linux-kernel - Re: [PATCH v3 2/3] x86/ldt: Make modify

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Date:	Thu, 23 Jul 2015 01:13:46 -0600
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Andy Lutomirski" <luto@...nel.org>
Cc:	"Borislav Petkov" <bp@...en8.de>,
	"Andrew Cooper" <andrew.cooper3@...rix.com>,
	"Steven Rostedt" <rostedt@...dmis.org>,
	"Peter Zijlstra" <peterz@...radead.org>,
	"security@...nel.org" <security@...nel.org>,
	"X86 ML" <x86@...nel.org>, "xen-devel" <xen-devel@...ts.xen.org>,
	"Boris Ostrovsky" <boris.ostrovsky@...cle.com>,
	"Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>,
	"Sasha Levin" <sasha.levin@...cle.com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 2/3] x86/ldt: Make modify_ldt optional

>>> On 22.07.15 at 21:23, <luto@...nel.org> wrote:
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1015,6 +1015,7 @@ config VM86
>  config X86_16BIT
>  	bool "Enable support for 16-bit segments" if EXPERT
>  	default y
> +	depends on MODIFY_LDT_SYSCALL
>  	---help---
>  	  This option is required by programs like Wine to run 16-bit
>  	  protected mode legacy code on x86 processors.  Disabling
> @@ -2053,6 +2054,22 @@ config CMDLINE_OVERRIDE
>  	  This is used to work around broken boot loaders.  This should
>  	  be set to 'N' under normal conditions.
>  
> +config MODIFY_LDT_SYSCALL
> +       bool "Enable the LDT (local descriptor table)" if EXPERT
> +       default y
> +       ---help---
> +         Linux can allow user programs to install a per-process x86
> +	 Local Descriptor Table (LDT) using the modify_ldt(2) system
> +	 call.  This is required to run 16-bit or segmented code such as
> +	 DOSEMU or some Wine programs.  It is also used by some very old
> +	 threading libraries.
> +
> +	 Enabling this feature adds a small amount of overhead to
> +	 context switches and increases the low-level kernel attack
> +	 surface.  Disabling it removes the modify_ldt(2) system call.
> +
> +	 Saying 'N' here may make sense for embedded or server kernels.
> +

I think it would be better to place this ahead of the one being
made dependent on it, to avoid the user being prompted for
X86_16BIT despite it possibly becoming unavailable (when this
one gets set to n).

Jan

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