Messages in this thread | | | Subject | Re: [PATCH v1 09/10] lkdtm: Fix lkdtm_EXEC_RODATA() | From | Christophe Leroy <> | Date | Wed, 13 Oct 2021 09:48:54 +0200 |
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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
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