linux-kernel - [PATCH] lkdtm: Fix content of section containing lkdtm_rodata_do

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Date:   Fri,  8 Oct 2021 18:58:40 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Kees Cook <keescook@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        stable@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>
Subject: [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@...r.kernel.org
Cc: Kees Cook <keescook@...omium.org>
Cc: Arnd Bergmann <arnd@...db.de>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Nick Desaulniers <ndesaulniers@...gle.com>
Cc: Nathan Chancellor <nathan@...nel.org>
Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
---
 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