linux-kernel - Re: [PATCH 11/70] x86/boot/compressed/64: Disable red-zone usage

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Date:   Tue, 31 Mar 2020 15:16:06 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Joerg Roedel <joro@...tes.org>
Cc:     x86@...nel.org, hpa@...or.com, Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Hellstrom <thellstrom@...are.com>,
        Jiri Slaby <jslaby@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Juergen Gross <jgross@...e.com>,
        Kees Cook <keescook@...omium.org>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        Joerg Roedel <jroedel@...e.de>
Subject: Re: [PATCH 11/70] x86/boot/compressed/64: Disable red-zone usage

On Thu, Mar 19, 2020 at 10:13:08AM +0100, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@...e.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. 10 Therefore, functions may use this area for
			^^

That 10 is the footnote number from the pdf. :)

>   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, so disable it for the

I could use some blurb as to what the problem is, for future reference.

> pre-decompression boot code.
> 
> Signed-off-by: Joerg Roedel <jroedel@...e.de>
> ---
>  arch/x86/boot/Makefile            | 2 +-
>  arch/x86/boot/compressed/Makefile | 4 ++--
>  2 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/boot/Makefile b/arch/x86/boot/Makefile
> index 012b82fc8617..8f55e4ce1ccc 100644
> --- a/arch/x86/boot/Makefile
> +++ b/arch/x86/boot/Makefile
> @@ -65,7 +65,7 @@ clean-files += 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)/=)
>  GCOV_PROFILE := n
> diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> index 26050ae0b27e..e186cc0b628d 100644
> --- a/arch/x86/boot/compressed/Makefile
> +++ b/arch/x86/boot/compressed/Makefile
> @@ -30,7 +30,7 @@ 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 += $(call cc-option,-ffreestanding)
> @@ -87,7 +87,7 @@ endif
>  
>  vmlinux-objs-$(CONFIG_ACPI) += $(obj)/acpi.o
>  
> -$(obj)/eboot.o: KBUILD_CFLAGS += -fshort-wchar -mno-red-zone
> +$(obj)/eboot.o: KBUILD_CFLAGS += -fshort-wchar
>  
>  vmlinux-objs-$(CONFIG_EFI_STUB) += $(obj)/eboot.o \
>  	$(objtree)/drivers/firmware/efi/libstub/lib.a

That last chunk is not needed anymore after

c2d0b470154c ("efi/libstub/x86: Incorporate eboot.c into libstub")

AFAICT.

-- 
Regards/Gruss,
    Boris.

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