[PATCH] lkdtm: Fix content of section containing lkdtm_rodata_do_nothing()

On a kernel without CONFIG_STRICT_KERNEL_RWX, running EXEC_RODATA
test leads to "Illegal instruction" failure.

Looking at the content of rodata_objcopy.o, we see that the
function content zeroes only:

	Disassembly of section .rodata:

	0000000000000000 <.lkdtm_rodata_do_nothing>:
	   0:	00 00 00 00 	.long 0x0

Add the contents flag in order to keep the content of the section
while renaming it.

	Disassembly of section .rodata:

	0000000000000000 <.lkdtm_rodata_do_nothing>:
	   0:	4e 80 00 20 	blr

Fixes: e9e08a07385e ("lkdtm: support llvm-objcopy")
Cc: stable@vger.kernel.org
Cc: Kees Cook <redacted>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Christophe Leroy <redacted>
---
 drivers/misc/lkdtm/Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/misc/lkdtm/Makefile b/drivers/misc/lkdtm/Makefile
index aa12097668d3..e2984ce51fe4 100644
--- a/drivers/misc/lkdtm/Makefile
+++ b/drivers/misc/lkdtm/Makefile

@@ -20,7 +20,7 @@ CFLAGS_REMOVE_rodata.o		+= $(CC_FLAGS_LTO)
 
 OBJCOPYFLAGS :=
 OBJCOPYFLAGS_rodata_objcopy.o	:= \
-			--rename-section .noinstr.text=.rodata,alloc,readonly,load
+			--rename-section .noinstr.text=.rodata,alloc,readonly,load,contents
 targets += rodata.o rodata_objcopy.o
 $(obj)/rodata_objcopy.o: $(obj)/rodata.o FORCE
 	$(call if_changed,objcopy)

-- 
2.31.1

On Fri, Oct 8, 2021 at 9:59 AM Christophe Leroy
[off-list ref] wrote:

On a kernel without CONFIG_STRICT_KERNEL_RWX, running EXEC_RODATA
test leads to "Illegal instruction" failure.

Looking at the content of rodata_objcopy.o, we see that the
function content zeroes only:

        Disassembly of section .rodata:

        0000000000000000 <.lkdtm_rodata_do_nothing>:
           0:   00 00 00 00     .long 0x0

Add the contents flag in order to keep the content of the section
while renaming it.

        Disassembly of section .rodata:

        0000000000000000 <.lkdtm_rodata_do_nothing>:
           0:   4e 80 00 20     blr

Fixes: e9e08a07385e ("lkdtm: support llvm-objcopy")

Thanks for the patch; sorry I broke this.
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

-- 
Thanks,
~Nick Desaulniers

On Fri, Oct 08, 2021 at 11:09:47AM -0700, Nick Desaulniers wrote:

On Fri, Oct 8, 2021 at 9:59 AM Christophe Leroy
[off-list ref] wrote:

quoted

On a kernel without CONFIG_STRICT_KERNEL_RWX, running EXEC_RODATA
test leads to "Illegal instruction" failure.

Looking at the content of rodata_objcopy.o, we see that the
function content zeroes only:

        Disassembly of section .rodata:

        0000000000000000 <.lkdtm_rodata_do_nothing>:
           0:   00 00 00 00     .long 0x0

Add the contents flag in order to keep the content of the section
while renaming it.

        Disassembly of section .rodata:

        0000000000000000 <.lkdtm_rodata_do_nothing>:
           0:   4e 80 00 20     blr

Fixes: e9e08a07385e ("lkdtm: support llvm-objcopy")

Thanks for the patch; sorry I broke this.
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

Hah! Whoops; sorry I don't have an inverted version of this test! I
should have caught this when it broke. :|

-Kees

-- 
Kees Cook

On Fri, 8 Oct 2021 18:58:40 +0200, Christophe Leroy wrote:

On a kernel without CONFIG_STRICT_KERNEL_RWX, running EXEC_RODATA
test leads to "Illegal instruction" failure.

Looking at the content of rodata_objcopy.o, we see that the
function content zeroes only:

	Disassembly of section .rodata:

[...]

Applied to for-next/lkdtm, thanks!

[1/1] lkdtm: Fix content of section containing lkdtm_rodata_do_nothing()
      https://git.kernel.org/kees/c/19c3069c5f5f

Also, can you take a moment and get "patatt" set up[1] for signing your
patches? I would appreciate that since b4 yells at me when patches aren't
signed. :)

-Kees

[1] https://github.com/mricon/patatt

-- 
Kees Cook