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From:   Andrey Konovalov <andreyknvl@google.com>
Date:   Tue, 29 Sep 2020 14:20:53 +0200
Message-ID: <CAAeHK+yMmGSTpwC1zPxaoBmXsfmmhuLJ3b2N3qUXUjO5U0tM3Q@mail.gmail.com>
Subject: Re: [PATCH v3 06/10] kfence, kasan: make KFENCE compatible with KASAN
To:     Marco Elver <elver@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Alexander Potapenko <glider@google.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Andy Lutomirski <luto@kernel.org>,
        Borislav Petkov <bp@alien8.de>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Christoph Lameter <cl@linux.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        David Rientjes <rientjes@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Eric Dumazet <edumazet@google.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Hillf Danton <hdanton@sina.com>,
        Ingo Molnar <mingo@redhat.com>, Jann Horn <jannh@google.com>,
        Jonathan.Cameron@huawei.com, Jonathan Corbet <corbet@lwn.net>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Kees Cook <keescook@chromium.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Pekka Enberg <penberg@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>, sjpark@amazon.com,
        Thomas Gleixner <tglx@linutronix.de>,
        Vlastimil Babka <vbabka@suse.cz>,
        Will Deacon <will@kernel.org>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        kasan-dev <kasan-dev@googlegroups.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Linux Memory Management List <linux-mm@kvack.org>
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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?