From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933060AbcKOR1v (ORCPT ); Tue, 15 Nov 2016 12:27:51 -0500 Received: from mail-wm0-f53.google.com ([74.125.82.53]:38799 "EHLO mail-wm0-f53.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752020AbcKOR1t (ORCPT ); Tue, 15 Nov 2016 12:27:49 -0500 MIME-Version: 1.0 In-Reply-To: <1479193352-486-1-git-send-email-mpe@ellerman.id.au> References: <1479193352-486-1-git-send-email-mpe@ellerman.id.au> From: Kees Cook Date: Tue, 15 Nov 2016 09:27:47 -0800 X-Google-Sender-Auth: ZfYBwxOX8OAbsESo1C6aXaZGGs0 Message-ID: Subject: Re: [PATCH] lkdtm: Prevent the compiler from optimising lkdtm_CORRUPT_STACK() To: Greg KH Cc: LKML , Michael Ellerman Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Nov 14, 2016 at 11:02 PM, Michael Ellerman 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 Acked-by: Kees Cook 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