diff mbox series

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

Message ID 7da92e59e148bd23564d63bdd8bcfaba0ba6d1f1.1633964380.git.christophe.leroy@csgroup.eu (mailing list archive)
State Superseded
Headers show
Series Fix LKDTM for PPC64/IA64/PARISC | expand

Commit Message

Christophe Leroy Oct. 11, 2021, 3:25 p.m. UTC
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(-)

Comments

Kees Cook Oct. 13, 2021, 7:09 a.m. UTC | #1
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().

I don't understand this. Why does the next patch not fix this?

-Kees

> 
> 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");
>  }
>  
>  void lkdtm_EXEC_USERSPACE(void)
> -- 
> 2.31.1
>
Kees Cook Oct. 13, 2021, 7:23 a.m. UTC | #2
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
Christophe Leroy Oct. 13, 2021, 7:35 a.m. UTC | #3
Le 13/10/2021 à 09:09, 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().
> 
> I don't understand this. Why does the next patch not fix this?

Well, probably it would, but it looked incorrect in my mind.

lkdtm_rodata_do_nothing() is a function which has its own function 
descriptor, it is not a copy of do_nothing().

If we use execute_location() as modified by next patch, then we will 
execute it using the function descriptor of do_nothing(). Allthough it 
most likely works (at least on powerpc as it uses the same TOC) it looks 
odd to me to do so.

Am I missing something ?

Christophe


> 
> -Kees
> 
>>
>> 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");
>>   }
>>   
>>   void lkdtm_EXEC_USERSPACE(void)
>> -- 
>> 2.31.1
>>
>
Christophe Leroy Oct. 13, 2021, 7:39 a.m. UTC | #4
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
Christophe Leroy Oct. 13, 2021, 7:48 a.m. UTC | #5
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 ?

Christophe
Christophe Leroy Oct. 13, 2021, 12:45 p.m. UTC | #6
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.
diff mbox series

Patch

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");
 }
 
 void lkdtm_EXEC_USERSPACE(void)