linux-kernel - [PATCH] lkdtm: Disable CFI checking for perms functions

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Message-Id: <20240430234953.work.760-kees@kernel.org>
Date: Tue, 30 Apr 2024 16:49:57 -0700
From: Kees Cook <keescook@...omium.org>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: Kees Cook <keescook@...omium.org>,
	kernel test robot <oliver.sang@...el.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org,
	linux-hardening@...r.kernel.org
Subject: [PATCH] lkdtm: Disable CFI checking for perms functions

The EXEC_RODATA test plays a lot of tricks to live in the .rodata section,
and once again ran into objtool's (completely reasonable) assumptions
that executable code should live in an executable section. However, this
manifested only under CONFIG_CFI_CLANG=y, as one of the .cfi_sites was
pointing into the .rodata section.

Since we're testing non-CFI execution properties in perms.c (and
rodata.c), we can disable CFI for the involved functions, and remove the
CFI arguments from rodata.c entirely.

Reported-by: kernel test robot <oliver.sang@...el.com>
Closes: https://lore.kernel.org/oe-lkp/202308301532.d7acf63e-oliver.sang@intel.com
Fixes: 6342a20efbd8 ("objtool: Add elf_create_section_pair()")
Signed-off-by: Kees Cook <keescook@...omium.org>
---
Cc: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
---
 drivers/misc/lkdtm/Makefile | 2 +-
 drivers/misc/lkdtm/perms.c  | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/misc/lkdtm/Makefile b/drivers/misc/lkdtm/Makefile
index 95ef971b5e1c..b28701138b4b 100644
--- a/drivers/misc/lkdtm/Makefile
+++ b/drivers/misc/lkdtm/Makefile
@@ -19,7 +19,7 @@ KASAN_SANITIZE_rodata.o			:= n
 KCSAN_SANITIZE_rodata.o			:= n
 KCOV_INSTRUMENT_rodata.o		:= n
 OBJECT_FILES_NON_STANDARD_rodata.o	:= y
-CFLAGS_REMOVE_rodata.o			+= $(CC_FLAGS_LTO) $(RETHUNK_CFLAGS)
+CFLAGS_REMOVE_rodata.o			+= $(CC_FLAGS_LTO) $(RETHUNK_CFLAGS) $(CC_FLAGS_CFI)
 
 OBJCOPYFLAGS :=
 OBJCOPYFLAGS_rodata_objcopy.o	:= \
diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
index b93404d65650..5b861dbff27e 100644
--- a/drivers/misc/lkdtm/perms.c
+++ b/drivers/misc/lkdtm/perms.c
@@ -61,7 +61,7 @@ static void *setup_function_descriptor(func_desc_t *fdesc, void *dst)
 	return fdesc;
 }
 
-static noinline void execute_location(void *dst, bool write)
+static noinline __nocfi void execute_location(void *dst, bool write)
 {
 	void (*func)(void);
 	func_desc_t fdesc;
-- 
2.34.1