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

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Date:   Tue, 29 Sep 2020 14:20:53 +0200
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Marco Elver <elver@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Potapenko <glider@...gle.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Christoph Lameter <cl@...ux.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Hillf Danton <hdanton@...a.com>,
        Ingo Molnar <mingo@...hat.com>, Jann Horn <jannh@...gle.com>,
        Jonathan.Cameron@...wei.com, Jonathan Corbet <corbet@....net>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Kees Cook <keescook@...omium.org>,
        Mark Rutland <mark.rutland@....com>,
        Pekka Enberg <penberg@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, sjpark@...zon.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Vlastimil Babka <vbabka@...e.cz>,
        Will Deacon <will@...nel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: [PATCH v3 06/10] kfence, kasan: make KFENCE compatible with KASAN

On Mon, Sep 21, 2020 at 3:26 PM Marco Elver <elver@...gle.com> wrote:
>
> From: Alexander Potapenko <glider@...gle.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@...gle.com>
> Co-developed-by: Marco Elver <elver@...gle.com>
> Signed-off-by: Marco Elver <elver@...gle.com>
> Signed-off-by: Alexander Potapenko <glider@...gle.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?