object into the quarantine, don't touch it for now. */ /*
> Put the object into the quarantine, don't touch it for now. */
> if (kasan_slab_free(cachep, objp, _RET_IP_)) if
> (kasan_slab_free(cachep, objp, _RET_IP_))
>   return; return;
>
>
> If it's not supposed to be triggered, it can make sense to replace
> with BUG/WARN.

It is required for SLUB. For SLAB, it seems it might not be necessary.
Making the check in kasan/common.c conditional on the allocator seems
ugly, so I propose we keep it there.

Thanks,
-- Marco

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