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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
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