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

Christophe Leroy
2021-10-13T12:45:11Z

Le 13/10/2021 à 09:48, Christophe Leroy a écrit :
>
>
> Le 13/10/2021 à 09:39, Christophe Leroy a écrit :
>>
>>
>> Le 13/10/2021 à 09:23, Kees Cook a écrit :
>>> 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?
>>>
>>
>> When LKDTM is built as a module I guess ?
>>
>
> To be more precise, dereference_symbol_descriptor() calls either
> dereference_kernel_function_descriptor() or
> dereference_module_function_descriptor()
>
> Both functions call dereference_function_descriptor() after checking
> that we want to dereference something that is in the OPD section.
>
> If we call dereference_function_descriptor() directly instead of
> dereference_symbol_descriptor() we skip the range verification and may
> dereference something that is not a function descriptor.
>
> Should we do that ?
>

Indeed we are using it only for well known functions so using
dereference_function_descriptor() is good enough. I'll use that in v2.