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From:   Arvind Sankar <nivedita@alum.mit.edu>
Date:   Fri, 24 Jul 2020 13:58:18 -0400
To:     Joerg Roedel <joro@8bytes.org>
Cc:     x86@kernel.org, Joerg Roedel <jroedel@suse.de>, hpa@zytor.com,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
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        Jiri Slaby <jslaby@suse.cz>,
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        Tom Lendacky <thomas.lendacky@amd.com>,
        Juergen Gross <jgross@suse.com>,
        Kees Cook <keescook@chromium.org>,
        David Rientjes <rientjes@google.com>,
        Cfir Cohen <cfir@google.com>,
        Erdem Aktas <erdemaktas@google.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
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        Sean Christopherson <sean.j.christopherson@intel.com>,
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Subject: Re: [PATCH v5 11/75] x86/boot/compressed/64: Disable red-zone usage
Message-ID: <20200724175818.GA732164@rani.riverdale.lan>
References: <20200724160336.5435-1-joro@8bytes.org>
 <20200724160336.5435-12-joro@8bytes.org>
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On Fri, Jul 24, 2020 at 06:02:32PM +0200, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@suse.de>
> 
> The x86-64 ABI defines a red-zone on the stack:
> 
>   The 128-byte area beyond the location pointed to by %rsp is considered
>   to be reserved and shall not be modified by signal or interrupt
>   handlers. Therefore, functions may use this area for temporary data
>   that is not needed across function calls. In particular, leaf
>   functions may use this area for their entire stack frame, rather than
>   adjusting the stack pointer in the prologue and epilogue. This area is
>   known as the red zone.
> 
> This is not compatible with exception handling, because the IRET frame
> written by the hardware at the stack pointer and the functions to handle
> the exception will overwrite the temporary variables of the interrupted
> function, causing undefined behavior. So disable red-zones for the
> pre-decompression boot code.
> 
> Signed-off-by: Joerg Roedel <jroedel@suse.de>
> ---
>  arch/x86/boot/Makefile            | 2 +-
>  arch/x86/boot/compressed/Makefile | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/boot/Makefile b/arch/x86/boot/Makefile
> index fe605205b4ce..4d6a16a47e9f 100644
> --- a/arch/x86/boot/Makefile
> +++ b/arch/x86/boot/Makefile
> @@ -66,7 +66,7 @@ targets += cpustr.h
>  
>  # ---------------------------------------------------------------------------
>  
> -KBUILD_CFLAGS	:= $(REALMODE_CFLAGS) -D_SETUP
> +KBUILD_CFLAGS	:= $(REALMODE_CFLAGS) -D_SETUP -mno-red-zone
>  KBUILD_AFLAGS	:= $(KBUILD_CFLAGS) -D__ASSEMBLY__
>  KBUILD_CFLAGS	+= $(call cc-option,-fmacro-prefix-map=$(srctree)/=)
>  KBUILD_CFLAGS	+= -fno-asynchronous-unwind-tables

This change seems unnecessary? REALMODE_CFLAGS means it uses -m16, so
this isn't 64-bit code. [As an aside, we can drop the check for -m16
support in arch/x86/Makefile now that we've bumbed to 4.9]

> diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> index 5a828fde7a42..416f52ab39ec 100644
> --- a/arch/x86/boot/compressed/Makefile
> +++ b/arch/x86/boot/compressed/Makefile
> @@ -32,7 +32,7 @@ KBUILD_CFLAGS := -m$(BITS) -O2
>  KBUILD_CFLAGS += -fno-strict-aliasing $(call cc-option, -fPIE, -fPIC)
>  KBUILD_CFLAGS += -DDISABLE_BRANCH_PROFILING
>  cflags-$(CONFIG_X86_32) := -march=i386
> -cflags-$(CONFIG_X86_64) := -mcmodel=small
> +cflags-$(CONFIG_X86_64) := -mcmodel=small -mno-red-zone
>  KBUILD_CFLAGS += $(cflags-y)
>  KBUILD_CFLAGS += -mno-mmx -mno-sse
>  KBUILD_CFLAGS += $(call cc-option,-ffreestanding)
> -- 
> 2.27.0
> 

This one is necessary.