Re: [PATCH v5 4/5] Add new file in LKDTM to test fortified strscpy.

[Kees Cook <keescook@chromium.org>]

On Wed, Nov 18, 2020 at 12:07:30PM +0100, laniel_francis@privacyrequired.com wrote:
> From: Francis Laniel <laniel_francis@privacyrequired.com>
> 
> This new test ensures that fortified strscpy has the same behavior than vanilla
> strscpy (e.g. returning -E2BIG when src content is truncated).
> Finally, it generates a crash at runtime because there is a write overflow in
> destination string.
> 
> Signed-off-by: Francis Laniel <laniel_francis@privacyrequired.com>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> ---
>  drivers/misc/lkdtm/Makefile             |  1 +
>  drivers/misc/lkdtm/core.c               |  1 +
>  drivers/misc/lkdtm/fortify.c            | 82 +++++++++++++++++++++++++
>  drivers/misc/lkdtm/lkdtm.h              |  3 +
>  tools/testing/selftests/lkdtm/tests.txt |  1 +
>  5 files changed, 88 insertions(+)
>  create mode 100644 drivers/misc/lkdtm/fortify.c
> 
> diff --git a/drivers/misc/lkdtm/Makefile b/drivers/misc/lkdtm/Makefile
> index c70b3822013f..d898f7b22045 100644
> --- a/drivers/misc/lkdtm/Makefile
> +++ b/drivers/misc/lkdtm/Makefile
> @@ -10,6 +10,7 @@ lkdtm-$(CONFIG_LKDTM)		+= rodata_objcopy.o
>  lkdtm-$(CONFIG_LKDTM)		+= usercopy.o
>  lkdtm-$(CONFIG_LKDTM)		+= stackleak.o
>  lkdtm-$(CONFIG_LKDTM)		+= cfi.o
> +lkdtm-$(CONFIG_LKDTM)		+= fortify.o
>  
>  KASAN_SANITIZE_stackleak.o	:= n
>  KCOV_INSTRUMENT_rodata.o	:= n
> diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c
> index b8c51a633fcc..3c0a67f072c0 100644
> --- a/drivers/misc/lkdtm/core.c
> +++ b/drivers/misc/lkdtm/core.c
> @@ -175,6 +175,7 @@ static const struct crashtype crashtypes[] = {
>  	CRASHTYPE(USERCOPY_KERNEL),
>  	CRASHTYPE(STACKLEAK_ERASING),
>  	CRASHTYPE(CFI_FORWARD_PROTO),
> +	CRASHTYPE(FORTIFIED_STRSCPY),
>  #ifdef CONFIG_X86_32
>  	CRASHTYPE(DOUBLE_FAULT),
>  #endif
> diff --git a/drivers/misc/lkdtm/fortify.c b/drivers/misc/lkdtm/fortify.c
> new file mode 100644
> index 000000000000..790d46591bf5
> --- /dev/null
> +++ b/drivers/misc/lkdtm/fortify.c
> @@ -0,0 +1,82 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright (c) 2020 Francis Laniel <laniel_francis@privacyrequired.com>
> + *
> + * Add tests related to fortified functions in this file.
> + */
> +#include <linux/string.h>
> +#include <linux/slab.h>
> +#include "lkdtm.h"

Ah, I just noticed one small nit here during build testing: lkdtm.h
needs to be included first to get the correct pr_fmt to avoid a warning:

In file included from drivers/misc/lkdtm/fortify.c:9:
drivers/misc/lkdtm/lkdtm.h:5: warning: "pr_fmt" redefined
    5 | #define pr_fmt(fmt) "lkdtm: " fmt

-Kees

-- 
Kees Cook