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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Thu, 22 Oct 2020 17:02:09 +0200
Message-ID: <CACT4Y+aY=Z4D+FzMUL2f0gda-PP1t5-HNXBcc22KW7OTYuZh4w@mail.gmail.com>
Subject: Re: [PATCH v2 2/2] kcsan: Never set up watchpoints on NULL pointers
To:     Marco Elver <elver@google.com>
Cc:     "Paul E. McKenney" <paulmck@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        kasan-dev <kasan-dev@googlegroups.com>,
        LKML <linux-kernel@vger.kernel.org>
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On Thu, Oct 22, 2020 at 1:45 PM Marco Elver <elver@google.com> wrote:
>
> Avoid setting up watchpoints on NULL pointers, as otherwise we would
> crash inside the KCSAN runtime (when checking for value changes) instead
> of the instrumented code.
>
> Because that may be confusing, skip any address less than PAGE_SIZE.
>
> Signed-off-by: Marco Elver <elver@google.com>

Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

> ---
>  kernel/kcsan/encoding.h | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/kcsan/encoding.h b/kernel/kcsan/encoding.h
> index f03562aaf2eb..64b3c0f2a685 100644
> --- a/kernel/kcsan/encoding.h
> +++ b/kernel/kcsan/encoding.h
> @@ -48,7 +48,11 @@
>
>  static inline bool check_encodable(unsigned long addr, size_t size)
>  {
> -       return size <= MAX_ENCODABLE_SIZE;
> +       /*
> +        * While we can encode addrs<PAGE_SIZE, avoid crashing with a NULL
> +        * pointer deref inside KCSAN.
> +        */
> +       return addr >= PAGE_SIZE && size <= MAX_ENCODABLE_SIZE;
>  }
>
>  static inline long
> --
> 2.29.0.rc1.297.gfa9743e501-goog
>