Re: [PATCH v2 11/13] lkdtm: Fix lkdtm_EXEC_RODATA()

Kees Cook
2021-10-15T21:33:02Z

On Thu, Oct 14, 2021 at 07:50:00AM +0200, Christophe Leroy wrote:
> Behind its location, lkdtm_EXEC_RODATA() executes
> lkdtm_rodata_do_nothing() which is a real function,
> not a copy of do_nothing().
>
> So executes it directly instead of using execute_location().
>
> This is necessary because following patch will fix execute_location()
> to use a copy of the function descriptor of do_nothing() and
> function descriptor of lkdtm_rodata_do_nothing() might be different.
>
> And fix displayed addresses by dereferencing the function descriptors.
>
> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>

I still don't understand this -- it doesn't look needed at all given the
changes in patch 12. (i.e. everything is using
dereference_function_descriptor() now)

Can't this patch be dropped?

-Kees

> ---
> drivers/misc/lkdtm/perms.c | 9 ++++++++-
> 1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
> index 035fcca441f0..5266dc28df6e 100644
> --- a/drivers/misc/lkdtm/perms.c
> +++ b/drivers/misc/lkdtm/perms.c
> @@ -153,7 +153,14 @@ void lkdtm_EXEC_VMALLOC(void)
>
> void lkdtm_EXEC_RODATA(void)
> {
> - execute_location(lkdtm_rodata_do_nothing, CODE_AS_IS);
> + pr_info("attempting ok execution at %px\n",
> + dereference_function_descriptor(do_nothing));
> + do_nothing();
> +
> + pr_info("attempting bad execution at %px\n",
> + dereference_function_descriptor(lkdtm_rodata_do_nothing));
> + lkdtm_rodata_do_nothing();
> + pr_err("FAIL: func returned\n");
> }
>
> void lkdtm_EXEC_USERSPACE(void)
> --
> 2.31.1
>

--
Kees Cook