From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christophe Leroy Date: Wed, 13 Oct 2021 12:45:11 +0000 Subject: Re: [PATCH v1 09/10] lkdtm: Fix lkdtm_EXEC_RODATA() Message-Id: List-Id: References: <7da92e59e148bd23564d63bdd8bcfaba0ba6d1f1.1633964380.git.christophe.leroy@csgroup.eu> <202110130018.7B2129375@keescook> <134b968f-f65f-cd74-3db1-fff60e5ebeb8@csgroup.eu> In-Reply-To: <134b968f-f65f-cd74-3db1-fff60e5ebeb8@csgroup.eu> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 8bit To: Kees Cook Cc: Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , Andrew Morton , "James E.J. Bottomley" , Helge Deller , Arnd Bergmann , Greg Kroah-Hartman , linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-ia64@vger.kernel.org, linux-parisc@vger.kernel.org, linux-arch@vger.kernel.org, linux-mm@kvack.org 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 >>>> --- >>>>   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.