linux-kernel - Re: [PATCH v2 2/3] x86/ldt: Make modify

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Date:	Tue, 21 Jul 2015 16:28:58 -0400
From:	Brian Gerst <brgerst@...il.com>
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>, Borislav Petkov <bp@...en8.de>,
	Sasha Levin <sasha.levin@...cle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: Re: [PATCH v2 2/3] x86/ldt: Make modify_ldt optional

On Tue, Jul 21, 2015 at 3:59 PM, Andy Lutomirski <luto@...nel.org> 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 55bced17dc95..a7ff3980bd65 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1009,6 +1009,7 @@ 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
> @@ -2047,6 +2048,22 @@ 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 believe Wine still uses the LDT for thread-local data, even for 32
and 64-bit programs.  This is separate from the Linux runtime TLS.

--
Brian Gerst
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