linux-kernel - Re: [PATCH v4 34/75] x86/head/64: Build k/head64.c with -fno-stack-protector

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Date:   Tue, 14 Jul 2020 18:34:24 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Joerg Roedel <joro@...tes.org>
Cc:     x86@...nel.org, Joerg Roedel <jroedel@...e.de>, hpa@...or.com,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Jiri Slaby <jslaby@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Juergen Gross <jgross@...e.com>,
        David Rientjes <rientjes@...gle.com>,
        Cfir Cohen <cfir@...gle.com>,
        Erdem Aktas <erdemaktas@...gle.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mike Stunes <mstunes@...are.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Martin Radev <martin.b.radev@...il.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v4 34/75] x86/head/64: Build k/head64.c with
 -fno-stack-protector

On Tue, Jul 14, 2020 at 02:08:36PM +0200, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@...e.de>
> 
> The code inserted by the stack protector does not work in the early
> boot environment because it uses the GS segment, at least with memory
> encryption enabled. Make sure the early code is compiled without this
> feature enabled.
> 
> Signed-off-by: Joerg Roedel <jroedel@...e.de>
> ---
>  arch/x86/kernel/Makefile | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
> index e77261db2391..1b166b866059 100644
> --- a/arch/x86/kernel/Makefile
> +++ b/arch/x86/kernel/Makefile
> @@ -39,6 +39,10 @@ ifdef CONFIG_FRAME_POINTER
>  OBJECT_FILES_NON_STANDARD_ftrace_$(BITS).o		:= y
>  endif
>  
> +# make sure head64.c is built without stack protector
> +nostackp := $(call cc-option, -fno-stack-protector)
> +CFLAGS_head64.o		:= $(nostackp)

Recent refactoring[1] for stack protector suggests this should just
unconditionally be:

CFLAGS_head64.o			+= -fno-stack-protector

But otherwise, yeah, this should be fine here -- it's all early init
stuff.

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

[1] https://lore.kernel.org/lkml/20200626185913.92890-1-masahiroy@kernel.org/

> +
>  # If instrumentation of this dir is enabled, boot hangs during first second.
>  # Probably could be more selective here, but note that files related to irqs,
>  # boot, dumpstack/stacktrace, etc are either non-interesting or can lead to
> -- 
> 2.27.0
> 

-- 
Kees Cook