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From:   Jann Horn <jannh@google.com>
Date:   Fri, 30 Oct 2020 03:49:47 +0100
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Subject: Re: [PATCH v6 6/9] 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>,
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        Andy Lutomirski <luto@kernel.org>,
        Borislav Petkov <bp@alien8.de>,
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        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>,
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On Thu, Oct 29, 2020 at 2:17 PM Marco Elver <elver@google.com> wrote:
> 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>

Reviewed-by: Jann Horn <jannh@google.com>

with one nit:

[...]
> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
[...]
> @@ -141,6 +142,14 @@ void kasan_unpoison_shadow(const void *address, size_t size)
>          */
>         address = reset_tag(address);
>
> +       /*
> +        * We may be called from SL*B internals, such as ksize(): with a size
> +        * not a multiple of machine-word size, avoid poisoning the invalid
> +        * portion of the word for KFENCE memory.
> +        */
> +       if (is_kfence_address(address))
> +               return;

It might be helpful if you could add a comment that explains that
kasan_poison_object_data() does not need a similar guard because
kasan_poison_object_data() is always paired with
kasan_unpoison_object_data() - that threw me off a bit at first.