Re: [PATCH] lkdtm: Keep the rodata test from causing retbleed WARNINGS

[Kees Cook <keescook@chromium.org>]

On Wed, Jul 20, 2022 at 03:55:07PM -0400, Steven Rostedt wrote:
> From: "Steven Rostedt (Google)" <rostedt@goodmis.org>
> 
> The rodata test checks if executable code in the .rodata section will not
> execute. But this also means that fix ups that the return hook code does
> will not be able to change this code at boot up, and this causes a
> warning.
> 
> By removing the RETHOOK_CFLAGS from the compilation of the rodata function
> makes it hidden from objtool and it will not add its return hook into the
> .returns section. This keeps the fix up code from trying and failing on
> modifying the rodata return call.
> 
> Link: https://lore.kernel.org/all/20220720125736.48164a14@gandalf.local.home/
> 
> Fixes: ee88d363d1561 ("x86,static_call: Use alternative RET encoding")
> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Yeah, this matches a portion of what was suggested:
https://lore.kernel.org/lkml/20220713213133.455599-1-keescook@chromium.org/

But it still needed objtool fixes:
https://lore.kernel.org/lkml/20220715032333.neywlazczbb35w3b@treble/

Perhaps objtool has already had patches landed for this? (Otherwise the
patch you suggested would fail to build on x86 with rethunk support.)

-Kees

> ---
> 
>  [ Note, my tests are still running with this, but it passed the test that
>    caused the failure, so I decided to post this now. ]
> 
>  arch/x86/Makefile           | 1 +
>  drivers/misc/lkdtm/Makefile | 2 +-
>  2 files changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index 1f40dad30d50..2dd61d8594f4 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -25,6 +25,7 @@ endif
>  ifdef CONFIG_RETHUNK
>  RETHUNK_CFLAGS		:= -mfunction-return=thunk-extern
>  RETPOLINE_CFLAGS	+= $(RETHUNK_CFLAGS)
> +export RETHUNK_CFLAGS
>  endif
>  
>  export RETPOLINE_CFLAGS
> diff --git a/drivers/misc/lkdtm/Makefile b/drivers/misc/lkdtm/Makefile
> index 2e0aa74ac185..fd96ac1617f7 100644
> --- a/drivers/misc/lkdtm/Makefile
> +++ b/drivers/misc/lkdtm/Makefile
> @@ -16,7 +16,7 @@ lkdtm-$(CONFIG_PPC_64S_HASH_MMU)	+= powerpc.o
>  KASAN_SANITIZE_rodata.o		:= n
>  KASAN_SANITIZE_stackleak.o	:= n
>  KCOV_INSTRUMENT_rodata.o	:= n
> -CFLAGS_REMOVE_rodata.o		+= $(CC_FLAGS_LTO)
> +CFLAGS_REMOVE_rodata.o		+= $(CC_FLAGS_LTO) $(RETHUNK_CFLAGS)
>  
>  OBJCOPYFLAGS :=
>  OBJCOPYFLAGS_rodata_objcopy.o	:= \
> -- 
> 2.35.1
> 

-- 
Kees Cook