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

[Christophe Leroy <christophe.leroy@csgroup.eu>]

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 ?

Christophe