linux-kernel - Re: [PATCH] x86: Disable CET instrumentation in the kernel

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Date:   Fri, 29 Jan 2021 11:21:05 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     x86@...nel.org, Masami Hiramatsu <masami.hiramatsu@...il.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>, bpf@...r.kernel.org,
        Steven Rostedt <rostedt@...dmis.org>,
        Nikolay Borisov <nborisov@...e.com>
Subject: Re: [PATCH] x86: Disable CET instrumentation in the kernel

On Thu, Jan 28, 2021 at 03:52:19PM -0600, Josh Poimboeuf wrote:
> 
> With retpolines disabled, some configurations of GCC will add Intel CET
> instrumentation to the kernel by default.  That breaks certain tracing
> scenarios by adding a superfluous ENDBR64 instruction before the fentry
> call, for functions which can be called indirectly.
> 
> CET instrumentation isn't currently necessary in the kernel, as CET is
> only supported in user space.  Disable it unconditionally.
> 
> Reported-by: Nikolay Borisov <nborisov@...e.com>
> Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
> ---
>  Makefile          | 6 ------
>  arch/x86/Makefile | 3 +++
>  2 files changed, 3 insertions(+), 6 deletions(-)
> 
> diff --git a/Makefile b/Makefile
> index e0af7a4a5598..51c2bf34142d 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -948,12 +948,6 @@ KBUILD_CFLAGS   += $(call cc-option,-Werror=designated-init)
>  # change __FILE__ to the relative path from the srctree
>  KBUILD_CPPFLAGS += $(call cc-option,-fmacro-prefix-map=$(srctree)/=)
>  
> -# ensure -fcf-protection is disabled when using retpoline as it is
> -# incompatible with -mindirect-branch=thunk-extern
> -ifdef CONFIG_RETPOLINE
> -KBUILD_CFLAGS += $(call cc-option,-fcf-protection=none)
> -endif
> -

Why is that even here, in the main Makefile if this cf-protection thing
is x86-specific?

Are we going to move it back there when some other arch gets CET or
CET-like support?

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette