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

[Kees Cook <keescook@xxxxxxxxxxxx>]

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@xxxxxxxxxx>
> ---
>  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