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

Christophe Leroy
2021-10-17T07:50:17Z

Le 16/10/2021 à 08:41, Christophe Leroy a écrit :
>
>
> Le 15/10/2021 à 23:32, Kees Cook a écrit :
>> 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)
>
> dereference_function_descriptor() only deals with the function address,
> not the function TOC.
>
> do_nothing() is a function. It has a function descriptor with a given
> address (address of .do_nothing) and a given TOC, say TOC1.
>
> lkdtm_rodata_do_nothing() is another function. It has its own function
> descriptor with a given address (address of .lkdtm_rodata_do_nothing)
> and a given TOC, say TOC2.
>
> If we use execute_location(), it will copy do_nothing() function
> descriptor and change the function address to the address of
> lkdtm_rodata_do_nothing(). So it will call lkdtm_rodata_do_nothing()
> with TOC1 instead of calling it with TOC2.
>
>>
>> Can't this patch be dropped?
>
> It is likely that the TOC will be the same for both functions, and
> anyway those functions are so simple that they don't use the TOC at all,
> so yes it would likely work without this patch but from my point of view
> it is incorrect to call one function with the TOC from the descriptor of
> another function.
>
> If you thing we can take the risk, then I'm happy to drop the patch and
> replace it by
>
>     execute_location(dereference_function_descriptor(lkdtm_rodata_do_nothing), CODE_AS_IS)
>

Once we have patch 12 EXEC_RODATA works well on powerpc without this
patch so I will drop this patch for now and will propose something else
as a follow-up to my series.

Christophe