linux-kernel - Re: [PATCH v1 09/10] lkdtm: Fix lkdtm_EXEC

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Message-ID: <134b968f-f65f-cd74-3db1-fff60e5ebeb8@csgroup.eu>
Date:   Wed, 13 Oct 2021 09:48:54 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Kees Cook <keescook@...omium.org>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
        Helge Deller <deller@....de>, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-ia64@...r.kernel.org, linux-parisc@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v1 09/10] lkdtm: Fix lkdtm_EXEC_RODATA()



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@...roup.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