linux-hardening - Re: [PATCH v3 11/16] x86/purgatory: Disable CFI

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Date:   Tue, 14 Sep 2021 13:02:01 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     x86@...nel.org, Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Nathan Chancellor <nathan@...nel.org>,
        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 11/16] x86/purgatory: Disable CFI

On Tue, Sep 14, 2021 at 12:11 PM Sami Tolvanen <samitolvanen@...gle.com> wrote:
>
> Disable CONFIG_CFI_CLANG for the stand-alone purgatory.ro.
>
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>

I kind of prefer the existing convention that has explicit guards on
specific configs (ie. CONFIG_FUNCTION_TRACER, CONFIG_STACKPROTECTOR,
CONFIG_STACKPROTECTOR_STRONG, CONFIG_RETPOLINE); it's more obvious
which configs may introduce which flags that are problematic. This
patch is ok as is, but it kind of makes this Makefile more
inconsistent.  I would prefer we had the explicit checks.

Does CFI actually do any instrumentation in these object files? I
guess issues in purgatory cause silent/hard to debug kexec failures?

> ---
>  arch/x86/purgatory/Makefile | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/purgatory/Makefile b/arch/x86/purgatory/Makefile
> index 95ea17a9d20c..ed46ad780130 100644
> --- a/arch/x86/purgatory/Makefile
> +++ b/arch/x86/purgatory/Makefile
> @@ -31,7 +31,7 @@ KCOV_INSTRUMENT := n
>  # These are adjustments to the compiler flags used for objects that
>  # make up the standalone purgatory.ro
>
> -PURGATORY_CFLAGS_REMOVE := -mcmodel=kernel
> +PURGATORY_CFLAGS_REMOVE := -mcmodel=kernel $(CC_FLAGS_CFI)
>  PURGATORY_CFLAGS := -mcmodel=large -ffreestanding -fno-zero-initialized-in-bss -g0
>  PURGATORY_CFLAGS += $(DISABLE_STACKLEAK_PLUGIN) -DDISABLE_BRANCH_PROFILING
>  PURGATORY_CFLAGS += -fno-stack-protector
> --
> 2.33.0.309.g3052b89438-goog
>


-- 
Thanks,
~Nick Desaulniers