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

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Date:   Wed, 13 Oct 2021 09:39:34 +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 09:23, 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 ?

Christophe