Re: [PATCH v1 09/10] lkdtm: Fix lkdtm_EXEC_RODATA()

Kees Cook
2021-10-13T07:23:53Z

On Mon, Oct 11, 2021 at 05:25:36PM +0200, Christophe Leroy wrote:
> Behind a location, lkdtm_EXEC_RODATA() executes a real function,
> not a copy of do_nothing().
>
> So do it directly instead of using execute_location().
>
> And fix displayed addresses by dereferencing the function descriptors.
>
> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
> ---
> 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 442d60ed25ef..da16564e1ecd 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_symbol_descriptor(do_nothing));
> + do_nothing();
> +
> + pr_info("attempting bad execution at %px\n",
> + dereference_symbol_descriptor(lkdtm_rodata_do_nothing));
> + lkdtm_rodata_do_nothing();
> + pr_err("FAIL: func returned\n");
> }

(In re-reading this more carefully, I see now why kallsyms.h is used
earlier: _function_ vs _symbol_ descriptor.)

In the next patch:

static noinline void execute_location(void *dst, bool write)
{
...
func = setup_function_descriptor(&fdesc, dst);
if (IS_ERR(func))
return;

pr_info("attempting bad execution at %px\n", dst);
func();
pr_err("FAIL: func returned\n");
}

What are the conditions for which dereference_symbol_descriptor works
but dereference _function_descriptor doesn't?

--
Kees Cook