linux-hardening - Re: [PATCH v3 14/16] x86, cpu: Use LTO for cpu.c with CFI

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Date:   Tue, 14 Sep 2021 12:44:59 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     x86@...nel.org, Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH v3 14/16] x86, cpu: Use LTO for cpu.c with CFI

On Tue, Sep 14, 2021 at 12:10:43PM -0700, Sami Tolvanen wrote:
> Allow LTO to be used for cpu.c when CONFIG_CFI_CLANG is enabled to avoid
> indirect call failures. CFI requires Clang >= 13, which doesn't have the
> stack protector inlining bug.
> 
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> ---
>  arch/x86/power/Makefile | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/arch/x86/power/Makefile b/arch/x86/power/Makefile
> index 379777572bc9..a0532851fed7 100644
> --- a/arch/x86/power/Makefile
> +++ b/arch/x86/power/Makefile
> @@ -4,9 +4,11 @@
>  # itself be stack-protected
>  CFLAGS_cpu.o	:= -fno-stack-protector
>  
> +ifndef CONFIG_CFI_CLANG
>  # Clang may incorrectly inline functions with stack protector enabled into
>  # __restore_processor_state(): https://bugs.llvm.org/show_bug.cgi?id=47479
>  CFLAGS_REMOVE_cpu.o := $(CC_FLAGS_LTO)
> +endif

This seems weird -- it's tangential that CONFIG_CFI_CLANG is only on
Clang >= 13, but it does seem better than adding a $(shell) to test the
Clang version. So:

Reviewed-by: Kees Cook <keescook@...omium.org>

>  
>  obj-$(CONFIG_PM_SLEEP)		+= cpu.o
>  obj-$(CONFIG_HIBERNATION)	+= hibernate_$(BITS).o hibernate_asm_$(BITS).o hibernate.o
> -- 
> 2.33.0.309.g3052b89438-goog
> 

-- 
Kees Cook