linux-kernel - Re: [PATCH v4 2/3] x86/ldt: Make modify

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Message-ID: <20150725091531.GE3427@nazgul.tnic>
Date:	Sat, 25 Jul 2015 11:15:31 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	"security@...nel.org" <security@...nel.org>,
	X86 ML <x86@...nel.org>, Sasha Levin <sasha.levin@...cle.com>,
	linux-kernel@...r.kernel.org,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Jan Beulich <jbeulich@...e.com>,
	xen-devel <xen-devel@...ts.xen.org>
Subject: Re: [PATCH v4 2/3] x86/ldt: Make modify_ldt optional

On Fri, Jul 24, 2015 at 10:36:45PM -0700, Andy Lutomirski wrote:
> The modify_ldt syscall exposes a large attack surface and is
> unnecessary for modern userspace.  Make it optional.
> 
> Signed-off-by: Andy Lutomirski <luto@...nel.org>
> ---
>  arch/x86/Kconfig                   | 17 +++++++++++++++++
>  arch/x86/include/asm/mmu.h         |  2 ++
>  arch/x86/include/asm/mmu_context.h | 31 +++++++++++++++++++++++--------
>  arch/x86/kernel/Makefile           |  3 ++-
>  arch/x86/kernel/cpu/perf_event.c   |  4 ++++
>  arch/x86/kernel/process_64.c       |  2 ++
>  arch/x86/kernel/step.c             |  2 ++
>  kernel/sys_ni.c                    |  1 +
>  8 files changed, 53 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index b3a1a5d77d92..ede52be845db 100644
> --- 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

	bool "Enable modify_ldt() for per-process Local Descriptor Table"

is how I'd call it.

> +       default y

Is that "default y" going to turn into a "default n" after a grace
period?

> +       ---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.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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