linux-kernel - Re: [PATCH] lkdtm: Prevent the compiler from optimising lkdtm_CORRUPT

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Message-ID: <CAGXu5j+V=+-=WHmc=XMDCRLovya7ZRkiu8p9znjOkRFWPNq5Rw@mail.gmail.com>
Date:   Tue, 15 Nov 2016 09:27:47 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH] lkdtm: Prevent the compiler from optimising lkdtm_CORRUPT_STACK()

On Mon, Nov 14, 2016 at 11:02 PM, Michael Ellerman <mpe@...erman.id.au> wrote:
> At least on powerpc with GCC 6, the compiler is smart enough to optimise
> lkdtm_CORRUPT_STACK() into an empty function that just returns.

Sneaky. Thanks!

> If we print the buffer after we've written to it that prevents the
> compiler from optimising away data and the memset().
>
> Signed-off-by: Michael Ellerman <mpe@...erman.id.au>

Acked-by: Kees Cook <keescook@...omium.org>

Greg, can you add this to the drivers/misc?

-Kees

> ---
>  drivers/misc/lkdtm_bugs.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/misc/lkdtm_bugs.c b/drivers/misc/lkdtm_bugs.c
> index 182ae1894b32..30e62dd7e7ca 100644
> --- a/drivers/misc/lkdtm_bugs.c
> +++ b/drivers/misc/lkdtm_bugs.c
> @@ -80,7 +80,8 @@ noinline void lkdtm_CORRUPT_STACK(void)
>         /* Use default char array length that triggers stack protection. */
>         char data[8];
>
> -       memset((void *)data, 0, 64);
> +       memset((void *)data, 'a', 64);
> +       pr_info("Corrupted stack with '%16s'...\n", data);
>  }
>
>  void lkdtm_UNALIGNED_LOAD_STORE_WRITE(void)
> --
> 2.7.4
>



-- 
Kees Cook
Nexus Security