linux-kernel - Re: [PATCH 2/2] x86: math-emu: limit MATH

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Date:   Tue, 1 Oct 2019 14:54:27 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Bill Metzenthen <billm@...bpc.org.au>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] x86: math-emu: limit MATH_EMULATION to 486SX
 compatibles

On Tue, Oct 01, 2019 at 04:23:35PM +0200, Arnd Bergmann wrote:
> The fpu emulation code is old and fragile in places, try to limit its
> use to builds for CPUs that actually use it. As far as I can tell,
> this is only true for i486sx compatibles, including the Cyrix 486SLC,
> AMD Am486SX and ÉLAN SC410, UMC U5S amd DM&P VortexSX86, all of which
> were relatively short-lived and got replaced with i486DX compatible
> processors soon after introduction, though the some of the embedded
> versions remained available much longer.
> 
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

Nice, I like carving out CONFIG space for 486SX; this makes sense to me.

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

-Kees

> ---
>  arch/x86/Kconfig              |  2 +-
>  arch/x86/Kconfig.cpu          | 25 ++++++++++++++++---------
>  arch/x86/Makefile_32.cpu      |  1 +
>  arch/x86/include/asm/module.h |  2 ++
>  4 files changed, 20 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index f4d9d1e55e5c..77b02387bd0c 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1751,7 +1751,7 @@ config X86_RESERVE_LOW
>  config MATH_EMULATION
>  	bool
>  	depends on MODIFY_LDT_SYSCALL
> -	prompt "Math emulation" if X86_32
> +	prompt "Math emulation" if X86_32 && (M486SX || MELAN)
>  	---help---
>  	  Linux can emulate a math coprocessor (used for floating point
>  	  operations) if you don't have one. 486DX and Pentium processors have
> diff --git a/arch/x86/Kconfig.cpu b/arch/x86/Kconfig.cpu
> index 228705a1232a..5f7bff9885a1 100644
> --- a/arch/x86/Kconfig.cpu
> +++ b/arch/x86/Kconfig.cpu
> @@ -50,12 +50,19 @@ choice
>  	  See each option's help text for additional details. If you don't know
>  	  what to do, choose "486".
>  
> +config M486SX
> +	bool "486SX"
> +	depends on X86_32
> +	---help---
> +	  Select this for an 486-class CPU without an FPU such as
> +	  AMD/Cyrix/IBM/Intel SL/SLC/SLC2/SLC3/SX/SX2 and UMC U5S.
> +
>  config M486
> -	bool "486"
> +	bool "486DX"
>  	depends on X86_32
>  	---help---
>  	  Select this for an 486-class CPU such as AMD/Cyrix/IBM/Intel
> -	  486DX/DX2/DX4 or SL/SLC/SLC2/SLC3/SX/SX2 and UMC U5D or U5S.
> +	  486DX/DX2/DX4 and UMC U5D.
>  
>  config M586
>  	bool "586/K5/5x86/6x86/6x86MX"
> @@ -313,20 +320,20 @@ config X86_L1_CACHE_SHIFT
>  	int
>  	default "7" if MPENTIUM4 || MPSC
>  	default "6" if MK7 || MK8 || MPENTIUMM || MCORE2 || MATOM || MVIAC7 || X86_GENERIC || GENERIC_CPU
> -	default "4" if MELAN || M486 || MGEODEGX1
> +	default "4" if MELAN || M486SX || M486 || MGEODEGX1
>  	default "5" if MWINCHIP3D || MWINCHIPC6 || MCRUSOE || MEFFICEON || MCYRIXIII || MK6 || MPENTIUMIII || MPENTIUMII || M686 || M586MMX || M586TSC || M586 || MVIAC3_2 || MGEODE_LX
>  
>  config X86_F00F_BUG
>  	def_bool y
> -	depends on M586MMX || M586TSC || M586 || M486
> +	depends on M586MMX || M586TSC || M586 || M486SX || M486
>  
>  config X86_INVD_BUG
>  	def_bool y
> -	depends on M486
> +	depends on M486SX || M486
>  
>  config X86_ALIGNMENT_16
>  	def_bool y
> -	depends on MWINCHIP3D || MWINCHIPC6 || MCYRIXIII || MELAN || MK6 || M586MMX || M586TSC || M586 || M486 || MVIAC3_2 || MGEODEGX1
> +	depends on MWINCHIP3D || MWINCHIPC6 || MCYRIXIII || MELAN || MK6 || M586MMX || M586TSC || M586 || M486SX || M486 || MVIAC3_2 || MGEODEGX1
>  
>  config X86_INTEL_USERCOPY
>  	def_bool y
> @@ -379,7 +386,7 @@ config X86_MINIMUM_CPU_FAMILY
>  
>  config X86_DEBUGCTLMSR
>  	def_bool y
> -	depends on !(MK6 || MWINCHIPC6 || MWINCHIP3D || MCYRIXIII || M586MMX || M586TSC || M586 || M486) && !UML
> +	depends on !(MK6 || MWINCHIPC6 || MWINCHIP3D || MCYRIXIII || M586MMX || M586TSC || M586 || M486SX || M486) && !UML
>  
>  menuconfig PROCESSOR_SELECT
>  	bool "Supported processor vendors" if EXPERT
> @@ -403,7 +410,7 @@ config CPU_SUP_INTEL
>  config CPU_SUP_CYRIX_32
>  	default y
>  	bool "Support Cyrix processors" if PROCESSOR_SELECT
> -	depends on M486 || M586 || M586TSC || M586MMX || (EXPERT && !64BIT)
> +	depends on M486SX || M486 || M586 || M586TSC || M586MMX || (EXPERT && !64BIT)
>  	---help---
>  	  This enables detection, tunings and quirks for Cyrix processors
>  
> @@ -471,7 +478,7 @@ config CPU_SUP_TRANSMETA_32
>  config CPU_SUP_UMC_32
>  	default y
>  	bool "Support UMC processors" if PROCESSOR_SELECT
> -	depends on M486 || (EXPERT && !64BIT)
> +	depends on M486SX || M486 || (EXPERT && !64BIT)
>  	---help---
>  	  This enables detection, tunings and quirks for UMC processors
>  
> diff --git a/arch/x86/Makefile_32.cpu b/arch/x86/Makefile_32.cpu
> index 1f5faf8606b4..cd3056759880 100644
> --- a/arch/x86/Makefile_32.cpu
> +++ b/arch/x86/Makefile_32.cpu
> @@ -10,6 +10,7 @@ else
>  tune		= $(call cc-option,-mcpu=$(1),$(2))
>  endif
>  
> +cflags-$(CONFIG_M486SX)		+= -march=i486
>  cflags-$(CONFIG_M486)		+= -march=i486
>  cflags-$(CONFIG_M586)		+= -march=i586
>  cflags-$(CONFIG_M586TSC)	+= -march=i586
> diff --git a/arch/x86/include/asm/module.h b/arch/x86/include/asm/module.h
> index 7948a17febb4..c215d2762488 100644
> --- a/arch/x86/include/asm/module.h
> +++ b/arch/x86/include/asm/module.h
> @@ -15,6 +15,8 @@ struct mod_arch_specific {
>  
>  #ifdef CONFIG_X86_64
>  /* X86_64 does not define MODULE_PROC_FAMILY */
> +#elif defined CONFIG_M486SX
> +#define MODULE_PROC_FAMILY "486SX "
>  #elif defined CONFIG_M486
>  #define MODULE_PROC_FAMILY "486 "
>  #elif defined CONFIG_M586
> -- 
> 2.20.0
> 

-- 
Kees Cook