Re: [PATCH v2] lkdtm: Add CFI_BACKWARD to test ROP mitigations

["Daniel Díaz" <daniel.diaz@linaro.org>]

Hello!

On Sat, 16 Apr 2022 at 00:30, Kees Cook <keescook@chromium.org> wrote:
> In order to test various backward-edge control flow integrity methods,
> add a test that manipulates the return address on the stack. Currently
> only arm64 Pointer Authentication and Shadow Call Stack is supported.
>
>  $ echo CFI_BACKWARD | cat >/sys/kernel/debug/provoke-crash/DIRECT
>
> Under SCS, successful test of the mitigation is reported as:
>
>  lkdtm: Performing direct entry CFI_BACKWARD
>  lkdtm: Attempting unchecked stack return address redirection ...
>  lkdtm: ok: redirected stack return address.
>  lkdtm: Attempting checked stack return address redirection ...
>  lkdtm: ok: control flow unchanged.
>
> Under PAC, successful test of the mitigation is reported by the PAC
> exception handler:
>
>  lkdtm: Performing direct entry CFI_BACKWARD
>  lkdtm: Attempting unchecked stack return address redirection ...
>  lkdtm: ok: redirected stack return address.
>  lkdtm: Attempting checked stack return address redirection ...
>  Unable to handle kernel paging request at virtual address bfffffc0088d0514
>  Mem abort info:
>    ESR = 0x86000004
>    EC = 0x21: IABT (current EL), IL = 32 bits
>    SET = 0, FnV = 0
>    EA = 0, S1PTW = 0
>    FSC = 0x04: level 0 translation fault
>  [bfffffc0088d0514] address between user and kernel address ranges
>  ...
>
> If the CONFIGs are missing (or the mitigation isn't working), failure
> is reported as:
>
>  lkdtm: Performing direct entry CFI_BACKWARD
>  lkdtm: Attempting unchecked stack return address redirection ...
>  lkdtm: ok: redirected stack return address.
>  lkdtm: Attempting checked stack return address redirection ...
>  lkdtm: FAIL: stack return address was redirected!
>  lkdtm: This is probably expected, since this kernel was built *without* CONFIG_ARM64_PTR_AUTH_KERNEL=y nor CONFIG_SHADOW_CALL_STACK=y
>
> Co-developed-by: Dan Li <ashimida@linux.alibaba.com>
> Signed-off-by: Dan Li <ashimida@linux.alibaba.com>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
> v1: https://lore.kernel.org/lkml/20220413213917.711770-1-keescook@chromium.org
> v2:
>  - add PAGE_OFFSET setting for PAC bits (Dan Li)
> ---
>  drivers/misc/lkdtm/cfi.c                | 134 ++++++++++++++++++++++++
>  tools/testing/selftests/lkdtm/tests.txt |   1 +
>  2 files changed, 135 insertions(+)
>
> diff --git a/drivers/misc/lkdtm/cfi.c b/drivers/misc/lkdtm/cfi.c
> index e88f778be0d5..804965a480b7 100644
> --- a/drivers/misc/lkdtm/cfi.c
> +++ b/drivers/misc/lkdtm/cfi.c
> @@ -3,6 +3,7 @@
>   * This is for all the tests relating directly to Control Flow Integrity.
>   */
>  #include "lkdtm.h"
> +#include <asm/page.h>
>
>  static int called_count;
>
> @@ -42,8 +43,141 @@ static void lkdtm_CFI_FORWARD_PROTO(void)
>         pr_expected_config(CONFIG_CFI_CLANG);
>  }
>
> +/*
> + * This can stay local to LKDTM, as there should not be a production reason
> + * to disable PAC && SCS.
> + */
> +#ifdef CONFIG_ARM64_PTR_AUTH_KERNEL
> +# ifdef CONFIG_ARM64_BTI_KERNEL
> +#  define __no_pac             "branch-protection=bti"
> +# else
> +#  define __no_pac             "branch-protection=none"
> +# endif
> +# define __no_ret_protection   __noscs __attribute__((__target__(__no_pac)))
> +#else
> +# define __no_ret_protection   __noscs
> +#endif

We're seeing this problem with allmodconfig on arm64 and GCC 8 (this
one observed on 6.0.12-rc3):

-----8<----------8<----------8<-----
make --silent --keep-going --jobs=8
O=/home/tuxbuild/.cache/tuxmake/builds/2/build
CROSS_COMPILE_COMPAT=arm-linux-gnueabihf- ARCH=arm64
CROSS_COMPILE=aarch64-linux-gnu- 'CC=sccache aarch64-linux-gnu-gcc'
'HOSTCC=sccache gcc'
/builds/linux/drivers/misc/lkdtm/cfi.c:67:1: error: pragma or
attribute 'target("branch-protection=none")' is not valid
 {
 ^
make[4]: *** [/builds/linux/scripts/Makefile.build:249:
drivers/misc/lkdtm/cfi.o] Error 1
make[4]: Target '__build' not remade because of errors.
make[3]: *** [/builds/linux/scripts/Makefile.build:465:
drivers/misc/lkdtm] Error 2
make[3]: Target '__build' not remade because of errors.
make[2]: *** [/builds/linux/scripts/Makefile.build:465: drivers/misc] Error 2
make[2]: Target '__build' not remade because of errors.
make[1]: *** [/builds/linux/Makefile:1852: drivers] Error 2
----->8---------->8---------->8-----

Reproducer: `tuxmake --runtime podman --target-arch arm64 --toolchain
gcc-8 --kconfig allmodconfig
CROSS_COMPILE_COMPAT=arm-linux-gnueabihf-`

Is this a legit problem?

Greetings!

Daniel Díaz
daniel.diaz@linaro.org